Android ListView源码学习
NelleVNLV
8年前
<h2><strong>ListView源码分析</strong></h2> <p>项目中使用ListView还是挺多的,之前看过几次,很是容易遗忘,今特做记录如下</p> <ul> <li>Android 6.0 & API Level 23</li> <li>Github: <a href="/misc/goto?guid=4959714730929315871" rel="nofollow,noindex">Nvsleep</a></li> <li>邮箱: lizhenqiao@126.com</li> <li>QQ: 522910000</li> </ul> <h3><strong>主要从以下几点进行源码分析</strong></h3> <ul> <li>构造函数初始化</li> <li>onMeasure()</li> <li>onLayout()</li> <li>listview.setAdapter() 以及 adapter.notifyDataSetChanged()</li> <li>onInterceptTouchEvent()和onTouchEvent()</li> </ul> <h3><strong>简要</strong></h3> <p>ListView继承之AbsListView抽象类,所以大部分分析的源码都在这两个类中</p> <h2><strong>构造函数初始化过程</strong></h2> <p><strong>父类AbsListView的初始化:</strong></p> <pre> <code class="language-java">public AbsListView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) { super(context, attrs, defStyleAttr, defStyleRes); // 初始化设置一些额外属性值 initAbsListView(); mOwnerThread = Thread.currentThread(); // 初始化XML文件中设置的某些默认属性值 final TypedArray a = context.obtainStyledAttributes( attrs, R.styleable.AbsListView, defStyleAttr, defStyleRes); final Drawable selector = a.getDrawable(R.styleable.AbsListView_listSelector); if (selector != null) { setSelector(selector); } mDrawSelectorOnTop = a.getBoolean(R.styleable.AbsListView_drawSelectorOnTop, false); // 初始化设置mStackFromBottom,这个影响到布局子view的顺序方式,默认为false setStackFromBottom(a.getBoolean( R.styleable.AbsListView_stackFromBottom, false)); setScrollingCacheEnabled(a.getBoolean( R.styleable.AbsListView_scrollingCache, true)); setTextFilterEnabled(a.getBoolean( R.styleable.AbsListView_textFilterEnabled, false)); setTranscriptMode(a.getInt( R.styleable.AbsListView_transcriptMode, TRANSCRIPT_MODE_DISABLED)); setCacheColorHint(a.getColor( R.styleable.AbsListView_cacheColorHint, 0)); setSmoothScrollbarEnabled(a.getBoolean( R.styleable.AbsListView_smoothScrollbar, true)); setChoiceMode(a.getInt( R.styleable.AbsListView_choiceMode, CHOICE_MODE_NONE)); setFastScrollEnabled(a.getBoolean( R.styleable.AbsListView_fastScrollEnabled, false)); setFastScrollStyle(a.getResourceId( R.styleable.AbsListView_fastScrollStyle, 0)); setFastScrollAlwaysVisible(a.getBoolean( R.styleable.AbsListView_fastScrollAlwaysVisible, false)); a.recycle(); } private void initAbsListView() { // Setting focusable in touch mode will set the focusable property to true // 设置ListView本身可以点击即可以消耗父View分发的事件 setClickable(true); setFocusableInTouchMode(true); // 因为向上父类还继承之ViewGroup,ViewGroup默认不需要重写draw()方法, // 从而setWillNotDraw(true),但是AbsListView为了滚动效果,自身重写了View的 // draw(),主要用于实现滚动到最底部或最顶部的非OVER_SCROLL_NEVER模式的效果 setWillNotDraw(false); setAlwaysDrawnWithCacheEnabled(false); setScrollingCacheEnabled(true); // 事件处理相关变量初始化 final ViewConfiguration configuration = ViewConfiguration.get(mContext); mTouchSlop = configuration.getScaledTouchSlop(); mMinimumVelocity = configuration.getScaledMinimumFlingVelocity(); mMaximumVelocity = configuration.getScaledMaximumFlingVelocity(); mOverscrollDistance = configuration.getScaledOverscrollDistance(); mOverflingDistance = configuration.getScaledOverflingDistance(); mDensityScale = getContext().getResources().getDisplayMetrics().density; }</code></pre> <p><strong>ListView的初始化:</strong></p> <pre> <code class="language-java">public ListView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) { super(context, attrs, defStyleAttr, defStyleRes); final TypedArray a = context.obtainStyledAttributes( attrs, R.styleable.ListView, defStyleAttr, defStyleRes); final CharSequence[] entries = a.getTextArray(R.styleable.ListView_entries); if (entries != null) { setAdapter(new ArrayAdapter<>(context, R.layout.simple_list_item_1, entries)); } // 获取item分割线的drawable对象 final Drawable d = a.getDrawable(R.styleable.ListView_divider); if (d != null) { // Use an implicit divider height which may be explicitly // overridden by android:dividerHeight further down. setDivider(d); } final Drawable osHeader = a.getDrawable(R.styleable.ListView_overScrollHeader); if (osHeader != null) { setOverscrollHeader(osHeader); } final Drawable osFooter = a.getDrawable(R.styleable.ListView_overScrollFooter); if (osFooter != null) { setOverscrollFooter(osFooter); } // Use an explicit divider height, if specified. // item分割线的高度 if (a.hasValueOrEmpty(R.styleable.ListView_dividerHeight)) { final int dividerHeight = a.getDimensionPixelSize( R.styleable.ListView_dividerHeight, 0); if (dividerHeight != 0) { setDividerHeight(dividerHeight); } } mHeaderDividersEnabled = a.getBoolean(R.styleable.ListView_headerDividersEnabled, true); mFooterDividersEnabled = a.getBoolean(R.styleable.ListView_footerDividersEnabled, true); a.recycle(); }</code></pre> <h2><strong>onMeasure()</strong></h2> <p><strong>ListView的onMeasure()方法</strong></p> <pre> <code class="language-java">@Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { // Sets up mListPadding super.onMeasure(widthMeasureSpec, heightMeasureSpec); final int widthMode = MeasureSpec.getMode(widthMeasureSpec); final int heightMode = MeasureSpec.getMode(heightMeasureSpec); int widthSize = MeasureSpec.getSize(widthMeasureSpec); int heightSize = MeasureSpec.getSize(heightMeasureSpec); int childWidth = 0; int childHeight = 0; int childState = 0; mItemCount = mAdapter == null ? 0 : mAdapter.getCount(); if (mItemCount > 0 && (widthMode == MeasureSpec.UNSPECIFIED || heightMode == MeasureSpec.UNSPECIFIED)) { final View child = obtainView(0, mIsScrap); // Lay out child directly against the parent measure spec so that // we can obtain exected minimum width and height. measureScrapChild(child, 0, widthMeasureSpec, heightSize); childWidth = child.getMeasuredWidth(); childHeight = child.getMeasuredHeight(); childState = combineMeasuredStates(childState, child.getMeasuredState()); if (recycleOnMeasure() && mRecycler.shouldRecycleViewType( ((LayoutParams) child.getLayoutParams()).viewType)) { mRecycler.addScrapView(child, 0); } } if (widthMode == MeasureSpec.UNSPECIFIED) { widthSize = mListPadding.left + mListPadding.right + childWidth + getVerticalScrollbarWidth(); } else { widthSize |= (childState & MEASURED_STATE_MASK); } if (heightMode == MeasureSpec.UNSPECIFIED) { heightSize = mListPadding.top + mListPadding.bottom + childHeight + getVerticalFadingEdgeLength() * 2; } // 有时候需要使ListView的高度等于所有子item view 可以重写onMeasure()方法使其调用以 // 下代码 if (heightMode == MeasureSpec.AT_MOST) { // TODO: after first layout we should maybe start at the first visible position, not 0 heightSize = measureHeightOfChildren(widthMeasureSpec, 0, NO_POSITION, heightSize, -1); } setMeasuredDimension(widthSize, heightSize); mWidthMeasureSpec = widthMeasureSpec; }</code></pre> <p>整体上ListView的onMeasure方法比较简单,普通.</p> <h2><strong>onLayout()</strong></h2> <p>ListView由adapter.getView()获取的子view的layout方式在此实现</p> <p><strong>父类AbsListView的onLayout():</strong></p> <pre> <code class="language-java">@Override protected void onLayout(boolean changed, int l, int t, int r, int b) { super.onLayout(changed, l, t, r, b); mInLayout = true; final int childCount = getChildCount(); if (changed) { for (int i = 0; i < childCount; i++) { getChildAt(i).forceLayout(); } mRecycler.markChildrenDirty(); } // 由子类ListView 和 GridView实现,是核心布局方法代码,也是listview与adapter交互数据 // 的主要入口函数 layoutChildren(); mInLayout = false; mOverscrollMax = (b - t) / OVERSCROLL_LIMIT_DIVISOR; // TODO: Move somewhere sane. This doesn't belong in onLayout(). if (mFastScroll != null) { mFastScroll.onItemCountChanged(getChildCount(), mItemCount); } }</code></pre> <p><strong>ListView的layoutChildren():</strong></p> <pre> <code class="language-java">@Override protected void layoutChildren() { final boolean blockLayoutRequests = mBlockLayoutRequests; if (blockLayoutRequests) { return; } mBlockLayoutRequests = true; try { super.layoutChildren(); invalidate(); if (mAdapter == null) { resetList(); invokeOnItemScrollListener(); return; } final int childrenTop = mListPadding.top; final int childrenBottom = mBottom - mTop - mListPadding.bottom; // 每次即将进行layout子item view的时候先记录当前listview已有的child view个数 final int childCount = getChildCount(); int index = 0; int delta = 0; View sel; View oldSel = null; View oldFirst = null; View newSel = null; // Remember stuff we will need down below switch (mLayoutMode) { case LAYOUT_SET_SELECTION: index = mNextSelectedPosition - mFirstPosition; if (index >= 0 && index < childCount) { newSel = getChildAt(index); } break; case LAYOUT_FORCE_TOP: case LAYOUT_FORCE_BOTTOM: case LAYOUT_SPECIFIC: case LAYOUT_SYNC: break; case LAYOUT_MOVE_SELECTION: default: // Remember the previously selected view index = mSelectedPosition - mFirstPosition; if (index >= 0 && index < childCount) { oldSel = getChildAt(index); } // Remember the previous first child oldFirst = getChildAt(0); if (mNextSelectedPosition >= 0) { delta = mNextSelectedPosition - mSelectedPosition; } // Caution: newSel might be null newSel = getChildAt(index + delta); } boolean dataChanged = mDataChanged; if (dataChanged) { handleDataChanged(); } // Handle the empty set by removing all views that are visible // and calling it a day if (mItemCount == 0) { resetList(); invokeOnItemScrollListener(); return; } else if (mItemCount != mAdapter.getCount()) { throw new IllegalStateException("The content of the adapter has changed but " + "ListView did not receive a notification. Make sure the content of " + "your adapter is not modified from a background thread, but only from " + "the UI thread. Make sure your adapter calls notifyDataSetChanged() " + "when its content changes. [in ListView(" + getId() + ", " + getClass() + ") with Adapter(" + mAdapter.getClass() + ")]"); } setSelectedPositionInt(mNextSelectedPosition); AccessibilityNodeInfo accessibilityFocusLayoutRestoreNode = null; View accessibilityFocusLayoutRestoreView = null; int accessibilityFocusPosition = INVALID_POSITION; // Remember which child, if any, had accessibility focus. This must // occur before recycling any views, since that will clear // accessibility focus. final ViewRootImpl viewRootImpl = getViewRootImpl(); if (viewRootImpl != null) { final View focusHost = viewRootImpl.getAccessibilityFocusedHost(); if (focusHost != null) { final View focusChild = getAccessibilityFocusedChild(focusHost); if (focusChild != null) { if (!dataChanged || isDirectChildHeaderOrFooter(focusChild) || focusChild.hasTransientState() || mAdapterHasStableIds) { // The views won't be changing, so try to maintain // focus on the current host and virtual view. accessibilityFocusLayoutRestoreView = focusHost; accessibilityFocusLayoutRestoreNode = viewRootImpl .getAccessibilityFocusedVirtualView(); } // If all else fails, maintain focus at the same // position. accessibilityFocusPosition = getPositionForView(focusChild); } } } View focusLayoutRestoreDirectChild = null; View focusLayoutRestoreView = null; // Take focus back to us temporarily to avoid the eventual call to // clear focus when removing the focused child below from messing // things up when ViewAncestor assigns focus back to someone else. final View focusedChild = getFocusedChild(); if (focusedChild != null) { // TODO: in some cases focusedChild.getParent() == null // We can remember the focused view to restore after re-layout // if the data hasn't changed, or if the focused position is a // header or footer. if (!dataChanged || isDirectChildHeaderOrFooter(focusedChild) || focusedChild.hasTransientState() || mAdapterHasStableIds) { focusLayoutRestoreDirectChild = focusedChild; // Remember the specific view that had focus. focusLayoutRestoreView = findFocus(); if (focusLayoutRestoreView != null) { // Tell it we are going to mess with it. focusLayoutRestoreView.onStartTemporaryDetach(); } } requestFocus(); } // Pull all children into the RecycleBin. // These views will be reused if possible final int firstPosition = mFirstPosition; final RecycleBin recycleBin = mRecycler; // 只有在调用adapter.notifyDatasetChanged()方法一直到layout()布局结束, // dataChanged为true,默认为false if (dataChanged) { // dataChanged为true,说明当前listview是有数据的了,把当前所有的item view // 存放到RecycleBin对象的mScrapViews中保存 for (int i = 0; i < childCount; i++) { recycleBin.addScrapView(getChildAt(i), firstPosition+i); } } else { // dataChanged默认为false,第一次执行此方法走这里 recycleBin.fillActiveViews(childCount, firstPosition); } // Clear out old views // 清除当前listview所有的子view detachAllViewsFromParent(); recycleBin.removeSkippedScrap(); // 在我们调用listview.setAdapter()时候,已经将mLayoutMode = LAYOUT_NORMAL; // 所以通常情况下可认为mLayoutMode == LAYOUT_NORMAL switch (mLayoutMode) { case LAYOUT_SET_SELECTION: if (newSel != null) { sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom); } else { sel = fillFromMiddle(childrenTop, childrenBottom); } break; case LAYOUT_SYNC: sel = fillSpecific(mSyncPosition, mSpecificTop); break; case LAYOUT_FORCE_BOTTOM: sel = fillUp(mItemCount - 1, childrenBottom); adjustViewsUpOrDown(); break; case LAYOUT_FORCE_TOP: mFirstPosition = 0; sel = fillFromTop(childrenTop); adjustViewsUpOrDown(); break; case LAYOUT_SPECIFIC: sel = fillSpecific(reconcileSelectedPosition(), mSpecificTop); break; case LAYOUT_MOVE_SELECTION: sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom); break; default: // 通常情况都走这里 if (childCount == 0) { // listview第一次布局childCount必然为0走这里 if (!mStackFromBottom) { // 通常我们没有外部调用listview.setStackFromBottom() // 成员变量mStackFromBottom均为false都走这里 final int position = lookForSelectablePosition(0, true); setSelectedPositionInt(position); // 从上到上布局listview能显示得下的子view sel = fillFromTop(childrenTop); } else { final int position = lookForSelectablePosition(mItemCount - 1, false); setSelectedPositionInt(position); sel = fillUp(mItemCount - 1, childrenBottom); } } else { // 非第一次layout,之前记录的存在的子view个数childCount不为0 // 包括两种情况:1.listview首次布局中的第二次执行的onlayout(); // 2.在后续listview已经显示存在子view然后数据改变时候调用 // adapter.nitifyDatasetChanged()方法时候 if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) { sel = fillSpecific(mSelectedPosition, oldSel == null ? childrenTop : oldSel.getTop()); } else if (mFirstPosition < mItemCount) { // 通常情况走这里,fillSpecific()会调用fillUp()和fillDown()布局子view sel = fillSpecific(mFirstPosition, oldFirst == null ? childrenTop : oldFirst.getTop()); } else { sel = fillSpecific(0, childrenTop); } } break; } // Flush any cached views that did not get reused above // 至此,listview已经布局完成能够显示得下的子view,将recycleBin可能剩余的 // mActiveViews中view移动到mScrapViews以便于listview滑动时候复用 recycleBin.scrapActiveViews(); if (sel != null) { // The current selected item should get focus if items are // focusable. if (mItemsCanFocus && hasFocus() && !sel.hasFocus()) { final boolean focusWasTaken = (sel == focusLayoutRestoreDirectChild && focusLayoutRestoreView != null && focusLayoutRestoreView.requestFocus()) || sel.requestFocus(); if (!focusWasTaken) { // Selected item didn't take focus, but we still want to // make sure something else outside of the selected view // has focus. final View focused = getFocusedChild(); if (focused != null) { focused.clearFocus(); } positionSelector(INVALID_POSITION, sel); } else { sel.setSelected(false); mSelectorRect.setEmpty(); } } else { positionSelector(INVALID_POSITION, sel); } mSelectedTop = sel.getTop(); } else { final boolean inTouchMode = mTouchMode == TOUCH_MODE_TAP || mTouchMode == TOUCH_MODE_DONE_WAITING; if (inTouchMode) { // If the user's finger is down, select the motion position. final View child = getChildAt(mMotionPosition - mFirstPosition); if (child != null) { positionSelector(mMotionPosition, child); } } else if (mSelectorPosition != INVALID_POSITION) { // If we had previously positioned the selector somewhere, // put it back there. It might not match up with the data, // but it's transitioning out so it's not a big deal. final View child = getChildAt(mSelectorPosition - mFirstPosition); if (child != null) { positionSelector(mSelectorPosition, child); } } else { // Otherwise, clear selection. mSelectedTop = 0; mSelectorRect.setEmpty(); } // Even if there is not selected position, we may need to // restore focus (i.e. something focusable in touch mode). if (hasFocus() && focusLayoutRestoreView != null) { focusLayoutRestoreView.requestFocus(); } } // Attempt to restore accessibility focus, if necessary. if (viewRootImpl != null) { final View newAccessibilityFocusedView = viewRootImpl.getAccessibilityFocusedHost(); if (newAccessibilityFocusedView == null) { if (accessibilityFocusLayoutRestoreView != null && accessibilityFocusLayoutRestoreView.isAttachedToWindow()) { final AccessibilityNodeProvider provider = accessibilityFocusLayoutRestoreView.getAccessibilityNodeProvider(); if (accessibilityFocusLayoutRestoreNode != null && provider != null) { final int virtualViewId = AccessibilityNodeInfo.getVirtualDescendantId( accessibilityFocusLayoutRestoreNode.getSourceNodeId()); provider.performAction(virtualViewId, AccessibilityNodeInfo.ACTION_ACCESSIBILITY_FOCUS, null); } else { accessibilityFocusLayoutRestoreView.requestAccessibilityFocus(); } } else if (accessibilityFocusPosition != INVALID_POSITION) { // Bound the position within the visible children. final int position = MathUtils.constrain( accessibilityFocusPosition - mFirstPosition, 0, getChildCount() - 1); final View restoreView = getChildAt(position); if (restoreView != null) { restoreView.requestAccessibilityFocus(); } } } } // Tell focus view we are done mucking with it, if it is still in // our view hierarchy. if (focusLayoutRestoreView != null && focusLayoutRestoreView.getWindowToken() != null) { focusLayoutRestoreView.onFinishTemporaryDetach(); } // 布局完成之后的操作 mLayoutMode = LAYOUT_NORMAL; mDataChanged = false; if (mPositionScrollAfterLayout != null) { post(mPositionScrollAfterLayout); mPositionScrollAfterLayout = null; } mNeedSync = false; setNextSelectedPositionInt(mSelectedPosition); updateScrollIndicators(); if (mItemCount > 0) { checkSelectionChanged(); } invokeOnItemScrollListener(); } finally { if (!blockLayoutRequests) { mBlockLayoutRequests = false; } } }</code></pre> <p>RecycleBin是AbsListView的一个非静态内部类,主要有一个数组成员变量View[] mActiveViews 和 ArrayList<View>[] mScrapViews.mActiveViews存放的是当前ListView可以使用的待激活的子item view,而mScrapViews存放的是在ListView滑动过程中滑出屏幕来回收以便下次利用的子item view</p> <p><strong>fillFromTop():</strong></p> <pre> <code class="language-java">/** * Fills the list from top to bottom, starting with mFirstPosition * * @param nextTop The location where the top of the first item should be * drawn * * @return The view that is currently selected */ private View fillFromTop(int nextTop) { mFirstPosition = Math.min(mFirstPosition, mSelectedPosition); mFirstPosition = Math.min(mFirstPosition, mItemCount - 1); if (mFirstPosition < 0) { mFirstPosition = 0; } return fillDown(mFirstPosition, nextTop); }</code></pre> <p>其实就是保证mFirstPosition>=0的情况下调用fillDown()从上到下依次布局子item view</p> <p><strong>fillDown():</strong></p> <pre> <code class="language-java">/** * Fills the list from pos down to the end of the list view. * * @param pos The first position to put in the list * * @param nextTop The location where the top of the item associated with pos * should be drawn * * @return The view that is currently selected, if it happens to be in the * range that we draw. */ private View fillDown(int pos, int nextTop) { View selectedView = null; int end = (mBottom - mTop); if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) { end -= mListPadding.bottom; } // while循环在listview范围内布局可见数量的子item view // nextTop == mListPadding.top,可认为是listview的mPaddingTop // end == mListPadding.bottom,可认为是listview的mPaddingBottom // nextTop < end说明下一个要装载的item view的getTop()依然可见,那当然要布局到listview中 while (nextTop < end && pos < mItemCount) { // is this the selected item? boolean selected = pos == mSelectedPosition; View child = makeAndAddView(pos, nextTop, true, mListPadding.left, selected); nextTop = child.getBottom() + mDividerHeight; if (selected) { selectedView = child; } pos++; } setVisibleRangeHint(mFirstPosition, mFirstPosition + getChildCount() - 1); return selectedView; }</code></pre> <p>第一次布局情况下,参数pos==0,即从0开始从上到下依次布局约定数量的从adapter.getView()获取的子view</p> <p><strong>makeAndAddView():</strong></p> <pre> <code class="language-java">/** * Obtain the view and add it to our list of children. The view can be made * fresh, converted from an unused view, or used as is if it was in the * recycle bin. * * @param position Logical position in the list * @param y Top or bottom edge of the view to add * @param flow If flow is true, align top edge to y. If false, align bottom * edge to y. * @param childrenLeft Left edge where children should be positioned * @param selected Is this position selected? * @return View that was added */ private View makeAndAddView(int position, int y, boolean flow, int childrenLeft, boolean selected) { View child; // 默认情况mDataChanged==false,在调用adapter.notifyDatasetChanged()之后的 // layout()阶段中mDataChanged == true if (!mDataChanged) { // Try to use an existing view for this position // 首先从mRecycler的mActiveViews数组中尝试获取可直接用的item view // 在前面layoutChildren()方法中首先如果mDataChanged==false会尝试把item view // 放到mRecycler的mActiveViews中保存,mRecycler.getActiveView()中对应positon // 的view被获取到之后本身就不再保存之 child = mRecycler.getActiveView(position); if (child != null) { // Found it -- we're using an existing child // This just needs to be positioned setupChild(child, position, y, flow, childrenLeft, selected, true); return child; } } // Make a new view for this position, or convert an unused view if possible child = obtainView(position, mIsScrap); // This needs to be positioned and measured setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]); return child; }</code></pre> <p>主要用于从RecycleBin或者adapter.getView()中获取子view并在setupChild()中设置以及布局之</p> <p><strong>setupChild():</strong></p> <pre> <code class="language-java">/** * Add a view as a child and make sure it is measured (if necessary) and * positioned properly. * * @param child The view to add * @param position The position of this child * @param y The y position relative to which this view will be positioned * @param flowDown If true, align top edge to y. If false, align bottom * edge to y. * @param childrenLeft Left edge where children should be positioned * @param selected Is this position selected? * @param recycled Has this view been pulled from the recycle bin? If so it * does not need to be remeasured. */ private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft, boolean selected, boolean recycled) { Trace.traceBegin(Trace.TRACE_TAG_VIEW, "setupListItem"); final boolean isSelected = selected && shouldShowSelector(); final boolean updateChildSelected = isSelected != child.isSelected(); final int mode = mTouchMode; final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL && mMotionPosition == position; final boolean updateChildPressed = isPressed != child.isPressed(); // 如果child view是曾经使用过的,已经测量measure过了不需要再次measure之 final boolean needToMeasure = !recycled || updateChildSelected || child.isLayoutRequested(); // Respect layout params that are already in the view. Otherwise make some up... // noinspection unchecked AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams(); if (p == null) { p = (AbsListView.LayoutParams) generateDefaultLayoutParams(); } p.viewType = mAdapter.getItemViewType(position); if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter && p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) { // 表明该child view是曾经使用过的,只需要attch一下就行了 attachViewToParent(child, flowDown ? -1 : 0, p); } else { p.forceAdd = false; if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) { p.recycledHeaderFooter = true; } // 表明不一定是曾经使用过的,需要addview到listview中 addViewInLayout(child, flowDown ? -1 : 0, p, true); } if (updateChildSelected) { child.setSelected(isSelected); } if (updateChildPressed) { child.setPressed(isPressed); } if (mChoiceMode != CHOICE_MODE_NONE && mCheckStates != null) { if (child instanceof Checkable) { ((Checkable) child).setChecked(mCheckStates.get(position)); } else if (getContext().getApplicationInfo().targetSdkVersion >= android.os.Build.VERSION_CODES.HONEYCOMB) { child.setActivated(mCheckStates.get(position)); } } if (needToMeasure) { // 测量item view,在viewgroup所有子view都需要测量执行view.measure()方法之后才能布局然后显示出来 final int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec, mListPadding.left + mListPadding.right, p.width); final int lpHeight = p.height; final int childHeightSpec; if (lpHeight > 0) { childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY); } else { childHeightSpec = MeasureSpec.makeSafeMeasureSpec(getMeasuredHeight(), MeasureSpec.UNSPECIFIED); } child.measure(childWidthSpec, childHeightSpec); } else { cleanupLayoutState(child); } final int w = child.getMeasuredWidth(); final int h = child.getMeasuredHeight(); final int childTop = flowDown ? y : y - h; // 放置该子view if (needToMeasure) { final int childRight = childrenLeft + w; final int childBottom = childTop + h; child.layout(childrenLeft, childTop, childRight, childBottom); } else { child.offsetLeftAndRight(childrenLeft - child.getLeft()); child.offsetTopAndBottom(childTop - child.getTop()); } if (mCachingStarted && !child.isDrawingCacheEnabled()) { child.setDrawingCacheEnabled(true); } if (recycled && (((AbsListView.LayoutParams)child.getLayoutParams()).scrappedFromPosition) != position) { child.jumpDrawablesToCurrentState(); } Trace.traceEnd(Trace.TRACE_TAG_VIEW); }</code></pre> <p><strong>obtainView():</strong></p> <pre> <code class="language-java">/** * Get a view and have it show the data associated with the specified * position. This is called when we have already discovered that the view is * not available for reuse in the recycle bin. The only choices left are * converting an old view or making a new one. * * @param position The position to display * @param isScrap Array of at least 1 boolean, the first entry will become true if * the returned view was taken from the scrap heap, false if otherwise. * * @return A view displaying the data associated with the specified position */ View obtainView(int position, boolean[] isScrap) { Trace.traceBegin(Trace.TRACE_TAG_VIEW, "obtainView"); isScrap[0] = false; // 此部分忽略 // Check whether we have a transient state view. Attempt to re-bind the // data and discard the view if we fail. final View transientView = mRecycler.getTransientStateView(position); if (transientView != null) { final LayoutParams params = (LayoutParams) transientView.getLayoutParams(); // If the view type hasn't changed, attempt to re-bind the data. if (params.viewType == mAdapter.getItemViewType(position)) { final View updatedView = mAdapter.getView(position, transientView, this); // If we failed to re-bind the data, scrap the obtained view. if (updatedView != transientView) { setItemViewLayoutParams(updatedView, position); mRecycler.addScrapView(updatedView, position); } } isScrap[0] = true; // Finish the temporary detach started in addScrapView(). transientView.dispatchFinishTemporaryDetach(); return transientView; } // 关键部分 // 先从mRecycler的mScrapViews数组中获取一个在滑动时候废弃保存的子view final View scrapView = mRecycler.getScrapView(position); // 平时写的adapter的getView()方法在此被调用了 final View child = mAdapter.getView(position, scrapView, this); if (scrapView != null) { if (child != scrapView) { // Failed to re-bind the data, return scrap to the heap. // 获取的不一样就把刚才的scrapView再次加入到mRecycler的mScrapViews数组中 mRecycler.addScrapView(scrapView, position); } else { // 保存已经获取到废弃view且可以使用 isScrap[0] = true; // Finish the temporary detach started in addScrapView(). child.dispatchFinishTemporaryDetach(); } } if (mCacheColorHint != 0) { child.setDrawingCacheBackgroundColor(mCacheColorHint); } if (child.getImportantForAccessibility() == IMPORTANT_FOR_ACCESSIBILITY_AUTO) { child.setImportantForAccessibility(IMPORTANT_FOR_ACCESSIBILITY_YES); } setItemViewLayoutParams(child, position); if (AccessibilityManager.getInstance(mContext).isEnabled()) { if (mAccessibilityDelegate == null) { mAccessibilityDelegate = new ListItemAccessibilityDelegate(); } if (child.getAccessibilityDelegate() == null) { child.setAccessibilityDelegate(mAccessibilityDelegate); } } Trace.traceEnd(Trace.TRACE_TAG_VIEW); return child; }</code></pre> <p>返回的要么是adapter.getView()中调用inflate获取的view,要么是mRecycler的mScrapViews数组中获取的.</p> <p><strong>fillSpecific():</strong></p> <pre> <code class="language-java">/** * Put a specific item at a specific location on the screen and then build * up and down from there. * * @param position The reference view to use as the starting point * @param top Pixel offset from the top of this view to the top of the * reference view. * * @return The selected view, or null if the selected view is outside the * visible area. */ private View fillSpecific(int position, int top) { boolean tempIsSelected = position == mSelectedPosition; // 首先获取并设置布局当前positon的item view View temp = makeAndAddView(position, top, true, mListPadding.left, tempIsSelected); // Possibly changed again in fillUp if we add rows above this one. mFirstPosition = position; View above; View below; final int dividerHeight = mDividerHeight; // 然后分别调用fillUp()和fillDown()向上和向下获取并设置布局其他item view if (!mStackFromBottom) { // 通常情况走这里 above = fillUp(position - 1, temp.getTop() - dividerHeight); // This will correct for the top of the first view not touching the top of the list // 保证listview的第一个或者最后一个item view在其paddingTop或是paddingBottom内 adjustViewsUpOrDown(); below = fillDown(position + 1, temp.getBottom() + dividerHeight); int childCount = getChildCount(); if (childCount > 0) { correctTooHigh(childCount); } } else { below = fillDown(position + 1, temp.getBottom() + dividerHeight); // This will correct for the bottom of the last view not touching the bottom of the list adjustViewsUpOrDown(); above = fillUp(position - 1, temp.getTop() - dividerHeight); int childCount = getChildCount(); if (childCount > 0) { correctTooLow(childCount); } } if (tempIsSelected) { return temp; } else if (above != null) { return above; } else { return below; } }</code></pre> <h2><strong>ListView.setAdapter()</strong></h2> <pre> <code class="language-java">/** * Sets the data behind this ListView. * * The adapter passed to this method may be wrapped by a {@link WrapperListAdapter}, * depending on the ListView features currently in use. For instance, adding * headers and/or footers will cause the adapter to be wrapped. * * @param adapter The ListAdapter which is responsible for maintaining the * data backing this list and for producing a view to represent an * item in that data set. * * @see #getAdapter() */ @Override public void setAdapter(ListAdapter adapter) { if (mAdapter != null && mDataSetObserver != null) { mAdapter.unregisterDataSetObserver(mDataSetObserver); } // 将一些成员变量还原设置为初始默认值 resetList(); // mRecycler的mScrapViews清空并执行listview.removeDetachedView mRecycler.clear(); if (mHeaderViewInfos.size() > 0|| mFooterViewInfos.size() > 0) { // 如果listview有headerView或者FooterView则会生成包装adapter mAdapter = new HeaderViewListAdapter(mHeaderViewInfos, mFooterViewInfos, adapter); } else { mAdapter = adapter; } mOldSelectedPosition = INVALID_POSITION; mOldSelectedRowId = INVALID_ROW_ID; // AbsListView#setAdapter will update choice mode states. super.setAdapter(adapter); if (mAdapter != null) { mAreAllItemsSelectable = mAdapter.areAllItemsEnabled(); mOldItemCount = mItemCount; // listview的数据源的item个数 mItemCount = mAdapter.getCount(); checkFocus(); // 重新生成一个内部类对象并将其注册到adapter中,用于通知回调数据源改变 mDataSetObserver = new AdapterDataSetObserver(); mAdapter.registerDataSetObserver(mDataSetObserver); // 设置listview的数据源类型,并在mRecycler中初始化对应个数的scrapViews list mRecycler.setViewTypeCount(mAdapter.getViewTypeCount()); int position; if (mStackFromBottom) { position = lookForSelectablePosition(mItemCount - 1, false); } else { position = lookForSelectablePosition(0, true); } setSelectedPositionInt(position); setNextSelectedPositionInt(position); if (mItemCount == 0) { // Nothing selected checkSelectionChanged(); } } else { mAreAllItemsSelectable = true; checkFocus(); // Nothing selected checkSelectionChanged(); } // 会调用顶层viewRootImpl.performTraversals(),导致视图重绘,listview重新 // mesaure layout ... requestLayout(); }</code></pre> <p><strong>BaseAdapter.registerDataSetObserver():</strong></p> <p><strong>BaseAdapter.notifyDataSetChanged():</strong></p> <pre> <code class="language-java">public abstract class BaseAdapter implements ListAdapter, SpinnerAdapter { private final DataSetObservable mDataSetObservable = new DataSetObservable(); public boolean hasStableIds() { return false; } // 注册监听回调 public void registerDataSetObserver(DataSetObserver observer) { mDataSetObservable.registerObserver(observer); } public void unregisterDataSetObserver(DataSetObserver observer) { mDataSetObservable.unregisterObserver(observer); } /** * Notifies the attached observers that the underlying data has been changed * and any View reflecting the data set should refresh itself. */ public void notifyDataSetChanged() { mDataSetObservable.notifyChanged(); } /** * Notifies the attached observers that the underlying data is no longer valid * or available. Once invoked this adapter is no longer valid and should * not report further data set changes. */ public void notifyDataSetInvalidated() { mDataSetObservable.notifyInvalidated(); } public boolean areAllItemsEnabled() { return true; } public boolean isEnabled(int position) { return true; } public View getDropDownView(int position, View convertView, ViewGroup parent) { return getView(position, convertView, parent); } public int getItemViewType(int position) { return 0; } public int getViewTypeCount() { return 1; } public boolean isEmpty() { return getCount() == 0; }</code></pre> <p>}</p> <p><strong>Observable.registerObserver():</strong></p> <pre> <code class="language-java">public abstract class Observable<T> { /** * The list of observers. An observer can be in the list at most * once and will never be null. */ protected final ArrayList<T> mObservers = new ArrayList<T>(); /** * Adds an observer to the list. The observer cannot be null and it must not already * be registered. * @param observer the observer to register * @throws IllegalArgumentException the observer is null * @throws IllegalStateException the observer is already registered */ public void registerObserver(T observer) { if (observer == null) { throw new IllegalArgumentException("The observer is null."); } synchronized(mObservers) { if (mObservers.contains(observer)) { throw new IllegalStateException("Observer " + observer + " is already registered."); } mObservers.add(observer); } } /** * Removes a previously registered observer. The observer must not be null and it * must already have been registered. * @param observer the observer to unregister * @throws IllegalArgumentException the observer is null * @throws IllegalStateException the observer is not yet registered */ public void unregisterObserver(T observer) { if (observer == null) { throw new IllegalArgumentException("The observer is null."); } synchronized(mObservers) { int index = mObservers.indexOf(observer); if (index == -1) { throw new IllegalStateException("Observer " + observer + " was not registered."); } mObservers.remove(index); } } /** * Remove all registered observers. */ public void unregisterAll() { synchronized(mObservers) { mObservers.clear(); } } }</code></pre> <p><strong>BaseAdapter.notifyDataSetChanged()最终会调用AdapterView中的内部类AdapterDataSetObserver.onChanged()</strong></p> <pre> <code class="language-java">class AdapterDataSetObserver extends DataSetObserver { private Parcelable mInstanceState = null; @Override public void onChanged() { mDataChanged = true; mOldItemCount = mItemCount; mItemCount = getAdapter().getCount(); // Detect the case where a cursor that was previously invalidated has // been repopulated with new data. if (AdapterView.this.getAdapter().hasStableIds() && mInstanceState != null && mOldItemCount == 0 && mItemCount > 0) { AdapterView.this.onRestoreInstanceState(mInstanceState); mInstanceState = null; } else { rememberSyncState(); } checkFocus(); // 同样,最终调用viewRootImpl.performTraversals(),导致视图重绘,执行listview的 // measure layout 方法等 requestLayout(); } @Override public void onInvalidated() { mDataChanged = true; if (AdapterView.this.getAdapter().hasStableIds()) { // Remember the current state for the case where our hosting activity is being // stopped and later restarted mInstanceState = AdapterView.this.onSaveInstanceState(); } // Data is invalid so we should reset our state mOldItemCount = mItemCount; mItemCount = 0; mSelectedPosition = INVALID_POSITION; mSelectedRowId = INVALID_ROW_ID; mNextSelectedPosition = INVALID_POSITION; mNextSelectedRowId = INVALID_ROW_ID; mNeedSync = false; checkFocus(); requestLayout(); }</code></pre> <h2><strong>onInterceptTouchEvent():</strong></h2> <pre> <code class="language-java">@Override public boolean onInterceptTouchEvent(MotionEvent ev) { final int actionMasked = ev.getActionMasked(); View v; if (mPositionScroller != null) { mPositionScroller.stop(); } if (mIsDetaching || !isAttachedToWindow()) { // Something isn't right. // Since we rely on being attached to get data set change notifications, // don't risk doing anything where we might try to resync and find things // in a bogus state. return false; } if (mFastScroll != null && mFastScroll.onInterceptTouchEvent(ev)) { return true; } switch (actionMasked) { case MotionEvent.ACTION_DOWN: { int touchMode = mTouchMode; // 如果手指放下时候 listview正出于fling滚动状态或者OVERSCROLL,则马上拦截事件交 // 由自身ontouchEvent()处理 if (touchMode == TOUCH_MODE_OVERFLING || touchMode == TOUCH_MODE_OVERSCROLL) { mMotionCorrection = 0; return true; } final int x = (int) ev.getX(); final int y = (int) ev.getY(); mActivePointerId = ev.getPointerId(0); // 获取当前触摸事件在对应的item view的positon,在listview中实现了该方法 int motionPosition = findMotionRow(y); if (touchMode != TOUCH_MODE_FLING && motionPosition >= 0) { // User clicked on an actual view (and was not stopping a fling). // Remember where the motion event started v = getChildAt(motionPosition - mFirstPosition); mMotionViewOriginalTop = v.getTop(); mMotionX = x; // 记录down事件时候的mMotionY初始值 mMotionY = y; mMotionPosition = motionPosition; // 设置触摸事件模式为TOUCH_MODE_DOWN mTouchMode = TOUCH_MODE_DOWN; clearScrollingCache(); } // 初试设置down事件时候mLastY值 mLastY = Integer.MIN_VALUE; initOrResetVelocityTracker(); mVelocityTracker.addMovement(ev); mNestedYOffset = 0; startNestedScroll(SCROLL_AXIS_VERTICAL); if (touchMode == TOUCH_MODE_FLING) { return true; } break; } case MotionEvent.ACTION_MOVE: { switch (mTouchMode) { case TOUCH_MODE_DOWN: int pointerIndex = ev.findPointerIndex(mActivePointerId); if (pointerIndex == -1) { pointerIndex = 0; mActivePointerId = ev.getPointerId(pointerIndex); } final int y = (int) ev.getY(pointerIndex); initVelocityTrackerIfNotExists(); mVelocityTracker.addMovement(ev); // 判断是否拦截事件自己处理,此为onInterceptTouchEvent()方法核心代码 if (startScrollIfNeeded((int) ev.getX(pointerIndex), y, null)) { return true; } break; } break; } case MotionEvent.ACTION_CANCEL: case MotionEvent.ACTION_UP: { // touchMode恢复默认状态 mTouchMode = TOUCH_MODE_REST; mActivePointerId = INVALID_POINTER; // 回收速度VelocityTracker相关资源 recycleVelocityTracker(); reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE); stopNestedScroll(); break; } case MotionEvent.ACTION_POINTER_UP: { onSecondaryPointerUp(ev); break; } } return false; } // 这个方法在onInterceptTouchEvent的move事件中调用,在onTouchEvent()的onTouchMove()方法 // 中开始时候也会调用 private boolean startScrollIfNeeded(int x, int y, MotionEvent vtev) { // Check if we have moved far enough that it looks more like a // scroll than a tap // 得到当前事件的y值与down事件时候设置的值的差值 final int deltaY = y - mMotionY; final int distance = Math.abs(deltaY); // mScrollY!=0即overscroll为true ,核心为distance > mTouchSlop即拦截事件自己处理 // mTouchSlop在构造函数中初始化并赋值了 final boolean overscroll = mScrollY != 0; if ((overscroll || distance > mTouchSlop) && (getNestedScrollAxes() & SCROLL_AXIS_VERTICAL) == 0) { createScrollingCache(); if (overscroll) { mTouchMode = TOUCH_MODE_OVERSCROLL; mMotionCorrection = 0; } else { // 设置触摸模式为TOUCH_MODE_SCROLL,在onTouchEvent()用到 mTouchMode = TOUCH_MODE_SCROLL; mMotionCorrection = deltaY > 0 ? mTouchSlop : -mTouchSlop; } // 取消子view的长按监听触发 removeCallbacks(mPendingCheckForLongPress); setPressed(false); final View motionView = getChildAt(mMotionPosition - mFirstPosition); // listview拦截了事件本身处理,所以恢复可能设置子view的press状态 if (motionView != null) { motionView.setPressed(false); } // 通知ScrollState状态变化回调 reportScrollStateChange(OnScrollListener.SCROLL_STATE_TOUCH_SCROLL); // Time to start stealing events! Once we've stolen them, don't let anyone // steal from us final ViewParent parent = getParent(); if (parent != null) { parent.requestDisallowInterceptTouchEvent(true); } // 作用如名,如果满足条件,滚动listview scrollIfNeeded(x, y, vtev); return true; } return false; }</code></pre> <p><strong>onTouchEvent()之onTouchDown():</strong></p> <pre> <code class="language-java">private void onTouchDown(MotionEvent ev) { mActivePointerId = ev.getPointerId(0); if (mTouchMode == TOUCH_MODE_OVERFLING) { // Stopped the fling. It is a scroll. // 如果已经正在出于TOUCH_MODE_OVERFLING则down事件瞬间中断fling mFlingRunnable.endFling(); if (mPositionScroller != null) { mPositionScroller.stop(); } mTouchMode = TOUCH_MODE_OVERSCROLL; mMotionX = (int) ev.getX(); mMotionY = (int) ev.getY(); mLastY = mMotionY; mMotionCorrection = 0; mDirection = 0; } else { final int x = (int) ev.getX(); final int y = (int) ev.getY(); int motionPosition = pointToPosition(x, y); if (!mDataChanged) { if (mTouchMode == TOUCH_MODE_FLING) { // Stopped a fling. It is a scroll. createScrollingCache(); mTouchMode = TOUCH_MODE_SCROLL; mMotionCorrection = 0; motionPosition = findMotionRow(y); // 根据y速度判断是否马上中断fling mFlingRunnable.flywheelTouch(); } else if ((motionPosition >= 0) && getAdapter().isEnabled(motionPosition)) { // User clicked on an actual view (and was not stopping a // fling). It might be a click or a scroll. Assume it is a // click until proven otherwise. // 设置touch模式为TOUCH_MODE_DOWN mTouchMode = TOUCH_MODE_DOWN; // FIXME Debounce if (mPendingCheckForTap == null) { mPendingCheckForTap = new CheckForTap(); } mPendingCheckForTap.x = ev.getX(); mPendingCheckForTap.y = ev.getY(); postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout()); } } if (motionPosition >= 0) { // Remember where the motion event started final View v = getChildAt(motionPosition - mFirstPosition); mMotionViewOriginalTop = v.getTop(); } mMotionX = x; // 记录mMotionY mMotionY = y; mMotionPosition = motionPosition; // 记录mLastY = Integer.MIN_VALUE mLastY = Integer.MIN_VALUE; } if (mTouchMode == TOUCH_MODE_DOWN && mMotionPosition != INVALID_POSITION && performButtonActionOnTouchDown(ev)) { removeCallbacks(mPendingCheckForTap); } }</code></pre> <p><strong>onTouchEvent()之onTouchMove():</strong></p> <pre> <code class="language-java">private void onTouchMove(MotionEvent ev, MotionEvent vtev) { int pointerIndex = ev.findPointerIndex(mActivePointerId); if (pointerIndex == -1) { pointerIndex = 0; mActivePointerId = ev.getPointerId(pointerIndex); } if (mDataChanged) { // Re-sync everything if data has been changed // since the scroll operation can query the adapter. layoutChildren(); } final int y = (int) ev.getY(pointerIndex); switch (mTouchMode) { // 刚开始进入这里 touchMode为TOUCH_MODE_DOWN case TOUCH_MODE_DOWN: case TOUCH_MODE_TAP: case TOUCH_MODE_DONE_WAITING: // Check if we have moved far enough that it looks more like a // scroll than a tap. If so, we'll enter scrolling mode. // 刚开始还么有处于可滚动状态,故进入判断是否可以滚动,核心判断调节为当然事件y // 与down事件mMotionY值的差值绝对值是否大于mTouchSlop if (startScrollIfNeeded((int) ev.getX(pointerIndex), y, vtev)) { break; } // Otherwise, check containment within list bounds. If we're // outside bounds, cancel any active presses. final View motionView = getChildAt(mMotionPosition - mFirstPosition); final float x = ev.getX(pointerIndex); if (!pointInView(x, y, mTouchSlop)) { setPressed(false); if (motionView != null) { motionView.setPressed(false); } removeCallbacks(mTouchMode == TOUCH_MODE_DOWN ? mPendingCheckForTap : mPendingCheckForLongPress); mTouchMode = TOUCH_MODE_DONE_WAITING; updateSelectorState(); } else if (motionView != null) { // Still within bounds, update the hotspot. final float[] point = mTmpPoint; point[0] = x; point[1] = y; transformPointToViewLocal(point, motionView); motionView.drawableHotspotChanged(point[0], point[1]); } break; case TOUCH_MODE_SCROLL: case TOUCH_MODE_OVERSCROLL: // 如果已经进入到listview的滚动状态,则直接执行scrollIfNeeded根据条件判断是否 // 进行滚动 scrollIfNeeded((int) ev.getX(pointerIndex), y, vtev); break; } } private void scrollIfNeeded(int x, int y, MotionEvent vtev) { int rawDeltaY = y - mMotionY; int scrollOffsetCorrection = 0; int scrollConsumedCorrection = 0; // mLastY==Integer.MIN_VALUE表明刚达到条件进入滚动状态 if (mLastY == Integer.MIN_VALUE) { // 保证了状态过度时候平稳滚动 rawDeltaY -= mMotionCorrection; } if (dispatchNestedPreScroll(0, mLastY != Integer.MIN_VALUE ? mLastY - y : -rawDeltaY, mScrollConsumed, mScrollOffset)) { rawDeltaY += mScrollConsumed[1]; scrollOffsetCorrection = -mScrollOffset[1]; scrollConsumedCorrection = mScrollConsumed[1]; if (vtev != null) { vtev.offsetLocation(0, mScrollOffset[1]); mNestedYOffset += mScrollOffset[1]; } } final int deltaY = rawDeltaY; // 两次连续下发事件的y差值.如果mLastY==Integer.MIN_VALUE表明刚达到条件进入滚动状态 // 此时incrementalDeltaY = rawDeltaY,而rawDeltaY已经在上面进行了 // (rawDeltaY -= mMotionCorrection),保证了平稳过度 int incrementalDeltaY = mLastY != Integer.MIN_VALUE ? y - mLastY + scrollConsumedCorrection : deltaY; int lastYCorrection = 0; if (mTouchMode == TOUCH_MODE_SCROLL) { if (PROFILE_SCROLLING) { if (!mScrollProfilingStarted) { Debug.startMethodTracing("AbsListViewScroll"); mScrollProfilingStarted = true; } } if (mScrollStrictSpan == null) { // If it's non-null, we're already in a scroll. mScrollStrictSpan = StrictMode.enterCriticalSpan("AbsListView-scroll"); } if (y != mLastY) { // We may be here after stopping a fling and continuing to scroll. // If so, we haven't disallowed intercepting touch events yet. // Make sure that we do so in case we're in a parent that can intercept. if ((mGroupFlags & FLAG_DISALLOW_INTERCEPT) == 0 && Math.abs(rawDeltaY) > mTouchSlop) { final ViewParent parent = getParent(); if (parent != null) { parent.requestDisallowInterceptTouchEvent(true); } } final int motionIndex; if (mMotionPosition >= 0) { motionIndex = mMotionPosition - mFirstPosition; } else { // If we don't have a motion position that we can reliably track, // pick something in the middle to make a best guess at things below. motionIndex = getChildCount() / 2; } int motionViewPrevTop = 0; View motionView = this.getChildAt(motionIndex); if (motionView != null) { motionViewPrevTop = motionView.getTop(); } // No need to do all this work if we're not going to move anyway boolean atEdge = false; // trackMotionScroll()方法真正进行滚动处理 if (incrementalDeltaY != 0) { atEdge = trackMotionScroll(deltaY, incrementalDeltaY); } // Check to see if we have bumped into the scroll limit motionView = this.getChildAt(motionIndex); if (motionView != null) { // Check if the top of the motion view is where it is // supposed to be final int motionViewRealTop = motionView.getTop(); // 滚动到最顶部或者最底部 if (atEdge) { // Apply overscroll int overscroll = -incrementalDeltaY - (motionViewRealTop - motionViewPrevTop); if (dispatchNestedScroll(0, overscroll - incrementalDeltaY, 0, overscroll, mScrollOffset)) { lastYCorrection -= mScrollOffset[1]; if (vtev != null) { vtev.offsetLocation(0, mScrollOffset[1]); mNestedYOffset += mScrollOffset[1]; } } else { final boolean atOverscrollEdge = overScrollBy(0, overscroll, 0, mScrollY, 0, 0, 0, mOverscrollDistance, true); if (atOverscrollEdge && mVelocityTracker != null) { // Don't allow overfling if we're at the edge mVelocityTracker.clear(); } final int overscrollMode = getOverScrollMode(); if (overscrollMode == OVER_SCROLL_ALWAYS || (overscrollMode == OVER_SCROLL_IF_CONTENT_SCROLLS && !contentFits())) { if (!atOverscrollEdge) { mDirection = 0; // Reset when entering overscroll. mTouchMode = TOUCH_MODE_OVERSCROLL; } if (incrementalDeltaY > 0) { // 顶部 OVER_SCROLL效果,draw()方法中实现 mEdgeGlowTop.onPull((float) -overscroll / getHeight(), (float) x / getWidth()); if (!mEdgeGlowBottom.isFinished()) { mEdgeGlowBottom.onRelease(); } invalidateTopGlow(); } else if (incrementalDeltaY < 0) { // 底部 OVER_SCROLL效果,draw()方法中实现 mEdgeGlowBottom.onPull((float) overscroll / getHeight(), 1.f - (float) x / getWidth()); if (!mEdgeGlowTop.isFinished()) { mEdgeGlowTop.onRelease(); } invalidateBottomGlow(); } } } } mMotionY = y + lastYCorrection + scrollOffsetCorrection; } // 记录当前事件的y mLastY = y + lastYCorrection + scrollOffsetCorrection; } }else if (mTouchMode == TOUCH_MODE_OVERSCROLL){.....}</code></pre> <p><strong>滚动实现核心代码trackMotionScroll(int deltaY, int incrementalDeltaY):</strong></p> <p>关键变量在第二个参数incrementalDeltaY,即两次连续事件的y差值</p> <pre> <code class="language-java">/** * Track a motion scroll * * @param deltaY Amount to offset mMotionView. This is the accumulated delta since the motion * began. Positive numbers mean the user's finger is moving down the screen. * @param incrementalDeltaY Change in deltaY from the previous event. * @return true if we're already at the beginning/end of the list and have nothing to do. */ boolean trackMotionScroll(int deltaY, int incrementalDeltaY) { final int childCount = getChildCount(); if (childCount == 0) { return true; } // listview 第一个item view的mTop值 final int firstTop = getChildAt(0).getTop(); // // listview 最后一个item view的mBottom值 final int lastBottom = getChildAt(childCount - 1).getBottom(); final Rect listPadding = mListPadding; // "effective padding" In this case is the amount of padding that affects // how much space should not be filled by items. If we don't clip to padding // there is no effective padding. int effectivePaddingTop = 0; int effectivePaddingBottom = 0; if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) { effectivePaddingTop = listPadding.top; effectivePaddingBottom = listPadding.bottom; } // FIXME account for grid vertical spacing too? final int spaceAbove = effectivePaddingTop - firstTop; final int end = getHeight() - effectivePaddingBottom; final int spaceBelow = lastBottom - end; final int height = getHeight() - mPaddingBottom - mPaddingTop; if (deltaY < 0) { deltaY = Math.max(-(height - 1), deltaY); } else { deltaY = Math.min(height - 1, deltaY); } // 限定incrementalDeltaY在合理的最值范围内 if (incrementalDeltaY < 0) { incrementalDeltaY = Math.max(-(height - 1), incrementalDeltaY); } else { incrementalDeltaY = Math.min(height - 1, incrementalDeltaY); } final int firstPosition = mFirstPosition; // Update our guesses for where the first and last views are if (firstPosition == 0) { mFirstPositionDistanceGuess = firstTop - listPadding.top; } else { mFirstPositionDistanceGuess += incrementalDeltaY; } if (firstPosition + childCount == mItemCount) { mLastPositionDistanceGuess = lastBottom + listPadding.bottom; } else { mLastPositionDistanceGuess += incrementalDeltaY; } // 判断是否在最顶部且手指向下滑动,是的话即不能向下滑动了 final boolean cannotScrollDown = (firstPosition == 0 && firstTop >= listPadding.top && incrementalDeltaY >= 0); // 判断是否在最底部且手指向上滑动,是的话即不能向上滑动了 final boolean cannotScrollUp = (firstPosition + childCount == mItemCount && lastBottom <= getHeight() - listPadding.bottom && incrementalDeltaY <= 0); // listview无法滚动即返回 if (cannotScrollDown || cannotScrollUp) { return incrementalDeltaY != 0; } // incrementalDeltaY<0说明手指是向上滑动的,即listview内容视图是向下移动显示的 final boolean down = incrementalDeltaY < 0; final boolean inTouchMode = isInTouchMode(); if (inTouchMode) { hideSelector(); } final int headerViewsCount = getHeaderViewsCount(); final int footerViewsStart = mItemCount - getFooterViewsCount(); int start = 0; int count = 0; if (down) { // 手指是向上滑动 incrementalDeltaY < 0 int top = -incrementalDeltaY; if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) { top += listPadding.top; } for (int i = 0; i < childCount; i++) { final View child = getChildAt(i); if (child.getBottom() >= top) { break; } else { // 最top的子view已经滑出listview count++; int position = firstPosition + i; if (position >= headerViewsCount && position < footerViewsStart) { // The view will be rebound to new data, clear any // system-managed transient state. child.clearAccessibilityFocus(); // 将最顶部滑出的子view 加入到mRecycler的mScrapViews中保存 mRecycler.addScrapView(child, position); } } } } else { // 手指是向下滑动 incrementalDeltaY > 0 int bottom = getHeight() - incrementalDeltaY; if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) { bottom -= listPadding.bottom; } for (int i = childCount - 1; i >= 0; i--) { final View child = getChildAt(i); if (child.getTop() <= bottom) { break; } else { // 最底部的子view已经滑出listview start = i; count++; int position = firstPosition + i; if (position >= headerViewsCount && position < footerViewsStart) { // The view will be rebound to new data, clear any // system-managed transient state. child.clearAccessibilityFocus(); // 将最底部滑出的子view 加入到mRecycler的mScrapViews中保存 mRecycler.addScrapView(child, position); } } } } mMotionViewNewTop = mMotionViewOriginalTop + deltaY; mBlockLayoutRequests = true; if (count > 0) { // 将上面滑出的子view 从listview中detach掉 detachViewsFromParent(start, count); mRecycler.removeSkippedScrap(); } // invalidate before moving the children to avoid unnecessary invalidate // calls to bubble up from the children all the way to the top if (!awakenScrollBars()) { invalidate(); } // 核心滚动便宜代码,根据incrementalDeltaY同步偏移所有的子view offsetChildrenTopAndBottom(incrementalDeltaY); if (down) { mFirstPosition += count; } // 根据条件判断是否填充滑动进入listview的子view final int absIncrementalDeltaY = Math.abs(incrementalDeltaY); if (spaceAbove < absIncrementalDeltaY || spaceBelow < absIncrementalDeltaY) { fillGap(down); } if (!inTouchMode && mSelectedPosition != INVALID_POSITION) { final int childIndex = mSelectedPosition - mFirstPosition; if (childIndex >= 0 && childIndex < getChildCount()) { positionSelector(mSelectedPosition, getChildAt(childIndex)); } } else if (mSelectorPosition != INVALID_POSITION) { final int childIndex = mSelectorPosition - mFirstPosition; if (childIndex >= 0 && childIndex < getChildCount()) { positionSelector(INVALID_POSITION, getChildAt(childIndex)); } } else { mSelectorRect.setEmpty(); } mBlockLayoutRequests = false; invokeOnItemScrollListener(); return false; }</code></pre> <p><strong>fillGap():</strong></p> <p>滚动过程判断需要加载填充滑动进的子view的处理部分</p> <pre> <code class="language-java">@Override void fillGap(boolean down) { final int count = getChildCount(); if (down) { // 手指是向上滑动,需要填充最底部滑动进的子view int paddingTop = 0; if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) { paddingTop = getListPaddingTop(); } final int startOffset = count > 0 ? getChildAt(count - 1).getBottom() + mDividerHeight : paddingTop; fillDown(mFirstPosition + count, startOffset); correctTooHigh(getChildCount()); } else { // 手指是向下滑动,需要填充最顶部滑动进的子view int paddingBottom = 0; if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) { paddingBottom = getListPaddingBottom(); } final int startOffset = count > 0 ? getChildAt(0).getTop() - mDividerHeight : getHeight() - paddingBottom; fillUp(mFirstPosition - 1, startOffset); correctTooLow(getChildCount()); } }</code></pre> <p>onTouchEvent()之onTouchUp():</p> <pre> <code class="language-java">private void onTouchUp(MotionEvent ev) { switch (mTouchMode) { .... case TOUCH_MODE_SCROLL: final int childCount = getChildCount(); if (childCount > 0) { final int firstChildTop = getChildAt(0).getTop(); final int lastChildBottom = getChildAt(childCount - 1).getBottom(); final int contentTop = mListPadding.top; final int contentBottom = getHeight() - mListPadding.bottom; if (mFirstPosition == 0 && firstChildTop >= contentTop && mFirstPosition + childCount < mItemCount && lastChildBottom <= getHeight() - contentBottom) { mTouchMode = TOUCH_MODE_REST; reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE); } else { final VelocityTracker velocityTracker = mVelocityTracker; velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity); final int initialVelocity = (int) (velocityTracker.getYVelocity(mActivePointerId) * mVelocityScale); // Fling if we have enough velocity and we aren't at a boundary. // Since we can potentially overfling more than we can overscroll, don't // allow the weird behavior where you can scroll to a boundary then // fling further. boolean flingVelocity = Math.abs(initialVelocity) > mMinimumVelocity; // 判断满足fling条件则进入 if (flingVelocity && !((mFirstPosition == 0 && firstChildTop == contentTop - mOverscrollDistance) || (mFirstPosition + childCount == mItemCount && lastChildBottom == contentBottom + mOverscrollDistance))) { if (!dispatchNestedPreFling(0, -initialVelocity)) { if (mFlingRunnable == null) { mFlingRunnable = new FlingRunnable(); } reportScrollStateChange(OnScrollListener.SCROLL_STATE_FLING); // fling走起 mFlingRunnable.start(-initialVelocity); dispatchNestedFling(0, -initialVelocity, true); } else { mTouchMode = TOUCH_MODE_REST; reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE); } } else { // 不满足fling条件 mTouchMode = TOUCH_MODE_REST; reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE); if (mFlingRunnable != null) { mFlingRunnable.endFling(); } if (mPositionScroller != null) { mPositionScroller.stop(); } if (flingVelocity && !dispatchNestedPreFling(0, -initialVelocity)) { dispatchNestedFling(0, -initialVelocity, false); } } } } else { mTouchMode = TOUCH_MODE_REST; reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE); } break; .... } setPressed(false); // over_scroll_mode 时候资源回收处理 if (mEdgeGlowTop != null) { mEdgeGlowTop.onRelease(); mEdgeGlowBottom.onRelease(); } // Need to redraw since we probably aren't drawing the selector anymore invalidate(); removeCallbacks(mPendingCheckForLongPress); recycleVelocityTracker(); mActivePointerId = INVALID_POINTER; if (PROFILE_SCROLLING) { if (mScrollProfilingStarted) { Debug.stopMethodTracing(); mScrollProfilingStarted = false; } } if (mScrollStrictSpan != null) { mScrollStrictSpan.finish(); mScrollStrictSpan = null; } }</code></pre> <p>Up事件的fling处理主要在AbsListView的内部类FlingRunnable中:</p> <p><strong>FlingRunnable.start():</strong></p> <pre> <code class="language-java">void start(int initialVelocity) { int initialY = initialVelocity < 0 ? Integer.MAX_VALUE : 0; mLastFlingY = initialY; mScroller.setInterpolator(null); // 设置scroller做fling动作 mScroller.fling(0, initialY, 0, initialVelocity, 0, Integer.MAX_VALUE, 0, Integer.MAX_VALUE); mTouchMode = TOUCH_MODE_FLING; // 在下一帧调用run()方法 postOnAnimation(this); if (PROFILE_FLINGING) { if (!mFlingProfilingStarted) { Debug.startMethodTracing("AbsListViewFling"); mFlingProfilingStarted = true; } } if (mFlingStrictSpan == null) { mFlingStrictSpan = StrictMode.enterCriticalSpan("AbsListView-fling"); } }</code></pre> <p><strong>FlingRunnable实现了Runnable,复写的run():</strong></p> <pre> <code class="language-java">@Override public void run() { switch (mTouchMode) { default: endFling(); return; case TOUCH_MODE_SCROLL: if (mScroller.isFinished()) { return; } // Fall through case TOUCH_MODE_FLING: { if (mDataChanged) { layoutChildren(); } if (mItemCount == 0 || getChildCount() == 0) { endFling(); return; } final OverScroller scroller = mScroller; boolean more = scroller.computeScrollOffset(); final int y = scroller.getCurrY(); // Flip sign to convert finger direction to list items direction // (e.g. finger moving down means list is moving towards the top) int delta = mLastFlingY - y; // Pretend that each frame of a fling scroll is a touch scroll if (delta > 0) { // List is moving towards the top. Use first view as mMotionPosition mMotionPosition = mFirstPosition; final View firstView = getChildAt(0); mMotionViewOriginalTop = firstView.getTop(); // Don't fling more than 1 screen delta = Math.min(getHeight() - mPaddingBottom - mPaddingTop - 1, delta); } else { // List is moving towards the bottom. Use last view as mMotionPosition int offsetToLast = getChildCount() - 1; mMotionPosition = mFirstPosition + offsetToLast; final View lastView = getChildAt(offsetToLast); mMotionViewOriginalTop = lastView.getTop(); // Don't fling more than 1 screen delta = Math.max(-(getHeight() - mPaddingBottom - mPaddingTop - 1), delta); } // Check to see if we have bumped into the scroll limit View motionView = getChildAt(mMotionPosition - mFirstPosition); int oldTop = 0; if (motionView != null) { oldTop = motionView.getTop(); } // Don't stop just because delta is zero (it could have been rounded) // fling滚动过程模拟的滑动处理 final boolean atEdge = trackMotionScroll(delta, delta); final boolean atEnd = atEdge && (delta != 0); if (atEnd) { if (motionView != null) { // Tweak the scroll for how far we overshot int overshoot = -(delta - (motionView.getTop() - oldTop)); overScrollBy(0, overshoot, 0, mScrollY, 0, 0, 0, mOverflingDistance, false); } if (more) { edgeReached(delta); } break; } if (more && !atEnd) { // fling没有结束 继续递归调用run()方法 if (atEdge) invalidate(); mLastFlingY = y; postOnAnimation(this); } else { // fling已经结束 endFling(); if (PROFILE_FLINGING) { if (mFlingProfilingStarted) { Debug.stopMethodTracing(); mFlingProfilingStarted = false; } if (mFlingStrictSpan != null) { mFlingStrictSpan.finish(); mFlingStrictSpan = null; } } } break; } .... } } }</code></pre> <p>至此,ListView源码分析完...</p> <p> </p> <p> </p> <p> </p> <p>来自：http://www.jianshu.com/p/7f95297b6271</p> <p> </p>