android中ListView的定位:使用setSelectionFromTop
TZL818
9年前
如果一个ListView太长,有时我们希望ListView在从其他界面返回的时候能够恢复上次查看的位置,这就涉及到ListView的定位问题:
解决的办法如下:
// 保存当前第一个可见的item的索引和偏移量 int index = mList.getFirstVisiblePosition(); View v = mList.getChildAt(0); int top = (v == null) ? 0 : v.getTop(); // ... //根据上次保存的index和偏移量恢复上次的位置 mList.setSelectionFromTop(index, top);
这里使用了setSelectionFromTop来定位ListView。其实还可以使用setSelection也可以定位,只是setSelectionFromTop要比setSelection更精准。因为通过getFirstVisiblePosition
得到的第一个item可能已经有一部分是不可见的了,如果用setSelection无法反映出这不可见的部分。
为了说明setSelectionFromTop的参数值的意义,以及与setSelection的区别,下面从源码上来分析:
看一下setSelectionFromTop()的具体实现,代码如下:
/** * Sets the selected item and positions the selection y pixels from the top edge * of the ListView. (If in touch mode, the item will not be selected but it will * still be positioned appropriately.) * * @param position Index (starting at 0) of the data item to be selected. * @param y The distance from the top edge of the ListView (plus padding) that the * item will be positioned. */ public void setSelectionFromTop(int position, int y) { if (mAdapter == null) { return; } if (!isInTouchMode()) { position = lookForSelectablePosition(position, true); if (position >= 0) { setNextSelectedPositionInt(position); } } else { mResurrectToPosition = position; } if (position >= 0) { mLayoutMode = LAYOUT_SPECIFIC; mSpecificTop = mListPadding.top + y; if (mNeedSync) { mSyncPosition = position; mSyncRowId = mAdapter.getItemId(position); } requestLayout(); } }
从上面的代码可以得知,setSelectionFromTop()的作用是设置ListView选中的位置,同时在Y轴设置一个偏移量。
而setSelection()方法,传入一个index整型数值,就可以让ListView定位到指定Item的位置。
这两个方法有什么区别呢?看一下setSelection()的具体实现,代码如下:
/** * Sets the currently selected item. If in touch mode, the item will not be selected * but it will still be positioned appropriately. If the specified selection position * is less than 0, then the item at position 0 will be selected. * * @param position Index (starting at 0) of the data item to be selected. */ @Override public void setSelection(int position) { setSelectionFromTop(position, 0); }
原来,setSelection()内部就是调用了setSelectionFromTop(),只不过是Y轴的偏移量是0而已。现在应该对setSelection()和setSelectionFromTop()有了更深刻的认识了。