SwipeDismissBehavior用法及实现原理

无意间发现design兼容库中有一个叫做SwipeDismissBehavior的类，顾名思义它就是用来实现滑动删除的了。莫非现在滑动删除又有更简单的解决办法了？鉴于之前RecyclerView中已经有ItemTouchHelper，而且也非常简单，这个意外发现真的让我很好奇，于是决定研究研究，看看它的实现原理以及应用场景：真的能替代其他的（不管是第三方还是RecyclerView自带的ItemTouchHelper）滑动删除吗？。

很不幸SwipeDismissBehavior现在的文档还很少，只有stackoverfolw上有点价值的讨论。

先来直接从API的角度使用SwipeDismissBehavior，然后再讲解SwipeDismissBehavior的原理。从而说明为什么SwipeDismissBehavior只能和CoordinatorLayout一起使用？为什么SwipeDismissBehavior对CoordinatorLayout中RecyclerView的item不起作用。

SwipeDismissBehavior的用法

SwipeDismissBehavior的用法非常简单。

第一步：引入design库：

compile 'com.android.support:appcompat-v7:23.1.0'  compile 'com.android.support:design:23+'

第二步：把要滑动删除的View放在CoordinatorLayout中：

xml代码：

<?xml version="1.0" encoding="utf-8"?>  <android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"   xmlns:app="http://schemas.android.com/apk/res-auto"   xmlns:tools="http://schemas.android.com/tools"   android:id="@+id/coordinatorLayout"   android:layout_width="match_parent"   android:layout_height="match_parent"   >   <TextView    android:id="@+id/swip"    android:layout_width="match_parent"    android:layout_height="200dip"    android:background="#32CD32"    android:text="别删我"    android:textSize="20dip"    android:gravity="center"    />  </android.support.design.widget.CoordinatorLayout>

第三步：在MainActivity中为View设置一个SwipeDismissBehavior对象：

package com.jcodecraeer.swipedismissbehaviordemo;  import android.os.Bundle;  import android.support.design.widget.CoordinatorLayout;  import android.support.design.widget.SwipeDismissBehavior;  import android.support.v7.app.AppCompatActivity;  import android.view.View;  import android.widget.TextView;  public class MainActivity extends AppCompatActivity {      @Override      protected void onCreate(Bundle savedInstanceState) {   super.onCreate(savedInstanceState);   setContentView(R.layout.activity_main);   TextView swipeView = (TextView)findViewById(R.id.swip);   final SwipeDismissBehavior<View> swipe    = new SwipeDismissBehavior();   swipe.setSwipeDirection(    SwipeDismissBehavior.SWIPE_DIRECTION_ANY);   swipe.setListener(    new SwipeDismissBehavior.OnDismissListener() {        @Override public void onDismiss(View view) {        }        @Override        public void onDragStateChanged(int state) {}    });   CoordinatorLayout.LayoutParams coordinatorParams =    (CoordinatorLayout.LayoutParams) swipeView.getLayoutParams();   coordinatorParams.setBehavior(swipe);      }  }

其中

swipe.setListener(          new SwipeDismissBehavior.OnDismissListener() {              @Override public void onDismiss(View view) {                }                @Override              public void onDragStateChanged(int state) {}          });

都可以去掉。

然后运行就能得到如下效果：

CoordinatorLayout与Behavior

SwipeDismissBehavior是design兼容库中的一个类。

定义

 public class SwipeDismissBehavior<V extends View> extends CoordinatorLayout.Behavior<V>

从这个定义可以看出SwipeDismissBehavior继承自CoordinatorLayout.Behavior。

而Behavior是CoordinatorLayout的一个内部类

public static abstract class Behavior<V extends View>

它只定义了一些抽象方法，其中最主要的当属下面两个（与本文相关）：

public boolean onInterceptTouchEvent(CoordinatorLayout parent, V child, MotionEvent ev) {      return false;  }    public boolean onTouchEvent(CoordinatorLayout parent, V child, MotionEvent ev) {      return false;  }

CoordinatorLayout会在自己的onInterceptTouchEvent()方法中调用Behavior的

onInterceptTouchEvent：

b.onInterceptTouchEvent(this, child, ev);

把自己(this)、与此Behavior对象相关的子view（child）以及MotionEvent ev传递过去。这三个参数对于实现一个Behavior都至关重要。

CoordinatorLayout遍历子view，判断子view的mLayoutParam变量中是否有Behavior成员，如果有则调用Behavior的onInterceptTouchEvent和onTouchEvent方法。上面的代码中，swipeView通过

CoordinatorLayout.LayoutParams coordinatorParams =          (CoordinatorLayout.LayoutParams) swipeView.getLayoutParams();    coordinatorParams.setBehavior(swipe);

给自己设置了一个类型为SwipeDismissBehavior的Behavior，而且它又是CoordinatorLayout的子view，因此当CoordinatorLayout遍历到了这个cardview的时候，会尝试从这个swipeView获得Behavior：

final LayoutParams lp = (LayoutParams) child.getLayoutParams();  final Behavior b = lp.getBehavior();

注：这里的LayoutParams是CoordinatorLayout.LayoutParams类，跟ViewGroup的还是有所区别，他是CoordinatorLayout的内部类。其实任何一个布局比如LinearLayout都有自己的LayoutParams类型，也都是定义在布局类的内部。

如果检测到这个Behavior不为空，就调用它的onInterceptTouchEvent和onTouchEvent方法。

if (!intercepted && b != null) {   switch (type) {    case TYPE_ON_INTERCEPT:     intercepted = b.onInterceptTouchEvent(this, child, ev);     break;    case TYPE_ON_TOUCH:     intercepted = b.onTouchEvent(this, child, ev);     break;   }   if (intercepted) {    mBehaviorTouchView = child;   }  }

自此CoordinatorLayout的任务完成，因为我们设置的这个Behavior是SwipeDismissBehavior对象，所以接下来该怎么处理就交给SwipeDismissBehavior了。

以上过程基本都在CoordinatorLayout的performIntercept(MotionEvent ev, final int type)方法里。

SwipeDismissBehavior

那么我们看看SwipeDismissBehavior的onInterceptTouchEvent和onTouchEvent方法到底做了什么呢？

public boolean onInterceptTouchEvent(CoordinatorLayout parent, V child, MotionEvent event) {   switch(MotionEventCompat.getActionMasked(event)) {   case 1:   case 3:    if(this.mIgnoreEvents) {     this.mIgnoreEvents = false;     return false;    }    break;   default:    this.mIgnoreEvents = !parent.isPointInChildBounds(child, (int)event.getX(), (int)event.getY());   }   if(this.mIgnoreEvents) {    return false;   } else {    this.ensureViewDragHelper(parent);    return this.mViewDragHelper.shouldInterceptTouchEvent(event);   }  }  public boolean onTouchEvent(CoordinatorLayout parent, V child, MotionEvent event) {   if(this.mViewDragHelper != null) {    this.mViewDragHelper.processTouchEvent(event);    return true;   } else {    return false;   }  }

在onInterceptTouchEvent(CoordinatorLayout parent, V child, MotionEvent event) 中，调用了ensureViewDragHelper(parent)：

private void ensureViewDragHelper(ViewGroup parent) {      if(this.mViewDragHelper == null) {          this.mViewDragHelper = this.mSensitivitySet?ViewDragHelper.create(parent, this.mSensitivity, this.mDragCallback):ViewDragHelper.create(parent, this.mDragCallback);      }    }

可以看到这里其实是用parent参数创建了一个ViewDragHelper，根据前面的分析，这里的parent其实就是CoordinatorLayout对象。如果你熟悉ViewDragHelper，那么基本上都能猜到SwipeDismissBehavior要做些什么了。

SwipeDismissBehavior就是根据 MotionEvent和parent创建了一个实现了滑动删除的ViewDragHelper。

具体实现的代码请看SwipeDismissBehavior的mDragCallback变量。

private final Callback mDragCallback = new Callback() {   private int mOriginalCapturedViewLeft;   public boolean tryCaptureView(View child, int pointerId) {    this.mOriginalCapturedViewLeft = child.getLeft();    return true;   }   public void onViewDragStateChanged(int state) {    if(SwipeDismissBehavior.this.mListener != null) {     SwipeDismissBehavior.this.mListener.onDragStateChanged(state);    }   }   public void onViewReleased(View child, float xvel, float yvel) {    int childWidth = child.getWidth();    boolean dismiss = false;    int targetLeft;    if(this.shouldDismiss(child, xvel)) {     targetLeft = child.getLeft() < this.mOriginalCapturedViewLeft?this.mOriginalCapturedViewLeft - childWidth:this.mOriginalCapturedViewLeft + childWidth;     dismiss = true;    } else {     targetLeft = this.mOriginalCapturedViewLeft;    }    if(SwipeDismissBehavior.this.mViewDragHelper.settleCapturedViewAt(targetLeft, child.getTop())) {     ViewCompat.postOnAnimation(child, SwipeDismissBehavior.this.new SettleRunnable(child, dismiss));    } else if(dismiss && SwipeDismissBehavior.this.mListener != null) {     SwipeDismissBehavior.this.mListener.onDismiss(child);    }   }   private boolean shouldDismiss(View child, float xvel) {    if(xvel != 0.0F) {     boolean distance1 = ViewCompat.getLayoutDirection(child) == 1;     return SwipeDismissBehavior.this.mSwipeDirection == 2?true:(SwipeDismissBehavior.this.mSwipeDirection == 0?(distance1?xvel < 0.0F:xvel > 0.0F):(SwipeDismissBehavior.this.mSwipeDirection == 1?(distance1?xvel > 0.0F:xvel < 0.0F):false));    } else {     int distance = child.getLeft() - this.mOriginalCapturedViewLeft;     int thresholdDistance = Math.round((float)child.getWidth() * SwipeDismissBehavior.this.mDragDismissThreshold);     return Math.abs(distance) >= thresholdDistance;    }   }   public int getViewHorizontalDragRange(View child) {    return child.getWidth();   }   public int clampViewPositionHorizontal(View child, int left, int dx) {    boolean isRtl = ViewCompat.getLayoutDirection(child) == 1;    int min;    int max;    if(SwipeDismissBehavior.this.mSwipeDirection == 0) {     if(isRtl) {      min = this.mOriginalCapturedViewLeft - child.getWidth();      max = this.mOriginalCapturedViewLeft;     } else {      min = this.mOriginalCapturedViewLeft;      max = this.mOriginalCapturedViewLeft + child.getWidth();     }    } else if(SwipeDismissBehavior.this.mSwipeDirection == 1) {     if(isRtl) {      min = this.mOriginalCapturedViewLeft;      max = this.mOriginalCapturedViewLeft + child.getWidth();     } else {      min = this.mOriginalCapturedViewLeft - child.getWidth();      max = this.mOriginalCapturedViewLeft;     }    } else {     min = this.mOriginalCapturedViewLeft - child.getWidth();     max = this.mOriginalCapturedViewLeft + child.getWidth();    }    return SwipeDismissBehavior.clamp(min, left, max);   }   public int clampViewPositionVertical(View child, int top, int dy) {    return child.getTop();   }   public void onViewPositionChanged(View child, int left, int top, int dx, int dy) {    float startAlphaDistance = (float)this.mOriginalCapturedViewLeft + (float)child.getWidth() * SwipeDismissBehavior.this.mAlphaStartSwipeDistance;    float endAlphaDistance = (float)this.mOriginalCapturedViewLeft + (float)child.getWidth() * SwipeDismissBehavior.this.mAlphaEndSwipeDistance;    if((float)left <= startAlphaDistance) {     ViewCompat.setAlpha(child, 1.0F);    } else if((float)left >= endAlphaDistance) {     ViewCompat.setAlpha(child, 0.0F);    } else {     float distance = SwipeDismissBehavior.fraction(startAlphaDistance, endAlphaDistance, (float)left);     ViewCompat.setAlpha(child, SwipeDismissBehavior.clamp(0.0F, 1.0F - distance, 1.0F));    }   }  };

问题

那么我们的问题来了？SwipeDismissBehavior可以替代RecyclerView的ItemTouchHelper或者其他列表滑动删除库吗？

答案是不能。

因为CoordinatorLayout遍历子View的时候，只遍历了第一层view，而列表的滑动删除对象是在RecyclerView的里面，不是CoordinatorLayout的直接子view。

再者，既然是RecyclerView的item，那么它的LayoutParams就是RecyclerView.LayoutParams 它无法强制转换成CoordinatorLayout.LayoutParams，所以运行的时候会报错：

java.lang.ClassCastException: android.support.v7.widget.RecyclerView$LayoutParams cannot be cast to android.support.design.widget.CoordinatorLayout$LayoutParams

因此SwipeDismissBehavior只适合本文开始的那种用法。