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。