android ViewGoup事件分发机制dispatchTouchEvent

做本章内容给大家分享一下ViewGroup的事件分发中dispatchTouchEvent事件，这个事件大家很熟悉，但平时重写最多的是onInterceptTouchEvent，onTouchEvent，说到点击事件大家都会明白有三个重要的方法，下面结合源码、例子、图来分析dispatchTouchEvent，onInterceptTouchEvent，onTouchEvent三者之间的关系

先上例子：

例子中，有4个ViewGroup，每个ViewGroup都重写了三个重要的方法，而且也重写了当前Activity的dispatchTouchEvent和onTouchEvent

ViewGroup之间的关系如下图

android ViewGoup事件分发机制dispatchTouchEvent

import android.os.Bundle;
import android.util.Log;
import android.view.MotionEvent;
import android.view.ViewGroup;
import android.widget.RelativeLayout;

public class MainActivity extends Activity {

    private final String TAG = "WXQ";
	
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        RelativeLayout viewA = new RelativeLayout(this){

            @Override
            public boolean dispatchTouchEvent(MotionEvent ev) {
                Log.i(TAG, "viewA dispatchTouchEvent action=" + ev.getAction());
                boolean value = super.dispatchTouchEvent(ev);
                Log.d(TAG, "viewA dispatchTouchEvent value=" + value);
                return value;
            }

            @Override
            public boolean onInterceptTouchEvent(MotionEvent ev) {
                Log.i(TAG, "viewA onInterceptTouchEvent action=" + ev.getAction());
                boolean value = super.onInterceptTouchEvent(ev);
                Log.d(TAG, "viewA onInterceptTouchEvent value=" + value);
                return value;
            }

            @Override
            public boolean onTouchEvent(MotionEvent event) {
                Log.i(TAG, "viewA onTouchEvent action=" + event.getAction());
                boolean value = super.onTouchEvent(event);
                Log.d(TAG, "viewA onTouchEvent value=" + value);
                return value;
            }
        };

        RelativeLayout viewB = new RelativeLayout(this) {

            @Override
            public boolean dispatchTouchEvent(MotionEvent ev) {
                Log.i(TAG, "viewB dispatchTouchEvent action=" + ev.getAction());
                boolean value = super.dispatchTouchEvent(ev);
                Log.d(TAG, "viewB dispatchTouchEvent value=" + value);
                return value;
            }

            @Override
            public boolean onInterceptTouchEvent(MotionEvent ev) {
                Log.i(TAG, "viewB onInterceptTouchEvent action=" + ev.getAction());
                boolean value = super.onInterceptTouchEvent(ev);
                Log.d(TAG, "viewB onInterceptTouchEvent value=" + value);
                return value;
            }

            @Override
            public boolean onTouchEvent(MotionEvent event) {
                Log.i(TAG, "viewB onTouchEvent action=" + event.getAction());
                boolean value = super.onTouchEvent(event);
                Log.d(TAG, "viewB onTouchEvent value=" + value);
                return value;
            }
        };
        RelativeLayout viewC = new RelativeLayout(this) {


            @Override
            public boolean dispatchTouchEvent(MotionEvent ev) {
                Log.i(TAG, "viewC dispatchTouchEvent action=" + ev.getAction());
                boolean value = super.dispatchTouchEvent(ev);
                Log.d(TAG, "viewC dispatchTouchEvent value=" + value);
                return value;
            }

            @Override
            public boolean onInterceptTouchEvent(MotionEvent ev) {
                Log.i(TAG, "viewC onInterceptTouchEvent action=" + ev.getAction());
                boolean value = super.onInterceptTouchEvent(ev);
                Log.d(TAG, "viewC onInterceptTouchEvent value=" + value);
                return value;
            }

            @Override
            public boolean onTouchEvent(MotionEvent event) {
                Log.i(TAG, "viewC onTouchEvent action=" + event.getAction());
                boolean value = super.onTouchEvent(event);
                Log.d(TAG, "viewC onTouchEvent value=" + value);
                return value;
            }
        };
        RelativeLayout viewD = new RelativeLayout(this) {

            @Override
            public boolean dispatchTouchEvent(MotionEvent ev) {
                Log.i(TAG, "viewD dispatchTouchEvent action=" + ev.getAction());
                boolean value = super.dispatchTouchEvent(ev);
                Log.d(TAG, "viewD dispatchTouchEvent value=" + value);
                return value;
            }

            @Override
            public boolean onInterceptTouchEvent(MotionEvent ev) {
                Log.i(TAG, "viewD onInterceptTouchEvent action=" + ev.getAction());
                boolean value = super.onInterceptTouchEvent(ev);
                Log.d(TAG, "viewD onInterceptTouchEvent value=" + value);
                return value;
            }

            @Override
            public boolean onTouchEvent(MotionEvent event) {
                Log.i(TAG, "viewD onTouchEvent action=" + event.getAction());
                boolean value = super.onTouchEvent(event);
                Log.d(TAG, "viewD onTouchEvent value=" + value);
                return value;
            }
        };
        viewC.addView(viewD, new RelativeLayout.LayoutParams(ViewGroup.LayoutParams.FILL_PARENT, ViewGroup.LayoutParams.FILL_PARENT));
        viewA.addView(viewC, new RelativeLayout.LayoutParams(ViewGroup.LayoutParams.FILL_PARENT, ViewGroup.LayoutParams.FILL_PARENT));
        viewA.addView(viewB, new RelativeLayout.LayoutParams(ViewGroup.LayoutParams.FILL_PARENT, ViewGroup.LayoutParams.FILL_PARENT));
        setContentView(viewA);

    }

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        Log.i(TAG, "activity dispatchTouchEvent action=" + ev.getAction());
        boolean value = super.dispatchTouchEvent(ev);
        Log.d(TAG, "activity dispatchTouchEvent value=" + value);
        return value;
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        Log.i(TAG, "activity onTouchEvent action=" + event.getAction());
        boolean value = super.onTouchEvent(event);
        Log.d(TAG, "activity onTouchEvent value=" + value);
        return value;
    }
}

情景1：首先所有的返回值都不做改动，我们运行一下看看效果

activity dispatchTouchEvent action=0
viewA dispatchTouchEvent action=0
viewA onInterceptTouchEvent action=0
viewA onInterceptTouchEvent value=false
viewB dispatchTouchEvent action=0
viewB onInterceptTouchEvent action=0
viewB onInterceptTouchEvent value=false
viewB onTouchEvent action=0
viewB onTouchEvent value=false
viewB dispatchTouchEvent value=false
viewC dispatchTouchEvent action=0
viewC onInterceptTouchEvent action=0
viewC onInterceptTouchEvent value=false
viewD dispatchTouchEvent action=0
viewD onInterceptTouchEvent action=0
viewD onInterceptTouchEvent value=false
viewD onTouchEvent action=0
viewD onTouchEvent value=false
viewD dispatchTouchEvent value=false
viewC onTouchEvent action=0
viewC onTouchEvent value=false
viewC dispatchTouchEvent value=false
viewA onTouchEvent action=0
viewA onTouchEvent value=false
viewA dispatchTouchEvent value=false
activity onTouchEvent action=0
activity onTouchEvent value=false
activity dispatchTouchEvent value=false
activity dispatchTouchEvent action=1
activity onTouchEvent action=1
activity onTouchEvent value=false
activity dispatchTouchEvent value=false

down事件，首先明白一点就是touch任何事件都是最先到Activity，Activity接收顺序为 dispatchTouchEvent然后是 onTouchEvent，然后再向下分发给ViewGroup，ViewGroup最先接收到的是 dispatchTouchEvent方法，然后经过 onInterceptTouchEvent 再到onTouchEvent ，先不要问为什么，一会儿咱们一起看源码就知道了。因为ViewGroup默认都是不可点击，所以所有的ViewGroup方法都返回了false，最终down事件又回到了Activity，所以Activity的dispatchTouchEvent的接收值为false

紧接着up事件，事件也是最先由当前Activity的dispatchTouchEvent来分发，然后就没再向下分发，为什么呢，再看一种情景就知道了

情景2：接下来我们做一点儿改动，我们将ViewB调用setClickable(true)，使ViewB变成一个可点击的VIEW，运行一下看看会发生什么

activity dispatchTouchEvent action=0

viewA dispatchTouchEvent action=0

viewA onInterceptTouchEvent action=0

viewA onInterceptTouchEvent value=false

viewB dispatchTouchEvent action=0

viewB onInterceptTouchEvent action=0

viewB onInterceptTouchEvent value=false

viewB onTouchEvent action=0

viewB onTouchEvent value=true

viewB dispatchTouchEvent value=true

viewA dispatchTouchEvent value=true

activity dispatchTouchEvent value=true

activity dispatchTouchEvent action=1

viewA dispatchTouchEvent action=1

viewA onInterceptTouchEvent action=1

viewA onInterceptTouchEvent value=false

viewB dispatchTouchEvent action=1

viewB onTouchEvent action=1

viewB onTouchEvent value=true

viewB dispatchTouchEvent value=true

viewA dispatchTouchEvent value=true

activity dispatchTouchEvent value=true

我们看到有了很大变化，，由于ViewB已经变成了可点击的View所以ViewB的onTouchEvent返回了true，同时我们也看到了Activity的dispatchTouchEvent也返回true，接着up事件，也是先经过Activity再往下传递给ViewA再传递给ViewB然后就没再向下传了，为什么呢，因为ViewB是可点击的View，既然ViewB收到了down事件，那么move、up事件也一定能收到才是合理的完整事件对吧。

那你可能会问两个问题1、事件为什么不再向ViewC和ViewD传递了呢，2、为什么ViewA能收到事件。原因很简单，抛开程序思维去想，ViewA收到事件后检索自己的子View，ViewB收到事件了，再往下传已经没意义了，ViewB是ViewA的子View，事件必定会先经过父结点

我们看一下源码粗体部分可知，当前ViewGroup的dispatchTouchEvent被调用后，会把事件交给了onInterceptTouchEvent来做拦截处理

public boolean dispatchTouchEvent(MotionEvent ev) {
        boolean handled = false;
        if (onFilterTouchEventForSecurity(ev)) {
	...........
            // Check for interception.
            final boolean intercepted;
            if (actionMasked == MotionEvent.ACTION_DOWN
                    || mFirstTouchTarget != null) {
                final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
                if (!disallowIntercept) {
                    intercepted = onInterceptTouchEvent(ev);
                    ev.setAction(action); // restore action in case it was changed
                } else {
                    intercepted = false;
                }
            } else {
                // There are no touch targets and this action is not an initial down
                // so this view group continues to intercept touches.
                intercepted = true;
            }
			.........
        }

		..........
        return handled;
    }

对子VIEW遍历，根据子VIEW的dispatchTouchEvent的返回值是否决定下一个事件继续分发

public boolean dispatchTouchEvent(MotionEvent ev) {

..................
                    if (newTouchTarget == null && childrenCount != 0) {
                        final float x = ev.getX(actionIndex);
                        final float y = ev.getY(actionIndex);
                        // Find a child that can receive the event.
                        // Scan children from front to back.
                        final ArrayList<View> preorderedList = buildOrderedChildList();
                        final boolean customOrder = preorderedList == null
                                && isChildrenDrawingOrderEnabled();
                        final View[] children = mChildren;
                        for (int i = childrenCount - 1; i >= 0; i--) {
                            final int childIndex = customOrder
                                    ? getChildDrawingOrder(childrenCount, i) : i;
                            final View child = (preorderedList == null)
                                    ? children[childIndex] : preorderedList.get(childIndex);
                            if (!canViewReceivePointerEvents(child)
                                    || !isTransformedTouchPointInView(x, y, child, null)) {
                                continue;
                            }

                            newTouchTarget = getTouchTarget(child);
                            if (newTouchTarget != null) {
                                // Child is already receiving touch within its bounds.
                                // Give it the new pointer in addition to the ones it is handling.
                                newTouchTarget.pointerIdBits |= idBitsToAssign;
                                break;
                            }

                            resetCancelNextUpFlag(child);
                            if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
                                // Child wants to receive touch within its bounds.
                                mLastTouchDownTime = ev.getDownTime();
                                if (preorderedList != null) {
                                    // childIndex points into presorted list, find original index
                                    for (int j = 0; j < childrenCount; j++) {
                                        if (children[childIndex] == mChildren[j]) {
                                            mLastTouchDownIndex = j;
                                            break;
                                        }
                                    }
                                } else {
                                    mLastTouchDownIndex = childIndex;
                                }
                                mLastTouchDownX = ev.getX();
                                mLastTouchDownY = ev.getY();
                                newTouchTarget = addTouchTarget(child, idBitsToAssign);
                                alreadyDispatchedToNewTouchTarget = true;
                                break;
                            }
                        }
                        if (preorderedList != null) preorderedList.clear();
                    }

...............
    }

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
            View child, int desiredPointerIdBits) {
        final boolean handled;

        // Canceling motions is a special case.  We don't need to perform any transformations
        // or filtering.  The important part is the action, not the contents.
        final int oldAction = event.getAction();
        if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
            event.setAction(MotionEvent.ACTION_CANCEL);
            if (child == null) {
                handled = super.dispatchTouchEvent(event);
            } else {
                handled = child.dispatchTouchEvent(event);
            }
            event.setAction(oldAction);
            return handled;
        }

        // Calculate the number of pointers to deliver.
        final int oldPointerIdBits = event.getPointerIdBits();
        final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;

        // If for some reason we ended up in an inconsistent state where it looks like we
        // might produce a motion event with no pointers in it, then drop the event.
        if (newPointerIdBits == 0) {
            return false;
        }

        // If the number of pointers is the same and we don't need to perform any fancy
        // irreversible transformations, then we can reuse the motion event for this
        // dispatch as long as we are careful to revert any changes we make.
        // Otherwise we need to make a copy.
        final MotionEvent transformedEvent;
        if (newPointerIdBits == oldPointerIdBits) {
            if (child == null || child.hasIdentityMatrix()) {
                if (child == null) {
                    handled = super.dispatchTouchEvent(event);
                } else {
                    final float offsetX = mScrollX - child.mLeft;
                    final float offsetY = mScrollY - child.mTop;
                    event.offsetLocation(offsetX, offsetY);

                    handled = child.dispatchTouchEvent(event);

                    event.offsetLocation(-offsetX, -offsetY);
                }
                return handled;
            }
            transformedEvent = MotionEvent.obtain(event);
        } else {
            transformedEvent = event.split(newPointerIdBits);
        }

        // Perform any necessary transformations and dispatch.
        if (child == null) {
            handled = super.dispatchTouchEvent(transformedEvent);
        } else {
            final float offsetX = mScrollX - child.mLeft;
            final float offsetY = mScrollY - child.mTop;
            transformedEvent.offsetLocation(offsetX, offsetY);
            if (! child.hasIdentityMatrix()) {
                transformedEvent.transform(child.getInverseMatrix());
            }

            handled = child.dispatchTouchEvent(transformedEvent);
        }

        // Done.
        transformedEvent.recycle();
        return handled;
    }
                       
 
对ViewGroup的dispatchTouchEvent方法的总结：方法中的super.dispatchTouchEvent去分发当前事件到自己的子View，而当前方法的返回值是告诉父View或Activity下一个事件(move、up)还需不需要给当前ViewGroup，它的返回值是由其所有子View来决定，如果所有子View都不可点返回false,如果有任何一个子View接收事件则返回true，回过头看看情景1知道为什么up事件Activity不再向ViewGroup传递，因为ViewA的dispatchTouchEvent在down事件时返回了false，说明ViewA的所有子View都没有接收down事件，所以ViewA告诉Activity说up事件不需要再给我了

android ViewGoup事件分发机制dispatchTouchEvent

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