Android事件传递机制(dispatchTouchEvent、onInterceptTouchEvent、onTouchEvent)

浏览数：25 / 时间：2015年06月11日

一、View的dispatchTouchEvent和onTouchEvent

探讨Android事件传递机制前，明确android的两大基础控件类型:View和ViewGroup。View即普通的控件，没有子布局的，如Button、TextView. ViewGroup继承自View，表示可以有子控件，如Linearlayout、Listview这些。而事件即MotionEvent，最重要的有3个:

(1)MotionEvent.ACTION_DOWN 按下View，是所有事件的开始

(2)MotionEvent.ACTION_MOVE 滑动事件

(3)MotionEvent.ACTION_UP 与down对应，表示抬起

另外，明确事件传递机制的最终目的都是为了触发执行View的点击监听和触摸监听:

******.setOnClickListener(new View.OnClickListener() {

@Override
public void onClick(View v) {
// TODO Auto-generated method stub
Log.i(tag, "testLinelayout---onClick...");
}
});

*******.setOnTouchListener(new View.OnTouchListener() {

@Override
public boolean onTouch(View v, MotionEvent event) {
// TODO Auto-generated method stub

return false;
}
});

我们简称为onClick监听和onTouch监听，一般程序会注册这两个监听。从上面可以看到，onTouch监听里默认return false。不要小看了这个return false，后面可以看到它有大用。

对于View来说，事件传递机制有两个函数：dispatchTouchEvent负责分发事件，在dispatch***里又会调用onTouchEvent表示执行事件，或者说消费事件，结合源码分析其流程。事件传递的入口是View的dispatchTouchEvent()函数:

/**
     * Pass the touch screen motion event down to the target view, or this
     * view if it is the target.
     *
     * @param event The motion event to be dispatched.
     * @return True if the event was handled by the view, false otherwise.
     */
    public boolean dispatchTouchEvent(MotionEvent event) {
        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onTouchEvent(event, 0);
        }

        if (onFilterTouchEventForSecurity(event)) {
            //noinspection SimplifiableIfStatement
            ListenerInfo li = mListenerInfo;
            if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
                    && li.mOnTouchListener.onTouch(this, event)) {
                return true;
            }

            if (onTouchEvent(event)) {
                return true;
            }
        }

        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
        }
        return false;
    }

找到这个判断:

if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
return true;
}

他会执行View的OnTouchListener.onTouch这个函数，也就是上面说的onTouch监听。里面有三个判断，如果三个都为1，就会执行return true，不往下走了。而默认的onTouch监听返回false，只要一个是false，就不会返回true。接着往下看，程序执行onTouchEvent：

            if (onTouchEvent(event)) {
                return true;
            }

onTouchEvent的源码比较多，贴最重要的:

                    if (!mHasPerformedLongPress) {
                            // This is a tap, so remove the longpress check
                            removeLongPressCallback();

                            // Only perform take click actions if we were in the pressed state
                            if (!focusTaken) {
                                // Use a Runnable and post this rather than calling
                                // performClick directly. This lets other visual state
                                // of the view update before click actions start.
                                if (mPerformClick == null) {
                                    mPerformClick = new PerformClick();
                                }
                                if (!post(mPerformClick)) {
                                   <span style="color:#ff0000;"> performClick();</span>
                                }
                            }
                        }

可以看到有个performClick(),它的源码里有这么一句 li.mOnClickListener.onClick(this);

    public boolean performClick() {
        sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);

        ListenerInfo li = mListenerInfo;
        if (li != null && li.mOnClickListener != null) {
            playSoundEffect(SoundEffectConstants.CLICK);
            <span style="color:#ff0000;"><strong>li.mOnClickListener.onClick(this);</strong></span>
            return true;
        }

        return false;
    }

终于对上了，它执行了我们注册的onClick监听。当然执行前会经过一系列判断，是否注册了监听等。
总结:

1、事件入口是dispatchTouchEvent(),它会先执行注册的onTouch监听,如果一切顺利的话，接着执行onTouchEvent，在onTouchEvent里会执行onClick监听。

2、无论是dispatchTouchEvent还是onTouchEvent，如果返回true表示这个事件已经被消费、处理了，不再往下传了。在dispathTouchEvent的源码里可以看到，如果onTouchEvent返回了true，那么它也返回true。如果dispatch***在执行onTouch监听的时候，onTouch返回了true，那么它也返回true，这个事件提前被onTouch消费掉了。就不再执行onTouchEvent了，更别说onClick监听了。

3、我们通常在onTouch监听了设置图片一旦被触摸就改变它的背景、透明度之类的，这个onTouch表示事件的时机。而在onClick监听了去具体干某些事。

二、ViewGroup的dispatchTouchEvent、onInterceptTouchEvent、onTouchEvent

再来看ViewGroup，在复写ViewGroup时可以发现它的onTouchEvent在在View里的，表示这两个方法是一样的。但dispatchTouchEvent是在ViewGroup里的，表示和View的dispatchTouchEvent不一样，多了一个onInterceptTouchEvent函数，表示拦截的意思。链接打个很形象的比喻，这玩意就像个秘书、谋士。为啥View没有呢，因为它级别不够，一个Button里面是不可能有子View的。但LinearLayout(继承ViewGroup)就有孩子(子布局)，这个onInterceptTouchEvent就会判断事件要不要通知它的孩子呢。它的源码如下：

public boolean dispatchTouchEvent(MotionEvent ev) {
        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
        }

        boolean handled = false;
        if (onFilterTouchEventForSecurity(ev)) {
            final int action = ev.getAction();
            final int actionMasked = action & MotionEvent.ACTION_MASK;

            // Handle an initial down.
            if (actionMasked == MotionEvent.ACTION_DOWN) {
                // Throw away all previous state when starting a new touch gesture.
                // The framework may have dropped the up or cancel event for the previous gesture
                // due to an app switch, ANR, or some other state change.
                cancelAndClearTouchTargets(ev);
                resetTouchState();
            }

            // Check for interception.
            final boolean intercepted;
            if (actionMasked == MotionEvent.ACTION_DOWN
                    || mFirstTouchTarget != null) {
                final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
                if (!disallowIntercept) {
                    <strong><span style="color:#ff0000;">intercepted = onInterceptTouchEvent(ev);</span></strong>
                    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;
            }

            // Check for cancelation.
            final boolean canceled = resetCancelNextUpFlag(this)
                    || actionMasked == MotionEvent.ACTION_CANCEL;

            // Update list of touch targets for pointer down, if needed.
            final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
            TouchTarget newTouchTarget = null;
            boolean alreadyDispatchedToNewTouchTarget = false;
            <strong><span style="color:#ff0000;">if (!canceled && !intercepted)</span></strong> {
                if (actionMasked == MotionEvent.ACTION_DOWN
                        || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
                        || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
                    final int actionIndex = ev.getActionIndex(); // always 0 for down
                    final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)
                            : TouchTarget.ALL_POINTER_IDS;

                    // Clean up earlier touch targets for this pointer id in case they
                    // have become out of sync.
                    removePointersFromTouchTargets(idBitsToAssign);

                    final int childrenCount = mChildrenCount;
                    if (childrenCount != 0) {
                        // Find a child that can receive the event.
                        // Scan children from front to back.
                        final View[] children = mChildren;
                        final float x = ev.getX(actionIndex);
                        final float y = ev.getY(actionIndex);

                        final boolean customOrder = isChildrenDrawingOrderEnabled();
                        for (int i = childrenCount - 1; i >= 0; i--) {
                            final int childIndex = customOrder ?
                                    getChildDrawingOrder(childrenCount, i) : i;
                            final View child = children[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();
                                mLastTouchDownIndex = childIndex;
                                mLastTouchDownX = ev.getX();
                                mLastTouchDownY = ev.getY();
                                newTouchTarget = addTouchTarget(child, idBitsToAssign);
                                alreadyDispatchedToNewTouchTarget = true;
                                break;
                            }
                        }
                    }

                    if (newTouchTarget == null && mFirstTouchTarget != null) {
                        // Did not find a child to receive the event.
                        // Assign the pointer to the least recently added target.
                        newTouchTarget = mFirstTouchTarget;
                        while (newTouchTarget.next != null) {
                            newTouchTarget = newTouchTarget.next;
                        }
                        newTouchTarget.pointerIdBits |= idBitsToAssign;
                    }
                }
            }

            // Dispatch to touch targets.
            if (mFirstTouchTarget == null) {
                // No touch targets so treat this as an ordinary view.
                handled = dispatchTransformedTouchEvent(ev, canceled, null,
                        TouchTarget.ALL_POINTER_IDS);
            } else {
                // Dispatch to touch targets, excluding the new touch target if we already
                // dispatched to it.  Cancel touch targets if necessary.
                TouchTarget predecessor = null;
                TouchTarget target = mFirstTouchTarget;
                while (target != null) {
                    final TouchTarget next = target.next;
                    if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
                        handled = true;
                    } else {
                        final boolean cancelChild = resetCancelNextUpFlag(target.child)
                        || intercepted;
                        if (dispatchTransformedTouchEvent(ev, cancelChild,
                                target.child, target.pointerIdBits)) {
                            handled = true;
                        }
                        if (cancelChild) {
                            if (predecessor == null) {
                                mFirstTouchTarget = next;
                            } else {
                                predecessor.next = next;
                            }
                            target.recycle();
                            target = next;
                            continue;
                        }
                    }
                    predecessor = target;
                    target = next;
                }
            }

            // Update list of touch targets for pointer up or cancel, if needed.
            if (canceled
                    || actionMasked == MotionEvent.ACTION_UP
                    || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
                resetTouchState();
            } else if (split && actionMasked == MotionEvent.ACTION_POINTER_UP) {
                final int actionIndex = ev.getActionIndex();
                final int idBitsToRemove = 1 << ev.getPointerId(actionIndex);
                removePointersFromTouchTargets(idBitsToRemove);
            }
        }

        if (!handled && mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
        }
        return handled;
    }

可以看到标红的有两句(intercepted = onInterceptTouchEvent(ev); if (!canceled && !intercepted) )，它会先调用 intercepted = onInterceptTouchEvent(ev);然后通过if判断。

  public boolean onInterceptTouchEvent(MotionEvent ev) {
        return false;
    }

它就一句话，默认false。也就是说这个谋士默认的意见是，永远不拦截！！！！只要有孩子，就交给孩子们处理吧。

最后的结论:

1、如果是自定义复合控件，如图片+文字，我再Activity里给你注册了onClick监听，期望点击它执行。那么最简单的方法就是将图片+文字的父布局，也即让其容器ViewGroup的秘书将事件拦下，这样父亲就可以执行onClick了。这时候的父亲就像一个独立的孩子一样了(View)，无官一身轻，再也不用管它的孩子了,可以正常onClick onTouch.

2、如果希望一个View只onTouch而不onClick，在onTouch里return true就ok了。

3、dispatch是为了onTouch监听，onTouchEvent是为了onClick监听。

4、自定义布局时，一般情况下：

@Override
public boolean onTouchEvent(MotionEvent event) {return super.onTouchEvent(event);}

@Override
public boolean dispatchTouchEvent(MotionEvent event) {return super.dispatchTouchEvent(event);

我们可以复写，但是最后的super.***是万万不能少滴。如果少了，表示连dispatch*** onTouchEvent压根就不调用了，事件就此打住。

转载：http://blog.csdn.net/yanzi1225627/article/details/22592831

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Android事件传递机制(dispatchTouchEvent、onInterceptTouchEvent、onTouchEvent)