android SDK-25事件分发机制--源码正确解析

android SDK-25事件分发机制–源码正确解析

Android 事件分发分为View和ViewGroup的事件分发，ViewGroup比View过一个拦截判断，viewgroup可以拦截事件，从而决定要不要把事件传递给子view，因为view没有子view所以不存在拦截事件的情况。

事件分发主要从事件的分发，拦截，和处理三个函数的调用逻辑关系来分析。

public boolean dispatchTouchEvent(MotionEvent event) {}public boolean onInterceptTouchEvent(MotionEvent ev) {}public boolean onTouchEvent(MotionEvent event) {}

首先，屏幕上面一个点击事件，通过传感器捕获到点击，然后知道把点击事件传递到activity 到PhoneWindow，再到，DecorView 最后就到我们自己在布局文件中的view或者viewgroup。

下面分析dispatchTouchEvent源码 ：（sdk-25）（==>这个标记为重点，不用全懂，只要把==>这个标记处的逻辑理清楚就行）

 @Overridepublic boolean dispatchTouchEvent(MotionEvent ev) {    //检验是不是利用辅助功能点击事件，根本不用管     if (mInputEventConsistencyVerifier != null) {        mInputEventConsistencyVerifier.onTouchEvent(ev, 1);    }    // If the event targets the accessibility focused view and this is it, start    // normal event dispatch. Maybe a descendant is what will handle the click.    if (ev.isTargetAccessibilityFocus() && isAccessibilityFocusedViewOrHost()) {        ev.setTargetAccessibilityFocus(false);    }    //辅助功能点击事件的判断和处理根本不用管，直接不要管，跳转到下一个重点    // ==> 一个标志，初始值为不处理，意识就是不处理MotionEvent    boolean handled = false;    if (onFilterTouchEventForSecurity(ev)) {        final int action = ev.getAction();        final int actionMasked = action & MotionEvent.ACTION_MASK;        // Handle an initial down.        // ==> 处理第一个点击事件（事件分为，点击事件，move事件，up事件，所以点击事件是第一个事件）        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.            // ==> 如果是点击事件，说明是一个新的事件序列开始了（按下，移动，手指抬起离开屏幕）            // 由于是新的事件序列开始，所以要重置一些参数，和清空一下数据等等操作。            cancelAndClearTouchTargets(ev);            resetTouchState();        }        // Check for interception.        final boolean intercepted;        // ==> 判断要不要打断，如果不是点击事件，并且mFirstTouchTarget为null，则该viewgroup则打断拦截事件。        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;        }        // If intercepted, start normal event dispatch. Also if there is already        // a view that is handling the gesture, do normal event dispatch.        if (intercepted || mFirstTouchTarget != null) {            ev.setTargetAccessibilityFocus(false);        }        // 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;         // ==>如果没有返回并且没有被拦截，则进行分发        if (!canceled && !intercepted) {            // If the event is targeting accessiiblity focus we give it to the            // view that has accessibility focus and if it does not handle it            // we clear the flag and dispatch the event to all children as usual.            // We are looking up the accessibility focused host to avoid keeping            // state since these events are very rare.            View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus()                    ? findChildWithAccessibilityFocus() : null;            // ==>如果是down事件，进行遍历子view，并且把事件分发给子view，前提是down事件的坐            标必须在子view中等等条件            ==>这句话，请把下面的源码分析完再一定回来看一看，你要思考一下，这里如果是down事件才            会进去，如果是move事件那么不会进去分发了，其实往后看源码会发现，在down事件分发给            一个child，如果这个child消费了这个down事件，那么这个child就会被保存起来，以后的            move（可能0到多次move），up都会直接分发给这个child，就不用每次再去遍历所有的Child            这些效率就提高和很多，但是你也会想到，如果一个down事件被某个child消费只有，其它            child就无法被分发事件了，除非我们手动调用child的分发方法，或者打断事件序列，从发分发一个down事件，这段话请看完后面的源码再来推敲一下，就完全理解事件分发机制了。            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 (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 = buildTouchDispatchChildList();                    final boolean customOrder = preorderedList == null                            && isChildrenDrawingOrderEnabled();                    final View[] children = mChildren;                    for (int i = childrenCount - 1; i >= 0; i--) {                        final int childIndex = getAndVerifyPreorderedIndex(                                childrenCount, i, customOrder);                        final View child = getAndVerifyPreorderedView(                                preorderedList, children, childIndex);                        // If there is a view that has accessibility focus we want it                        // to get the event first and if not handled we will perform a                        // normal dispatch. We may do a double iteration but this is                        // safer given the timeframe.                        if (childWithAccessibilityFocus != null) {                            if (childWithAccessibilityFocus != child) {                                continue;                            }                            childWithAccessibilityFocus = null;                            i = childrenCount - 1;                        }                    // ==>判断这个子view能否接受点击事件和子view是否包含这个点击的坐标，具体的判断点击这两个方法进去看                        if (!canViewReceivePointerEvents(child)                                || !isTransformedTouchPointInView(x, y, child, null)) {                            ev.setTargetAccessibilityFocus(false);                            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);                    // ==>  关键之处，这里将事件传递给child进行处理了，马上进入这个方法看看吧                        if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) 

// ==>dispatchTransformedTouchEvent方法

 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.    //根本都不是什么返回事件，所以跳过，直接到重点    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) {    // ==>如果child为空，直接调用该viewgroup自己父类的dispatchTouchEvent，也就是viewdispatchTouchEvent方法，点击进去会发现它会调用onTouchEvent，也就是说如果viewGroup如果没有child那么他就会调用自己的onTouchEvent方法来消费这个事件，这个handle就表明了这个事件viewgroup自己是否消费        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());        }        // ==>我们的child不为空的时候，就调用child事件分发方法，于是到这里可以看到事件的传递了，先来分析简单的情况，如果child是view不是viewgroup，那么dispatchTouchEvent流程就简单了，会调用onTouchEvent来告诉父view，child它是否消费父亲分发给他的事件，这个handle就表明了这个事件clild是否消费        handled = child.dispatchTouchEvent(transformedEvent);    }    // Done.    transformedEvent.recycle();    return handled;}

//这个重要的方法分析完之后，马上返回刚才源码的地方，继续。。。

//如果刚才，我们的child不为空并且child消费down事件，那么就很棒了，说明有child处理事件了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();            // ==>重点，从上面的if条件可以看出，有child消费down事件才会执行到这里，这个方法点        进去发现mFirstTouchTarget = target;             这句代码，很重要哦。很明显就是把消费down事件的child赋值给mFirstTouchTarget，从而保存起来                            newTouchTarget = addTouchTarget(child, idBitsToAssign);            // ==> alreadyDispatchedToNewTouchTarget = true; 已经把事件分发给新的触摸目标                下面在分发的时候会用到                            alreadyDispatchedToNewTouchTarget = true;                            break;                        }                        // The accessibility focus didn't handle the event, so clear                        // the flag and do a normal dispatch to all children.                        ev.setTargetAccessibilityFocus(false);                    }                    if (preorderedList != null) preorderedList.clear();                }            //这里，如果是down事件，并且mFirstTouchTarget != null则加入链表newTouchTarget，这里不要管                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;                }            }        }        // ==> 由于刚才mFirstTouchTarget被赋值为消息了down事件的child，所以不为空了        // Dispatch to touch targets.        if (mFirstTouchTarget == null) {        // 这里非常重要，如果child为空，表明viewgroup在分发事件给child的dispatchTouchEvent被返回了false，说明子view都不消费down事件，那么这里会调用dispatchTransformedTouchEvent这个重要的方法，点进去就会发现，当child为null的时候，会调用viewgroup自己的父类View的dispatchTouchEvent，从而调用onTouchEvent，就是说儿子不消费，给老子再看看要不要消费。            // 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;        // ==>看到这里，你是不是发现怎么又调用了这个重要的，吧事件分发给child的方法，刚才不是已            经分发了，这不是第二次又来分发吗？，当然不会，你看前面的，第一次分发事件的时候已经讲            alreadyDispatchedToNewTouchTarget=true；了                    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;}

总结：事件分发，viewgroup自己先判断要不要拦截事件，和有没有调用过requestDisallowInterceptTouchEvent方法来不拦截事件，如果不拦截，当down事件的时候，遍历child，看child是否消费，child如果消息，则被保存下来，后面的事件就不遍历直接分发给他，如果child不消费，那么viewgruop继续执行，调用自己onTouchEvent方法来判断自己是不是要消费事件。