Android事件分发机制

 最近在学习Android事件分发机制,也参考了网上许多文章。特别是Android事件分发机制详解:史上最全面、最易懂这篇文章,分析的特别全面、详细,所以在此文章基础上对分发机制做个总结。

1、activity事件分发机制

 在activity中,事件分发是从dispatchTouchEvent开始的,这个方法实现也很简单,代码如下:

    public boolean dispatchTouchEvent(MotionEvent ev) {
        // 监听手指按下事件
        if (ev.getAction() == MotionEvent.ACTION_DOWN) {
            onUserInteraction();
        }
        //交给PhoneWindow去处理,如果返回true则代表已经消费事件,否则activity自己处理事件。
        if (getWindow().superDispatchTouchEvent(ev)) {
            return true;
        }
        return onTouchEvent(ev);
    }

 这里有一个很有意思的方法onUserInteraction,它是一个空实现,但它可以用来实现屏保功能且当点击Home、Back、Recent 时会调用栈顶activity的onUserInteraction方法。然后调用getWindow().superDispatchTouchEvent(ev)将事件交给DecorView处理,DecorView继承自FrameLayout,所以就将事件交给了ViewGroup处理。

Android事件分发机制_第1张图片
来自于Android事件分发机制详解:史上最全面、最易懂

2、ViewGroup事件分发机制

 来看ViewGroup中dispatchTouchEvent的实现。

    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        ...
        //默认返回false
        boolean handled = false;
        if (onFilterTouchEventForSecurity(ev)) {
            final int action = ev.getAction();
            final int actionMasked = action & MotionEvent.ACTION_MASK;
            //当手指按下时
            if (actionMasked == MotionEvent.ACTION_DOWN) {	  
                //在开始新的触摸手势时丢弃所有先前的状态。 由于应用程序切换,ANR或某些其他状态更改,framework可能已经删除了先前手势的up或cancel事件。
                cancelAndClearTouchTargets(ev);
                resetTouchState();
            }
            //检查是否拦截事件
            final boolean intercepted;
            //监听手指按下、移动、抬起等事件,如果mFirstTouchTarget为null则可以认为没有子View响应事件了
            if (actionMasked == MotionEvent.ACTION_DOWN
                    || mFirstTouchTarget != null) {
                final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
                if (!disallowIntercept) {
                    //调用onInterceptTouchEvent来判断是否需要拦截事件,默认是返回false
                    intercepted = onInterceptTouchEvent(ev);
                    ev.setAction(action); // restore action in case it was changed
                } else {
                    intercepted = false;
                }
            } else {
                //当前ViewGroup消费事件
                intercepted = true;
            }
            ...
            if (!canceled && !intercepted) {
                ...
                if (actionMasked == MotionEvent.ACTION_DOWN
                        || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
                        || actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
                    ...
                    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.
                        //拿到处于重叠状态的所有View并倒序排列,因为我们是要响应最上面的View
                        final ArrayList<View> preorderedList = buildTouchDispatchChildList();
                        final boolean customOrder = preorderedList == null
                                && isChildrenDrawingOrderEnabled();
                        final View[] children = mChildren;
                        //倒序遍历ViewGroup的所有子View
                        for (int i = childrenCount - 1; i >= 0; i--) {
                            ...
                            //进行事件分发
                            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();
                                //找到对应的子View,这里之所以是一个链表,是因为有时候会多手指触发事件
                                newTouchTarget = addTouchTarget(child, idBitsToAssign);
                                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();
                    }
                    ...
                }
            }
            //mFirstTouchTarget == null可以理解为没有子View响应事件,需要当前ViewGroup自己处理事件
            if (mFirstTouchTarget == null) {
                //进行事件分发,传递的子View为null,则代表需要当前ViewGroup来处理事件
                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 {
                        //判断子View是否需要取消事件
                        final boolean cancelChild = resetCancelNextUpFlag(target.child)
                                || intercepted;
                        //进行事件分发
                        if (dispatchTransformedTouchEvent(ev, cancelChild,
                                target.child, target.pointerIdBits)) {
                            handled = true;
                        }
                        //子View取消事件
                        if (cancelChild) {
                            if (predecessor == null) {
                                mFirstTouchTarget = next;
                            } else {
                                predecessor.next = next;
                            }
                            target.recycle();
                            target = next;
                            continue;
                        }
                    }
                    predecessor = target;
                    target = next;
                }
            }
            ...
        }
        ...
        return handled;
    }

 从上面代码可以看出,按下事件是无法被拦截的,其他后续事件都有可能被拦截,当不拦截的时候,都会调用onInterceptTouchEvent方法。当拦截事件时,会给子View传一个取消的事件,且将mFirstTouchTarget设置为null,后续的事件就都不会调用onInterceptTouchEventbuildTouchDispatchChildList这个方法需要注意一下,它主要处理View叠加的问题,具体实现是遍历所有子View然后将这个列表倒序返回,这是为什么尼?如果多个View叠加在一起的话,按照正常逻辑应该响应最下面的那个子View,但实际上应该响应的是最上面的子View,所以需要倒序。最后都是调用dispatchTransformedTouchEvent来进行事件分发,先来看看这个方法的实现。

    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;
    }

 可以看出dispatchTransformedTouchEvent的实现逻辑很简单,就是将事件传递给当前ViewGroup来处理或者让子View继续进行分发,如果要处理事件就返回true,否则返回false。细心一点可以发现mFirstTouchTarget是一个单链表,那这里为什么要使用单链表尼?先来看看TouchTarget这个类,在这个类的开始部分有一段注释,大意就是最多支持32点在屏幕上触发事件,这也就是mFirstTouchTarget是一个单链表的原因了(如果是单点触摸则该链表只有一个元素)。

    /* Describes a touched view and the ids of the pointers that it has captured.
     *
     * This code assumes that pointer ids are always in the range 0..31 such that
     * it can use a bitfield to track which pointer ids are present.
     * As it happens, the lower layers of the input dispatch pipeline also use the
     * same trick so the assumption should be safe here...
     */
    private static final class TouchTarget {
        private static final int MAX_RECYCLED = 32;
        private static final Object sRecycleLock = new Object[0];
        private static TouchTarget sRecycleBin;
        private static int sRecycledCount;

        public static final int ALL_POINTER_IDS = -1; // all ones

        // The touched child view.
        public View child;

        // The combined bit mask of pointer ids for all pointers captured by the target.
        public int pointerIdBits;

        // The next target in the target list.
        public TouchTarget next;

        private TouchTarget() {
        }

        public static TouchTarget obtain(@NonNull View child, int pointerIdBits) {
            if (child == null) {
                throw new IllegalArgumentException("child must be non-null");
            }

            final TouchTarget target;
            synchronized (sRecycleLock) {
                if (sRecycleBin == null) {
                    target = new TouchTarget();
                } else {
                    target = sRecycleBin;
                    sRecycleBin = target.next;
                     sRecycledCount--;
                    target.next = null;
                }
            }
            target.child = child;
            target.pointerIdBits = pointerIdBits;
            return target;
        }

        public void recycle() {
            if (child == null) {
                throw new IllegalStateException("already recycled once");
            }

            synchronized (sRecycleLock) {
                if (sRecycledCount < MAX_RECYCLED) {
                    next = sRecycleBin;
                    sRecycleBin = this;
                    sRecycledCount += 1;
                } else {
                    next = null;
                }
                child = null;
            }
        }
    }

 到此ViewGroup的事件分发就分析完毕了,一般在使用过程中都会ViewGroup里放ViewGroup这样嵌套,但流程都是不变的。

Android事件分发机制_第2张图片
来自于Android事件分发机制详解:史上最全面、最易懂

3、View事件事件分发机制

 View的事件分发主要是dispatchTouchEvent里实现。先来看一下该方法的具体实现

    public boolean dispatchTouchEvent(MotionEvent event) {
        ...
        boolean result = false;
        ...
        if (onFilterTouchEventForSecurity(event)) {
            if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
                result = true;
            }
            //noinspection SimplifiableIfStatement
            ListenerInfo li = mListenerInfo;
            //从这里判断可以看出onTouch在onTouchEvent之前调用且如果onTouch返回了true,onTouchEvent就不会执行
            if (li != null && li.mOnTouchListener != null
                    && (mViewFlags & ENABLED_MASK) == ENABLED
                    && li.mOnTouchListener.onTouch(this, event)) {
                result = true;
            }
            if (!result && onTouchEvent(event)) {
                result = true;
            }
        }
        ...
        return result;
    }

 View的事件分发实现还是比较简单的,没有ViewGroup那么复杂,从上面可以看出onTouch方法在onTouchEvent之前调用且如果onTouch返回true则onTouchEvent就不会执行。

public boolean onTouchEvent(MotionEvent event) {
        ...
        if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {
            switch (action) {
                case MotionEvent.ACTION_UP:
                    ...
                    if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {
                        ...
                        if (!mHasPerformedLongPress && !mIgnoreNextUpEvent) {
                            // 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();
                                }
                                //点击事件
                                if (!post(mPerformClick)) {
                                    performClick();
                                }
                            }
                        }
                        ...
                        removeTapCallback();
                    }
                    mIgnoreNextUpEvent = false;
                    break;
                 ...
                //当ViewGroup拦截事件时,会传递给子View一个ACTION_CANCEL事件
                case MotionEvent.ACTION_CANCEL:
                    ...
                    break;
            }

            return true;
        }

        return false;
    }

 从上面可以看出点击事件及长按事件优先级别是最低的,如果子类重写了onTouchEvent则该View的点击、长按等事件就无效了,需要在子类里调用主动调用super.onTouchEvent或者performClick

Android事件分发机制_第3张图片
来自于Android事件分发机制详解:史上最全面、最易懂

 到此事件分发机制就总结完毕了,最后来一张总的流程图。

Android事件分发机制_第4张图片

参考

Android事件分发机制详解:史上最全面、最易懂

你可能感兴趣的:(Android,源码)