Android事件分发机制源码分析

前面写过一篇文章,说了下事件分发机制的方法和大致流程,本文尝试从源码的角度一层一层的看下分发机制。

源码的查看:https://www.androidos.net.cn/sourcecode(可能是我下的源码有问题,部分方法我是在线查看的)

Activity的事件分发机制

我们从activity的dispatchTouchEvent方法进行源码分析:

    public boolean dispatchTouchEvent(MotionEvent ev) {
        if (ev.getAction() == MotionEvent.ACTION_DOWN) {
            onUserInteraction();
        }
        if (getWindow().superDispatchTouchEvent(ev)) {
            return true;
        }
        return onTouchEvent(ev);
    }

点进去看到,onUserInteraction是个空方法,这里是用来实现屏保功能的,当activity位于栈顶时,触屏点击home、menu、back会触发。

    public void onUserInteraction() {
    }

然后我们来看第二个判断

 if (getWindow().superDispatchTouchEvent(ev)) {
            return true;
        }

继续往下看,

    public abstract boolean superDispatchTouchEvent(MotionEvent event);

这玩意是个抽象方法,对应的Window也是个抽象类,我们能找到PhoneWindow,

    @Override
    public boolean superDispatchTouchEvent(MotionEvent event) {
        return mDecor.superDispatchTouchEvent(event);
    }

然后我们在看下这个mDecor是啥,

找到这个类

public class DecorView extends FrameLayout implements RootViewSurfaceTaker, WindowCallbacks

他是继承FrameLayout的,也就是mDecor.SuperDispatchTouchEvent即等同于Viegroup的分发机制。(即事件从activity传递到了viewgroup)

再看最后一个onTouch方法,

    public boolean onTouchEvent(MotionEvent event) {
        if (mWindow.shouldCloseOnTouch(this, event)) {
            finish();
            return true;
        }

        return false;
    }

这里默认返回false,就是不处理,向下层分发,那我们来看下这个判断里面。

    public boolean shouldCloseOnTouch(Context context, MotionEvent event) {
        final boolean isOutside =
                event.getAction() == MotionEvent.ACTION_DOWN && isOutOfBounds(context, event)
                || event.getAction() == MotionEvent.ACTION_OUTSIDE;
        if (mCloseOnTouchOutside && peekDecorView() != null && isOutside) {
            return true;
        }
        return false;
    }

这里主要判断是否在边界外,在即消费事件,返回true,分发结束,反之返回false,activity层的分发也结束,扔给viewgroup继续分发,直到被消费。

viewgroup的分发机制

我们从ViewGroup的dispatchTouchEvent方法进行源码分析:

if (!canceled && !intercepted) {

                // If the event is targeting accessibility 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;

                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 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);

代码很长,挑一点看看(不完整,感兴趣的自己查看),就是判断事件没有取消也没有被拦截,然后给viewgroup内的子view进行遍历,继续看重点

 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;
                            }

我们给这个dispatchTransformedTouchEvent点进去,

  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;
        }

这里面进行了判断,事件传递给了view(即child)进行分发,或者给他的上一层viewgroup进行分发,直到事件分发结束,即被消费。

回过头来,我们看拦截的判断

 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;
            }

这里面的事件被拦截即 mFirsTouchTarget!=null不成立,即不拦截,mFirsTouchTarget!=null,disallowIntercept表示是否允许被拦截,是可以用代码来控制的,经过判断,允许被拦截再调用拦截的方法,

    public boolean onInterceptTouchEvent(MotionEvent ev) {
        if (ev.isFromSource(InputDevice.SOURCE_MOUSE)
                && ev.getAction() == MotionEvent.ACTION_DOWN
                && ev.isButtonPressed(MotionEvent.BUTTON_PRIMARY)
                && isOnScrollbarThumb(ev.getX(), ev.getY())) {
            return true;
        }
        return false;
    }

这里面进行多个判断,默认返回false,即不拦截。拦截的要求大致上,这个事件是来自使用者的输入、是down事件、这个是按钮、点击的事件只在viewgroup内。一旦拦截了,它就会向它的父类分发,也就是view的分发,但执行ViewGroup的onTouch() ->> onTouchEvent() ->> performClick() ->> onClick(),即自己处理该事件,事件不会往下传递(具体请参考View事件的分发机制中的View.dispatchTouchEvent())。(可以理解为虽然是个viewgroup,但里面并没有子view,所以事件分发相当于view的事件分发),这个地方要注意。

View的事件分发机制

我们从View的dispatchTouchEvent方法进行源码分析:(源码可能下的有点问题,我们在线看下)

 boolean result = false;

        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onTouchEvent(event, 0);
        }

        final int actionMasked = event.getActionMasked();
        if (actionMasked == MotionEvent.ACTION_DOWN) {
            // Defensive cleanup for new gesture
            stopNestedScroll();
        }

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

            if (!result && onTouchEvent(event)) {
                result = true;
            }
        }

它返回的是result,即决定是否继续分发,返回true,即事件被消费,false即要调用onTouch方法。返回true有三种情况

1. view是可点击的且handleScrollBarDragging

   看下源码(太长,看下什么时候返回true)

 * @return true if the event was handled as a scroll bar dragging, false otherwise.
     */
    protected boolean handleScrollBarDragging(MotionEvent event) {

翻译:如果作为一个滚动条拖动事件处理返回true

2. view可点击且onTouch事件不为空

3. onTouch事件返回true

下面看下onTouchEvent方法

 if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {
            switch (action) {
                case MotionEvent.ACTION_UP:
                    mPrivateFlags3 &= ~PFLAG3_FINGER_DOWN;
                    if ((viewFlags & TOOLTIP) == TOOLTIP) {
                        handleTooltipUp();
                    }
                    if (!clickable) {
                        removeTapCallback();
                        removeLongPressCallback();
                        mInContextButtonPress = false;
                        mHasPerformedLongPress = false;
                        mIgnoreNextUpEvent = false;
                        break;
                    }
                    boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0;
                    if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) {
                        // take focus if we don't have it already and we should in
                        // touch mode.
                        boolean focusTaken = false;
                        if (isFocusable() && isFocusableInTouchMode() && !isFocused()) {
                            focusTaken = requestFocus();
                        }

即view可点击进入switch判断具体DOWN、UP等事件,重点看下这个

Android事件分发机制源码分析_第1张图片

    public boolean performClick() {
        final boolean result;
        final ListenerInfo li = mListenerInfo;
        if (li != null && li.mOnClickListener != null) {
            playSoundEffect(SoundEffectConstants.CLICK);
            li.mOnClickListener.onClick(this);
            result = true;
        } else {
            result = false;
        }

        sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);

        notifyEnterOrExitForAutoFillIfNeeded(true);

        return result;
    }

根据代码我们知道只要我们通过setOnClickListener()为控件View注册1个点击事件,那么就会给mOnClickListener变量赋值(即不为空),则会往下回调onClick() & performClick()返回true。即调用onTouch事件要调用performClick事件,当这些执行完才能执行我们常见的onClick事件,至此,事件分发结束。

核心结论

事件逐层分发,判断是否拦截,拦截就本层消费,否则向下分发,直至被消费。

写在最后

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Android事件分发机制源码分析_第2张图片

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