事件分发机制

事件分发

分发对象

事件

事件种类

• MotionEvent.ACTION_DOWN：按下View（所有事件的开始）
• MotionEvent.ACTION_MOVE：滑动View
• MotionEvent.ACTION_CANCEL：非人为原因结束本次事件
• MotionEvent.ACTION_UP：抬起View（与DOWN对应）

事件流程

事件分发本质

当一个点击事件发生后，系统要讲该事件传递至具体的view进行处理的过程，这个传递的过程，就是事件分发的本质

传递方向

从activity到phonewindow到decorview再到开发者的viewgroup，view

响应方向

以传递相反的方向进行响应

如何传递

方法作用

一个事件列中，只要viewgroup中的onInterceptTouchEvent返回值有一次为true，后续事件列将不会在走onIntercepetTouchEvent,直接进入viewgroup中的onTouchEvent中。在move中的intercepet，该move事件会被当做cancel，传递view中。

Activity分发机制

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

• 一般事件列开始都是DOWN（按下按钮），所以这里返回true，执行onUserInteraction()
• onUserInteraction()源码为空，当此activity在栈顶时，触屏点击按home，back，menu键等都会触发此方法。onUserInteraction()主要用于屏保
• Window类是抽象类，且PhoneWindow是Window类的唯一实现类，即执行phoneWindow中的superDispatchTouchEvent。

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

mDecor为DecorView的事例，即执行DecorView中的superDispatchTouchEvent。

public boolean superDispatchTouchEvent(MotionEvent event) {
        return super.dispatchTouchEvent(event);
    }

DecorView继承FrameLayout,即执行ViewGroup中的dispatchTouchEvent。这样事件就进入了viewgroup中。

ViewGroup分发机制

@Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        if (!onFilterTouchEventForSecurity(ev)) {
            return false;
        }

        final int action = ev.getAction();
        final float xf = ev.getX();
        final float yf = ev.getY();
        final float scrolledXFloat = xf + mScrollX;
        final float scrolledYFloat = yf + mScrollY;
        final Rect frame = mTempRect;

        boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;

        if (action == MotionEvent.ACTION_DOWN) {
            if (mMotionTarget != null) {
                // this is weird, we got a pen down, but we thought it was
                // already down!
                // XXX: We should probably send an ACTION_UP to the current
                // target.
                mMotionTarget = null;
            }
            // If we're disallowing intercept or if we're allowing and we didn't
            // intercept
            if (disallowIntercept || !onInterceptTouchEvent(ev)) {
                // reset this event's action (just to protect ourselves)
                ev.setAction(MotionEvent.ACTION_DOWN);
                // We know we want to dispatch the event down, find a child
                // who can handle it, start with the front-most child.
                final int scrolledXInt = (int) scrolledXFloat;
                final int scrolledYInt = (int) scrolledYFloat;
                final View[] children = mChildren;
                final int count = mChildrenCount;

                for (int i = count - 1; i >= 0; i--) {
                    final View child = children[i];//*
                    if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE
                            || child.getAnimation() != null) {
                        child.getHitRect(frame);
                        if (frame.contains(scrolledXInt, scrolledYInt)) {
                            // offset the event to the view's coordinate system
                            final float xc = scrolledXFloat - child.mLeft;
                            final float yc = scrolledYFloat - child.mTop;
                            ev.setLocation(xc, yc);
                            child.mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;
                            if (child.dispatchTouchEvent(ev))  {
                                // Event handled, we have a target now.
                                mMotionTarget = child;
                                return true;
                            }
                            // The event didn't get handled, try the next view.
                            // Don't reset the event's location, it's not
                            // necessary here.
                        }
                    }
                }
            }
        }

        boolean isUpOrCancel = (action == MotionEvent.ACTION_UP) ||
                (action == MotionEvent.ACTION_CANCEL);

        if (isUpOrCancel) {
            // Note, we've already copied the previous state to our local
            // variable, so this takes effect on the next event
            mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
        }

        // The event wasn't an ACTION_DOWN, dispatch it to our target if
        // we have one.
        final View target = mMotionTarget;//*
        if (target == null) {
            // We don't have a target, this means we're handling the
            // event as a regular view.
            ev.setLocation(xf, yf);
            if ((mPrivateFlags & CANCEL_NEXT_UP_EVENT) != 0) {
                ev.setAction(MotionEvent.ACTION_CANCEL);
                mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;
            }
            return super.dispatchTouchEvent(ev);
        }

        // if have a target, see if we're allowed to and want to intercept its
        // events
        if (!disallowIntercept && onInterceptTouchEvent(ev)) {
            final float xc = scrolledXFloat - (float) target.mLeft;
            final float yc = scrolledYFloat - (float) target.mTop;
            mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;
            ev.setAction(MotionEvent.ACTION_CANCEL);
            ev.setLocation(xc, yc);
            if (!target.dispatchTouchEvent(ev)) {
                // target didn't handle ACTION_CANCEL. not much we can do
                // but they should have.
            }
            // clear the target
            mMotionTarget = null;
            // Don't dispatch this event to our own view, because we already
            // saw it when intercepting; we just want to give the following
            // event to the normal onTouchEvent().
            return true;
        }

        if (isUpOrCancel) {
            mMotionTarget = null;
        }

        // finally offset the event to the target's coordinate system and
        // dispatch the event.
        final float xc = scrolledXFloat - (float) target.mLeft;
        final float yc = scrolledYFloat - (float) target.mTop;
        ev.setLocation(xc, yc);

        if ((target.mPrivateFlags & CANCEL_NEXT_UP_EVENT) != 0) {
            ev.setAction(MotionEvent.ACTION_CANCEL);
            target.mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;
            mMotionTarget = null;
        }

        return target.dispatchTouchEvent(ev);
    }

View分发机制

public boolean dispatchTouchEvent(MotionEvent event) {
        if (!onFilterTouchEventForSecurity(event)) {
            return false;
        }

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

小细节

• 为啥viewgroup中的onInterceptTouchEvent在一个事件列中，只走一次？如果viewgroup中的子view不消费事件，就会这样。因为在事件列后续的move、cancel、up等中，viewgroup被当做view执行了。