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Android 的事件分发机制(二)---ViewGroup的事件传递

2015-12-03 22:11 375 查看
先前分析了View的事件分发机制,但是我们现在的APP应用界面上的东西可不少,都是很多布局相互嵌套的,可不是只有几个孤零零的View而已,比如各种SlidingMenu侧滑控件以及ListView上下滑动控件,所以分析ViewGroup中的事件分发机制也是必不可少的,废话不多说,我们立刻开始。

代码测试

结合之前的代码,我加了一个自定义LinearLayout,重写它的dispatchTouchEvent方法、onIntercetTouchEvent方法和onTouchEvent方法,这三个方法是有关于事件分发的。

public class MyLayout extends LinearLayout{
    public MyLayout(Context context, AttributeSet attrs) {
        super(context, attrs);
    }
    @Override
    public boolean dispatchTouchEvent(MotionEvent ev) {
        Log.v("EventTest", "dispatchTouchEvent(MyLayout) ");
        return super.dispatchTouchEvent(ev);
    }
    @Override
    public boolean onTouchEvent(MotionEvent event) {
        Log.v("EventTest","onTouchEvent (MyLayout)");
        return super.onTouchEvent(event);
    }
    @Override
    public boolean onInterceptTouchEvent(MotionEvent ev) {
        Log.v("EventTest","onInterceptTouchEvent (MyLayout)");
        return super.onInterceptTouchEvent(ev);
    }
}
同时在布局文件中,将上次的自定义Button放到我们这次的自定义布局中,同时在activity中增加layout的处理代码以及重写了activity的dispatchTouchEvent和onTouchEvent方法。

layout.setOnTouchListener(new View.OnTouchListener() {
    @Override
    public boolean onTouch(View v, MotionEvent event) {
        Log.v("EventTest","onTouch (MyLayout)");
        return true;
    }
});
button.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        Log.v("EventTest", "onClick");
    }
});
button.setOnTouchListener(new View.OnTouchListener() {
    @Override
    public boolean onTouch(View v, MotionEvent event) {
        Log.v("EventTest","onTouch(MyButton)"+event.getAction());
        return false;
    }
});
……
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
    Log.v("EventTest","dispatchTouchEvent (activity)");
    return super.dispatchTouchEvent(ev);
}
@Override
public boolean onTouchEvent(MotionEvent event) {
    Log.v("EventTest","onTouchEvent (activity)");
    return super.onTouchEvent(event);
}


为了方便,我们再次把自定义Button的代码也贴出来

public class MyButton extends Button {
    public MyButton(Context context, AttributeSet attrs) {
        super(context, attrs);
    }
    public MyButton(Context context) {
        super(context);
    }
    public MyButton(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
    }
    @Override
    public boolean dispatchTouchEvent(MotionEvent event) {
        Log.v("EventTest", "dispatchTouchEvent(MyButton) ");
        return super.dispatchTouchEvent(event);
    }
    @Override
    public boolean onTouchEvent(MotionEvent event) {
        Log.v("EventTest", "onTouchEvent(MyButton) ");
        return super.onTouchEvent(event);
    }
}

运行程序,点击一下按钮,结果如下:



通过日志我们可以看到,事件传递从activity开始,进而执行了layout的dispatchTouchEvent方法,可以看到,事件分发是从activity先到viewgroup的,然后在layout中,我们执行了onInterceptTouchEvent方法,再然后却直接执行了MyButton的dispatchTouchEvent方法,而我们layout的ontouch方法和onTouchEvent方法却没有执行,为什么呢,从过程上看,问题应该在onInterceptTouchEvent上,按字面意思看,这个方法是拦截的意思,而从结果看,我们的事件传递到了Button(从ViewGroup传到了View)里面,说明layout并没有拦截,
onInterceptTouchEvent的默认返回值我们不知道是什么,但是我们可以尝试一下返回true会是什么样,改动代码,重新运行:



可以看到,拦截之后,事件成功的传递到了layout上面的ontouch事件,不知道大家有没有注意到一个细节,就是onInterceptTouchEvent方法只在down事件的时候执行了一次,后面并没有再次执行。而上面的结果却是onInterceptTouchEvent执行了三次(因为执行了1次move事件)这个我们先记下来等等分析。通过这次测试,我们大致可以判断viewgroup的onInterceptTouchEvent方法默认返回的应该是false,这里不再演示了,下面让我们结合源码看看为什么会出现这种情况。

源码分析

首先我们知道,事件传递是从activity开始的,这点从前面就可以看到,每次事件都是从activity的dispatchTouchEvent方法开始的,所以我们先来看看activity的dispatchTouchEvent方法源码,看看里面执行了什么:

/**
 * Called to process touch screen events.  You can override this to
 * intercept all touch screen events before they are dispatched to the
 * window.  Be sure to call this implementation for touch screen events
 * that should be handled normally.
 * 
 * @param ev The touch screen event.
 * 
 * @return boolean Return true if this event was consumed.
 */
public boolean dispatchTouchEvent(MotionEvent ev) {
    if (ev.getAction() == MotionEvent.ACTION_DOWN) {
        onUserInteraction();
    }
    if (getWindow().superDispatchTouchEvent(ev)) {
        return true;
    }
    return onTouchEvent(ev);
}
代码不多,我们一条条分析,首先第一个if语句判断了当事件是down的时候,就是最开始的时候,执行onUserInteraction方法,我们点进去看看

public void onUserInteraction() {
}


显然这是一条空方法,是让我们子类去实现的,由于这个方法在事件分发之前调用,所以我们可以通过实现这个方法,来知道当activity正在运行的时候用户是怎么和设备进行交互的,由于这个方法并没有返回值,所以对我们整个流程没什么影响,所以不要在意这些细节。。。。

继续看代码,第二个if语句里面判断,getWindow().superDispatchTouchEvent()这个可以肯定就是我们的重点了,分发应该就是在这里进行的,首先第一个getWindow(),这是个什么鬼,通过方法名字可以判断应该获取一个window,点进去看看,确实是获取了一个Window,然后window继续调用superDispatchTouchEvent()方法,我们点开window类,却发现这个是一个抽象类,当然这个方法就是个抽象方法了,那么我们就要找到window类的实现类才行,执行过程肯定在那里,通过Window类源码的注释我们可以知道,window的实现类是PhoneWindow。



我们进PhoneWindow里面看看对应的方法

public boolean superDispatchKeyEvent(KeyEvent event)
{
        return mDecor.superDispatchKeyEvent(event);
}
代码很简单,就是调用了mDecor的相应方法,这个mDecor又是个什么鬼,继续看

public final View getDecorView()
{
        if(mDecor == null)
            installDecor();
        return mDecor;
}
installDecor()方法里面又会执行 generateDecor(),藏的好深,我们继续看

protected DecorView generateDecor()
    {
        int flags = getAttributes().flags;
        boolean coverStatusBar = true;
        if(getContainer() != null)
            coverStatusBar = false;
        else
        if((flags & 0x10) == 0 || (flags & 0x80) != 0)
            coverStatusBar = false;
        return new DecorView(getContext(), -1, coverStatusBar);
}
这里会发现,这其实就是个DecorView,而DecorView是一个内部类,继承了FrameLayout,就是说这是一个ViewGroup,是
一个顶级布局,其实我们代码里面通过setContentView设置的view布局就是在它里面的,现在
事件就传递到了ViewGroup中,下面就是ViewGroup的传递了。。。
ViewGroup中的事件分发肯定也是先从dispatchTouchEvent开始的,还是先看看这个方法:

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) {
                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;
        }

        // 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;
        if (!canceled && !intercepted) {
            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 View[] children = mChildren;

                    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;
}

代码还是有点长的,不过不要在意那些细节。。。。我们还是关注流程即可,看21行,有个布尔类型的intercepted,我们可以看到,当viewgroup在两种情况下会判断是否进行拦截,一种是当事件类型为Action_down的时候,二是当mFirstTouchTarget!=null的时候,mFirstTouchTarget是个啥玩意儿,这个我们暂且先留着,继续看,下面有一个布尔类型的disallowIntercept,这个的意思就是禁止拦截,默认是false,这个标记位可以在子View中通过requestDisallowInterceptTouchEvent方法来设置,设置之后ViewGroup就无法拦截除了down事件外的其他事件,为什么说是除了down事件呢,因为前面第12行,当事件是down的时候,就会进入resetTouchState方法,这个方法中会重置标记位。所以面对down事件的时候,ViewGroup总会调用onInterceptTouchEvent来询问自己是否要拦截。进入if块中,发现执行了我们的onInterceptTouchEvent方法,我们进入这个方法看一下:

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

这方法相当简单,就是直接返回了false,证实了我们之前的推测,ViewGroup默认是不拦截事件的,继续往下看,发现一个canceled,这个又是啥,这个其实就是事件被取消,这个不常见(比如一个ViewGroup中包含了一个View,在down事件传递的时候,ViewGroup不拦截,View处理了down事件返回了true,但是当move事件传递过来的时候,ViewGroup拦截了,这时候ViewGroup的onTouch和onTouchEvent被触发,在触发之前View的dispatchTouchEvent就会收到ACTION_CANCEL事件,了解一下即可)。我们继续看,如果事件不被取消并且不被拦截的时候,进入if块中,第66行,遍历子元素,判断子元素是否可以接收到点击事件,主要判断两点,一个是子元素是否有动画在播放,还有就是点击事件的坐标是否在子元素的View区域内,如果条件满足就会交给子元素处理,下面有个dispatchTransformedTouchEvent方法,进去看看

/**
 * Transforms a motion event into the coordinate space of a particular child view,
 * filters out irrelevant pointer ids, and overrides its action if necessary.
 * If child is null, assumes the MotionEvent will be sent to this ViewGroup instead.
 */
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;
}
我们发现,当child是空的时候,执行的就是super.dispatchTouchEvent()方法,如果child不为空,执行的就是child.dispatchTouchEvent(),这样就将事件传递到子元素View中了。下面继续看,有个addTouchTarget()方法,进去瞧瞧

/**
 * Adds a touch target for specified child to the beginning of the list.
 * Assumes the target child is not already present.
 */
private TouchTarget addTouchTarget(View child, int pointerIdBits) {
    TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
    target.next = mFirstTouchTarget;
    mFirstTouchTarget = target;
    return target;
}


这里会将mFirstTouchTarget赋值,也就是说如果子元素处理了事件,那么mFirstTouchTarget就不会是空,如果viewgroup拦截了事件,那么mFirstTouchTarget就会是空,所以我们刚才测试的结果是down事件的时候执行了onInterceptTouchEvent方法,并且返回了true,使得viewGroup拦截了事件,那么在move和up事件的时候,mFirstTouchTarget为null, onInterceptTouchEvent方法就不会被调用;如果onInterceptTouchEvent返回了false,那么viewGroup不拦截事件,子元素会处理事件,mFirstTouchTarget被赋值,所以onInterceptTouchEvent会每次都执行。

如果遍历了所有的子元素后事件都没有被处理完毕,包含两种情况,第一就是没有子元素,第二就是子元素处理了事件,但是在最后返回了false,那么这时候ViewGroup就会自己处理事件,看110行,注意第三个参数是null,经过前面的分析,这时候会执行super.dispatchTouchEvent方法,ViewGroup的super就是View,所以事件就会交给View的dispatchTouchEvent方法来处理。

而如果顶级ViewGroup的dispatchTouchEvent方法最后也返回了false,那么根据activity中dispatchTouchEvent方法的源码来看,最后就交给了activity的onTouchEvent方法来处理。这样,整个ViewGroup的事件分发流程执行了一次。

总结

1、viewGroup默认不拦截任何事件,但是如果拦截了事件,那么此次事件序列中的后续其他事件就会交给viewGroup处理,并且onInterceptTouchEvent不会被再次调用;如果没有拦截事件,那么每次事件传递的时候,onInterceptTouchEvent都会被调用。
2、和ViewGroup相比,View没有onInterceptTouchEvent方法,一旦事件传递过来,那么View的onTouchEvent方法就会被调用(或者onTouch)

3、事件传递过程是从外向内的,事件总是先传给父元素,然后再分发给子元素,在子元素中可以通过requestDisallowInterceptTouchEvent方法干预父元素的分发过程,除了ACTION_DOWN事件。

图示



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