View的事件处理
1.现象
我们分别创建一个ImageView和一个Button，并监听其onTouchListener事件。
我认为所谓能响应事件，简单地理解就是说能监听到事件的发生，判断能否监听到事件的依据就是能否调用到相应的回调函数。
实验代码
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Button btn = (Button) findViewById(R.id.btn);
ImageView iv = (ImageView) findViewById(R.id.iv);
iv.setOnTouchListener(new OnTouchListener() {
@Override
public boolean onTouch(View v, MotionEvent event) {
System.out.println("***********iv onTouch ** "+event.getAction()+" *********");
return false; //ImageView返回false:只能响应第一个事件，之后的事件不会响应
//return true; //ImageView返回true:一直响应各种事件
}
});
btn.setOnTouchListener(new OnTouchListener() {
@Override
public boolean onTouch(View v, MotionEvent event) {
System.out.println("***********btn onTouch ** "+event.getAction()+" *********");
return false; //button都能响应
//return true; //button都能响应
}
});
现在ImageView的onTouch方法不同返回值响应事件的结果是不一样的，但是Button的却一样。
2.View的事件处理函数
dispatchTouchEvent 事件处理关键函数
/**
* Pass the touch screen motion event down to the target view, or this
* view if it is the target.
*
* @param event The motion event to be dispatched.
* @return True if the event was handled by the view, false otherwise. 返回true事件将被本view处理
*/
public boolean dispatchTouchEvent(MotionEvent event) {
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(event, 0);
}
if (onFilterTouchEventForSecurity(event)) {
//noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo;
//1.判断是否传递进了实现mOnTouchListener的类(也就是是否设置了Touch监听)，如果没有传，直接跳到后面
//2.判断当前控件是否可用(是否为enable)
//3.传入实现类中的onTouch是否为true
//三条都满足，直接返回ture，当前控件响应事件。
/*另外，此处会调用一次onTouch方法进行判断返回值，所以无论返回值是什么，第一次
点击的事件(ACTION_DOWN总会被响应),但是如果返回true怎么连续响应事件还不清楚
*/
if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
return true;
}
//如果上述判断条件有没有满足的，则执行onTouchEven方法
//另外要说的一点，如果我们要禁止当前控件响应任何事件，
//直接不做监听(上面判断第一条为false)，重写onTouchEvent并直接返回false，
//那么dispatchTouchEvent永远为false，当前控件不响应任何事件
//onTouchEvent源码在下面
if (onTouchEvent(event)) {
return true;
}
}
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
}
return false;
}
onTouchEvent
/**
* Implement this method to handle touch screen motion events.
*
* @param event The motion event.
* @return True if the event was handled, false otherwise.
*/
public boolean onTouchEvent(MotionEvent event) {
final int viewFlags = mViewFlags;
if ((viewFlags & ENABLED_MASK) == DISABLED) {
if (event.getAction() == MotionEvent.ACTION_UP && (mPrivateFlags & PFLAG_PRESSED) != 0) {
setPressed(false);
}
// A disabled view that is clickable still consumes the touch
// events, it just doesn't respond to them.
return (((viewFlags & CLICKABLE) == CLICKABLE ||
(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE));
}
if (mTouchDelegate != null) {
if (mTouchDelegate.onTouchEvent(event)) {
return true;
}
}
/*
此处需要注意，当前控件必须是CLICKABLE的才能进入，进入之后返回的都是true，这样调用它的
dispatchTouchEvent函数返回true，否则dispatchTouchEvent函数返回false。
*/
if (((viewFlags & CLICKABLE) == CLICKABLE ||
(viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)) {
switch (event.getAction()) {
case MotionEvent.ACTION_UP:
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();
}
if (prepressed) {
// The button is being released before we actually
// showed it as pressed. Make it show the pressed
// state now (before scheduling the click) to ensure
// the user sees it.
setPressed(true);
}
if (!mHasPerformedLongPress) {
// 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();
}
if (!post(mPerformClick)) {
/************************************************
下面这个函数会调用OnClickListener()中的onClick方法，从而实现点击回调
************************************************/
performClick();
}
}
}
if (mUnsetPressedState == null) {
mUnsetPressedState = new UnsetPressedState();
}
if (prepressed) {
postDelayed(mUnsetPressedState,
ViewConfiguration.getPressedStateDuration());
} else if (!post(mUnsetPressedState)) {
// If the post failed, unpress right now
mUnsetPressedState.run();
}
removeTapCallback();
}
break;
case MotionEvent.ACTION_DOWN:
mHasPerformedLongPress = false;
if (performButtonActionOnTouchDown(event)) {
break;
}
// Walk up the hierarchy to determine if we're inside a scrolling container.
boolean isInScrollingContainer = isInScrollingContainer();
// For views inside a scrolling container, delay the pressed feedback for
// a short period in case this is a scroll.
if (isInScrollingContainer) {
mPrivateFlags |= PFLAG_PREPRESSED;
if (mPendingCheckForTap == null) {
mPendingCheckForTap = new CheckForTap();
}
postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
} else {
// Not inside a scrolling container, so show the feedback right away
setPressed(true);
checkForLongClick(0);
}
break;
case MotionEvent.ACTION_CANCEL:
setPressed(false);
removeTapCallback();
removeLongPressCallback();
break;
case MotionEvent.ACTION_MOVE:
final int x = (int) event.getX();
final int y = (int) event.getY();
// Be lenient about moving outside of buttons
if (!pointInView(x, y, mTouchSlop)) {
// Outside button
removeTapCallback();
if ((mPrivateFlags & PFLAG_PRESSED) != 0) {
// Remove any future long press/tap checks
removeLongPressCallback();
setPressed(false);
}
}
break;
}
return true;
}
return false;
}
结论
根据上面的dispatchTouchEvent函数，我们可以发现，如果一个控件可点击(实现了
监听onClickEvent就可以置位可点击标志),那么无论onTouc返回什么内容，最终
dispatchTouchEvent函数可以都响应事件。
如果不可点击，那么onTouchEvent函数必定会返回false，只能寄希望于onTouch函数了，onTouch必
须由接口传入并且
返回true才能保证dispatchTouchEvent函数返回true，才能响应事件。
现在我们为上位的iv设置一个setOnClickListener方法，传入一个OnClickListener，这样即使
iv的onTouch返回false，也一样可以进入下面的onTouchEvent函数，并在这个函数中让返回值变为
true。
iv.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
System.out.println("***********iv OnClickListener ***********"); //鼠标弹起时候被触发
}
});
ViewGroup 的事件处理与传递
ViewGroup中会重写dispatchTouchEvent函数，可以说此时dispatchTouchEvent函数是用来传递事
件的，那么谁来处理事件呢？先看一下ViewGroup中dispatchTouchEvent做了些什么。
ViewGroup的事件处理函数
dispatchTouchEvent
@Override
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) {
/**********************************************************
此处调用onInterceptTouchEvent，即是否当前ViewGroup拦截事件，如果拦截，则事件由
当前ViewGroup处理，不再向下分发。
**********************************************************/
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;
/**************************************************************
如果不拦截事件(false)，则进入这个if内部
如果拦截，则直接在下面调用ViewGroup的父类的dispatchTouchEvent方法，由当前ViewGrop处理事件，而不再向下传递
**************************************************************/
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();
/****************************************************************
遍历ViewGroup中的每个子控件，看看这个事件落到谁的头上
****************************************************************/
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);
/***************************************************************
dispatchTransformedTouchEvent函数中调用dispatchTouchEvent，相当于递归
调用dispatchTouchEvent，查找当前孩子的子控件。
***************************************************************/
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.
/**************************************************************************
如果当前ViewGrop的孩子没有控件可以响应事件，那么当前ViewGrop只能调用他父类的
dispatchTouchEvent去进行响应。可以看出ViewGroup的dispatchTouchEven方法只是
进行传递，而真正是否能响应事件还是需要用View的dispatchTouchEvent的返回值。
**************************************************************************/
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;
}
###onInterceptTouchEvent 是否拦截事件
public boolean onInterceptTouchEvent(MotionEvent ev) {
return false;
}
结论
ViewGroup的事件传递是从外部向内部传递的。 其中每一级可以调用onInterceptTouchEvent(true)拦截事件，
让当前的ViewGroup处理。如果不拦截，则会递归到最下面的控件，如果它可以处理这个事件，就处理
(dispatchTouchEvent返回true)，否则还会交还给父ViewGroup。这时，递归会出来，从而调用父ViewGroup的
父类的dispatchTouchEvent方法，尝试处理事件。