Android事件分发详解
2017-03-30 13:06
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Android事件分发
1.事件传递的流程是从外到内,即事件总是由父元素分发给子元素:Activity->ViewGroup-View,但是通过requestDisallowInterceptTouchEvent可以使子view要求父view不拦截事件2.当一个事件发生时,首先由Activity接收,然后activity会交由他的window处理,window的处理方式即是调用顶级容器DecorView(即setContentView所设置的View的父容器,是一个FramLayout)的dispatchTouchEvent方法,事件就进入了一个ViewGroup,开始按照事件分发机制去分发事件,若DecorView不处理,则activity会调用自身的onTouchEvent()方法消费事件
3.三个最主要方法:
public boolean dispatchTouchEvent(MotionEvent ev)
此方法是事件进入某个view和ViewGroup的入口,返回true表示在此层或者下层已经消费了事件,返回false则表示此层即下层都没处理
public boolean onTouchEvent(MotionEvent event)
此方法是view和ViewGroup自身事件的处理方法,返回true表示自己处理事件,返回false表示自己不处理
public boolean onInterceptTouchEvent(MotionEvent ev)
此方法是ViewGroup拦截事件的方法,返回true表示拦截事件不再向下层传递事件,返回false表示不拦截
4.整个ViewGroup的事件分发机制大致可以用下面的伪代码来表示:
public boolean dispatchTouchEvent(MotionEvent ev) { boolean handled = false; if(onInterceptTouchEvent(ev)){ handled = super.dispatchTouchEvent(ev); }else{ if(child.dispatchTouchEvent(ev)){ handled = true; }else{ handled = super.dispatchTouchEvent(ev); } } return handled; }
ViewGroup的dispatchTouchEvent方法的功能可分为两部分,即事件下发部分和自身事件处理部分,ViewGroup是继承于View,所以它的自身事件就是view的dispatchTouchEvent方法().
由两种情况来决定走什么功能:
1.是否拦截,拦截,则直接调用super.dispatchTouchEvent()方法处理身为一个view的特性处理自身事件。不拦截则调用child.dispatchTouchEvent()下发事件 2.所有被下发事件的子view的dispatchTouchEvent()在ACTION_DOWN就返回false,表示下面无view处理,则再调用super.dispatchTouchEvent()处理自身事件。这就完成了这一轮的事件分发,其他层同理,是一个递归的过程
3、一个事件从手指按下屏幕的那一刻到离开屏幕,中间还可能有多次滑动,这个过程以一个ACTION_DOWN开始,中间可能有0及以上个ACTION_MOVE,然后以一个ACTION_UP结束。这是一个事件序列。
4.事件的分发是一层一层分发下来的,一旦某个ViewGrou成功拦截了事件序列中的某个事件,那么此事件序列之后的所有事件都会默认被拦截(只要后续事件能传递到它这里),不会再去调用onInterceptTouchEvent询问是否拦截,当然也不会再向下分发,
5.如果一个事件传递到某个View或ViewGroup,一旦在ACTION_DOWN返回了false,那么此事件序列后续的事件都不会传递到这里了,并且父View的onTouchEvent方法会被调用即事件会再次向上传递,若一旦开始消费某个事件,那么即便在后续事件选择不消费了(比如此View的onTouchEvent方法在ACTION_DOWN返回true,但是ACTION_MOVE返回false),后续事件也会持续的传到这里,并且父View的onTouchEvent方法不会被调用即事件不会再次向上传递,这些后续被返回false的事件最终会直接传递给Activity处理
6.View.dispatchTouchEvent()关键代码,onTouchListener的优先级要高于onTouchEvent(),onClickListener在onTouchEvent()里面调用。:
if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED && li.mOnTouchListener.onTouch(this, event)) { result = true; } if (!result && onTouchEvent(event)) { result = true; }
onTouchEvent方法里如果clickable和longClickable有一个为真,就会调用
performClick(),就是点击事件:
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); return result; }
7.Enable属性不影响默认的系统控件的onTouchEvent的返回值,只要view的clickable或longClickAble有一个为true,默认的onTouchEvent方法便会返回true,否则返回false(例如,当TextView的onTouchEvent默认返回false,因为它默认就是不可点击的,在设置了clickable为true后,它的onTouchEvent就返回true了)
if ((viewFlags & ENABLED_MASK) == DISABLED) { if (action == 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) || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE); }
8.以ViewGroup为例,当事件传进来时,dispatchTouchEvent中的代码:
// 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; }
这里的条件判断的决定是否直接将拦截标志位设为true,不再调用onInterceptTouchEvent方法去判断。满足条件为这个事件是ACTION_DOWN事件,或者mFirstTouchTarget不为空。ACTION_DOWN是一个新事件序列的开始肯定不能直接将拦截标志位设为true,而是要经过onInterceptTouchEvent去决定,mFirstTouchTarget是只要下面有子view消耗了事件就会赋值,所以不为空就是下面有子view处理了事件,而如果是后续事件但是mFirstTouchTarget为空说明子view在ACTION_DOWN事件就返回了false不消耗事件,那么后续的事件也就无需再传递给下面了,直接将拦截标志为设为true,而不再通过onInterceptTouchEvent去决定,这里需结合4和5仔细体会。
若进入了条件为真的的代码块,这个disallowIntercept一般是在子view中被用parent.requestDisallowInterceptTouchEvent后结果为true,就不再走onInterceptTouchEvent判断了,直接使拦截标志位位true,使得本parentView不拦截事件。虽然事件的分发是从外到里,但是这是特殊的子view干预父view的事件分发的办法,另外在MotionEvent.ACTION_DOWN事件到来时,disallowIntercept标志会在时被重置,mFirstTouchTarget会被清除,因为这是新事件序列的开始,所以这也更好理解为什么在MotionEvent.ACTION_DOWN时不能直接拦截了
// 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(); }
如果不拦截,就会遍历子view,通过是否在播放动画和事件是否落在它的范围内来获得合适的 View,如果存在就为mFirstTouchTarget赋值,注意这里能找到的合适子view可能不止一个,因此如果第一个合适的view不处理事件,那么会继续为下面合适的继续赋值,以此类推
if (!canceled && !intercepted) { // If the event is targeting accessiiblity 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<View> preorderedList = buildOrderedChildList(); final boolean customOrder = preorderedList == null && isChildrenDrawingOrderEnabled(); final View[] children = mChildren; for (int i = childrenCount - 1; i >= 0; i--) { final int childIndex = customOrder ? getChildDrawingOrder(childrenCount, i) : i; final View child = (preorderedList == null) ? children[childIndex] : preorderedList.get(childIndex); // If there is a view that has accessibility focus we want it // to get the event first and if not handled we will perform a // normal dispatch. We may do a double iteration but this is // safer given the timeframe. if (childWithAccessibilityFocus != null) { if (childWithAccessibilityFocus != child) { continue; } childWithAccessibilityFocus = null; i = childrenCount - 1; } if (!canViewReceivePointerEvents(child) || !isTransformedTouchPointInView(x, y, child, null)) { ev.setTargetAccessibilityFocus(false); 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(); 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; } // 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(); } 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; } } }
接着检验mFirstTouchTarget是否为空,为空的可能有两种,一是没有合适的view,二是view不处理事件:
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; } }
可以看到,不管是否为空都调用了dispatchTransformedTouchEvent()这个方法,看看它的关键代码
if (child == null) { handled = super.dispatchTouchEvent(event); } else { handled = child.dispatchTouchEvent(event); } return handled;
看到这里就能明白了,关键在于第三个参数,mFirstTouchTarget为空则将child赋值空,就会调用了父view的dispatchTouchEvent方法,事件再次向上传递,否则直接调用了子view的dispatchTouchEvent方法,事件进入下一轮
最后事件分发大致图:
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