Android中View的事件分发和拦截机制
1.点击事件，就是MotionEvent,所谓点击事件的事件分发，其实就是对MotionEvent事件的分发过程。点击事件的分发过程由三个很重要的方法来共同完成：dispatchTouchEvent、 onInterceptTouchEvent和onTouchEvent.
下列的伪代码(Pseudo code)形象生动地展示了这三个方法之间的逻辑关系:
public boolean dispatchTouchEvent(MotionEvent event){
boolean consumed = false;
if(onInterceptTouchEvent(event)){
consumed= onTouchEvent(event);
}else{
consumed = child.dispatchTouchEvent(event);
}
return consumed;
}
2.传递规则:
对于一个根ViewGroup来说，点击事件产生后，首先会传递给他，这时它的dispatchTouchEvent就会被调用，如果这个ViewGroup的onInterceptTouchEvent方法返回true,就表示它要拦截当前事件，接着这个ViewGroup就会处理这个事件，即onTouchEvent方法被调用，事件得到消耗；如果这个ViewGroup的onInterceptTouchEvent方法返回false就表示它不拦截当前事件，这时当前事件就会继续传递给他的子元素，接着子元素的dispatchTouchEvent方法就会被调用，如此反复直到事件被最终处理。
当一个点击事件产生后，它的传递过程遵循如下顺序Activity->Window->View,如果最终所有元素都没能处理这个时间，那么该事件最终将会被传递给Activity处理，即Activity的onTouchEvent方法被调用
注意:
1) ViewGroup默认不拦截任何事件， ViewGroup的onInterceptTouchEvent方法默认返回false
2) View没有onInterceptTouchEvent方法，一旦有点击事件传递给它，那么它的onTouchEvent方法就会被调用
3) View的onTouchEvent默认会消耗事件(返回true), 除非它是不可点击的
4) View的enable属性不影响onTouchEvent的默认返回值，哪怕一个View是disable状态，主要它的clickable或者longClickable有一个为true,那么它的onTouchEvent就返回true
5)事件传递过程是由外向内的，即事件总是先传递给父元素，然后再由父元素分发给子View,通过requestDisallowInterceptTouchEvent方法可以在子元素中干预父元素的事件分发过程，但是ACTION_DOWN事件除外
6)当某个View已经决定对事件进行拦截，那么同一事件序列内的所有事件都将交给它来处理，而且它的onInterceptTouchEvent方法将不再被调用
简而概之，事件的分发路径就是：activity -> window -> decor view -> view
3.源码分析
ViewGroup源码分析
public boolean dispatchTouchEvent(MotionEvent ev) {
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(); //ACTION_DOWN事件将重置触控状态，所以FLAG_DISALLOW_INTERCEPT标记不会影响对该事件的拦截 }// Check for interception. final boolean intercepted;
// First touch target in the linked list of touch targets. 触控对象列表的首个对象
//mFirstTouchTarget 当ViewGroup的子元素成功处理事件时，该对象将被赋值并指向该子元素 if (actionMasked == MotionEvent.ACTION_DOWN || mFirstTouchTarget != null) { final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0; if (!disallowIntercept) { // FLAG_DISALLOW_INTERCEPT标记一旦被设置，则ViewGroup将无法拦截除了ACTION_DOWN事件外的所有触控事件 intercepted = onInterceptTouchEvent(ev); ev.setAction(action); // restore action in case it was changed } else { intercepted = false; } } else { //当触控事件未发生时或事件由ViewGroup本身拦截时，则ViewGroup的onInterceptTouchEvent方法不会再被调用 // There are no touch targets and this action is not an initial down // so this view group continues to intercept touches. intercepted = true; } ... 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 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 ArrayListpreorderedList = buildTouchDispatchChildList(); final boolean customOrder = preorderedList == null && isChildrenDrawingOrderEnabled();
//当ViewGroup不拦截事件时，事件将下发给它的子View去处理 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); ... } if (preorderedList != null) preorderedList.clear(); } ... } } // Dispatch to touch targets. if (mFirstTouchTarget == null) { //ViewGroup没有子元素或者处理事件的子元素在OnTouch方法中返回了false,此时ViewGroup自行处理事件 // No touch targets so treat this as an ordinary view. handled = dispatchTransformedTouchEvent(ev, canceled, null, TouchTarget.ALL_POINTER_IDS); } ... } ... return handled; }
/** * Adds a touch target for specified child to the beginning of the list. * Assumes the target child is not already present. */
private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) { //对mFirstTouchTarget进行赋值
final TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
target.next = mFirstTouchTarget;
mFirstTouchTarget = target;
return target;
}
View的源码分析：
//view是单独的一个元素，它没有子元素，所以不能向下继续分发事件，只能自己处理事件
public boolean dispatchTouchEvent(MotionEvent event) {
...
boolean result = false;
...
if (onFilterTouchEventForSecurity(event)) {
...
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) { //这里先对OnTouchListener是否被设置进行判断
result = true;
}
if (!result && onTouchEvent(event)) { //这里表明OnTouchListener优先级高于onTouchEvent,设置了listener,就不调用onTouchEvent方法
result = true;
}
}
...
return result;
}
public boolean onTouchEvent(MotionEvent event) {
...
final int action = event.getAction();
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.
//这里表明即使处于disabled状态的view依然可以消耗触碰事件,只要Clickable或long_clickable其中之一为true return (((viewFlags & CLICKABLE) == CLICKABLE || (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) || (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE); } ... return false; }