網上看到過很多人寫的事件分發機制解析,感覺表述都不是很清楚,也可能沒有看到寫得好的文章,所以自己重新看了一遍源碼,來徹底搞清楚Android事件分發機制.
觸摸事件有哪些以及怎麼從Activity傳遞到DecorView大家可以上網查下，幾個重要方法的基本調用順序,這些很容易搜到,我們重點關注事件從ViewGroup到View的事件具體的執行過程.
1 Android事件分發涉及到的方法主要有
dispatchTouchEvent()事件分發
onInterceptTouchEvent()攔截事件
onTouchEvent()處理事件
requestDisallowInterceptTouchEvent()請求不要攔截事件
2 測試類介紹
BigGroup類型ViewGroup
SmallGroup類型ViewGroup
TestView類型View*(clickable默認false)
3 默認情況下的事件分發流程(View.onTouchEvent()返回super.onTouchEvent())
log如下所示：
BigGroup: dispatchTouchEvent() called with: ev = [ACTION_DOWN]
BigGroup: onInterceptTouchEvent() called with: ev = [ACTION_DOWN]
SmallGroup: dispatchTouchEvent() called with: ev = [ACTION_DOWN]
SmallGroup: onInterceptTouchEvent() called with: ev = [ACTION_DOWN]
TestView: dispatchTouchEvent() called with: ev = [ACTION_DOWN]
TestView: onTouchEvent() called with: event = [ACTION_DOWN]
SmallGroup: onTouchEvent() called with: event = [ACTION_DOWN]
BigGroup: onTouchEvent() called with: event = [ACTION_DOWN]
我們看到不管怎麼點擊滑動，都只會觸發DOWN事件的流程，我們來看下這是爲什麼，首先我們來看下ViewGroup的dispatchTouchEvent()
///////////ViewGroup
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
...省略部分代碼
boolean handled = false;
if (onFilterTouchEventForSecurity(ev)) {
final int action = ev.getAction();
final int actionMasked = action & MotionEvent.ACTION_MASK;
//如果是DOWN事件,清除標記,恢復一些標記狀態
if (actionMasked == MotionEvent.ACTION_DOWN) {
cancelAndClearTouchTargets(ev);
resetTouchState();
}
// Check for interception.
//判斷是否攔截
final boolean intercepted;
//觸發DOWN事件或者mFirstTouchTarget不是Null,纔會走if裏面的邏輯
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.
//不是DOWN事件 mFirstTouchTarget爲空 攔截後續事件
intercepted = true;
}
...省略部分代碼
// 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;
//事件沒有被取消或者沒有被攔截執行if中邏輯
if (!canceled && !intercepted) {
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 = buildTouchDispatchChildList();
final boolean customOrder = preorderedList == null
&& isChildrenDrawingOrderEnabled();
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 (childWithAccessibilityFocus != null) {
if (childWithAccessibilityFocus != child) {
continue;
}
childWithAccessibilityFocus = null;
i = childrenCount - 1;
}
//判斷child是否可以響應事件 是否點擊在了View的範圍之內
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
ev.setTargetAccessibilityFocus(false);
continue;
}
//獲取上次響應事件的View,DOWN事件時候newTouchTarget爲Null
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);
//很重要的一個方法,在下面我們會看這個方法做了什麼
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();
//在這個方法中把child賦值給了mFirstTouchTarget
newTouchTarget = addTouchTarget(child, idBitsToAssign);
alreadyDispatchedToNewTouchTarget = true;
break;
}
ev.setTargetAccessibilityFocus(false);
}
if (preorderedList != null) preorderedList.clear();
}
...
}
}
// 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;
}
}
...
}
...
return handled;
}
//最重要的幾行代碼的邏輯
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
View child, int desiredPointerIdBits) {
final boolean handled;
...省略部分代碼
// Perform any necessary transformations and dispatch.
//child爲空調用父View的dispatchTouchEvent
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());
}
//child不爲空調用child的dispatchTouchEvent方法
handled = child.dispatchTouchEvent(transformedEvent);
}
// Done.
transformedEvent.recycle();
return handled;
}
//默認返回false
public boolean onInterceptTouchEvent(MotionEvent ev) {
if (ev.isFromSource(InputDevice.SOURCE_MOUSE)
&& ev.getAction() == MotionEvent.ACTION_DOWN
&& ev.isButtonPressed(MotionEvent.BUTTON_PRIMARY)
&& isOnScrollbarThumb(ev.getX(), ev.getY())) {
return true;
}
return false;
}
//////////View
public boolean dispatchTouchEvent(MotionEvent event) {
...
boolean result = false;
...
if (onFilterTouchEventForSecurity(event)) {
if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
result = true;
}
//noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo;
//我們看到有mOnTouchListener,並且onTouch返回true,就沒有onTouchEvent啥事了
if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
result = true;
}
if (!result && onTouchEvent(event)) {
result = true;
}
}
...
return result;
}
public boolean onTouchEvent(MotionEvent event) {
final float x = event.getX();
final float y = event.getY();
final int viewFlags = mViewFlags;
final int action = event.getAction();
final boolean clickable = ((viewFlags & CLICKABLE) == CLICKABLE
|| (viewFlags & LONG_CLICKABLE) == LONG_CLICKABLE)
|| (viewFlags & CONTEXT_CLICKABLE) == CONTEXT_CLICKABLE;
...
if (clickable || (viewFlags & TOOLTIP) == TOOLTIP) {
switch (action) {
case MotionEvent.ACTION_UP:
...
break;
case MotionEvent.ACTION_DOWN:
...
break;
case MotionEvent.ACTION_CANCEL:
...
break;
}
return true;
}
return false;
}
我們梳理下DOWN事件的執行流程:
1 觸發BigGroup的dispatchTouchEvent()方法
2 給intercepted賦值,disallowIntercept默認爲false,這是就會觸發onInterceptTouchEvent()方法,onInterceptTouchEvent默認返回false.intercepted賦值爲false
3 接下來走到if (!canceled && !intercepted)的邏輯裏面去了,注意注意intercepted爲true的話這個方法直接就跳過去了.在這段邏輯裏遍歷所有的子View,接下來進入淘汰機制,child不能獲得焦點,下一個,然後看看View不可見,下一個,沒有點到child範圍之內,下一個.剩下來的娃會執行dispatchTransformedTouchEvent方法
4 接下來dispatchTransformedTouchEvent中調用了child的dispatchTouchEvent方法
5 執行dispatchTouchEvent的child就是我們的SmallGroup,執行邏輯同2-4
6 這次執行dispatchTouchEvent的child就是我們的TestView了
7 TestView dispatchTouchEvent中首先判斷了有沒有onTouchListener,判斷onTouch方法的返回值.我們肯定沒寫的了,接下來會執行onTouchEvent
8 在TestView onTouchEvent中我們看到如果這個View可點擊就返回true,不可點擊就返回false.TestView不可點擊,所以返回了false
9 觸發了連鎖反應,TestView dispatchTouchEvent返回false
10SmallGroup dispatchTransformedTouchEvent返回了false, mFirstTouchTarget爲null
11SmallGroup再次執行dispatchTransformedTouchEvent child參數爲null,執行super.dispatchTouchEvent(),ViewGroup的super最後就是View,所以會執行邏輯7-9,不同的事主角事SmallGroup
12BigGroup dispatchTransformedTouchEvent返回了false, mFirstTouchTarget爲null,重複步驟11,主角BigGroup,DOWN事件結束
mFirstTouchTarget爲null會有什麼影響呢?後續事件中intercepted爲true,執行dispatchTransformedTouchEvent child參數爲null,執行super.dispatchTouchEvent(),事件直接傳遞不下來了,後續事件都接收不到了
所以log只能看到有關於DOWN事件的．