定義：
使多個(gè)對(duì)象都有機(jī)會(huì)處理請(qǐng)求赦拘，從而避免了請(qǐng)求的發(fā)送者和接收者之間的耦合聯(lián)系他膳，將這些對(duì)象連成一條鏈惦银，并沿著這條鏈傳遞該請(qǐng)求，知道有對(duì)象處理它為止辣垒。
使用場景：
（1）多個(gè)對(duì)象可以同時(shí)處理同一請(qǐng)求望侈，但具體由哪個(gè)對(duì)象處理則在運(yùn)行時(shí)動(dòng)態(tài)決定。
（2）再請(qǐng)求處理者不明確的情況下向多個(gè)對(duì)象中的一個(gè)提交一個(gè)請(qǐng)求
（3）需要?jiǎng)討B(tài)指定一組對(duì)象處理請(qǐng)求
責(zé)任鏈的在android源碼中的應(yīng)用就是事件的分發(fā)機(jī)制
點(diǎn)擊事件用MotionEvent來表示勋桶，當(dāng)一個(gè)點(diǎn)擊操作發(fā)生時(shí)脱衙，事件最先傳遞給Activity，由Activity的dispatchTouchEvent來進(jìn)行事件分發(fā)例驹，具體工作將由Activity內(nèi)部的Window來完成的捐韩，Window將事件傳遞給decor view，decor view一般就是當(dāng)前界面的底層容器（即setContentView所設(shè)置的view的容器）
Activiity的dispatchTouchEvent
/**
* 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);
}
事件開始一般開始都是down開始的鹃锈，所有從代碼中我們可以看出荤胁，先執(zhí)行的是 onUserInteraction();，我們onUserInteraction的源碼：
/**
* Called whenever a key, touch, or trackball event is dispatched to the
* activity. Implement this method if you wish to know that the user has
* interacted with the device in some way while your activity is running.
* This callback and {@link #onUserLeaveHint} are intended to help
* activities manage status bar notifications intelligently; specifically,
* for helping activities determine the proper time to cancel a notification.
*
* <p>All calls to your activity's {@link #onUserLeaveHint} callback will
* be accompanied by calls to {@link #onUserInteraction}. This
* ensures that your activity will be told of relevant user activity such
* as pulling down the notification pane and touching an item there.
*
* <p>Note that this callback will be invoked for the touch down action
* that begins a touch gesture, but may not be invoked for the touch-moved
* and touch-up actions that follow.
*
* @see #onUserLeaveHint()
*/
public void onUserInteraction() {
}
這個(gè)方法實(shí)現(xiàn)體是空的屎债，從注釋中可以看出仅政，當(dāng)key,touch,trackball事件分發(fā)到當(dāng)前Activity就會(huì)調(diào)用。
我們接下來看第二個(gè)if語句盆驹，getWindow().superDispatchTouchEvent(ev)圆丹，
每一個(gè)Activity都包含一個(gè)Window對(duì)象，Window對(duì)象通常由PhoneWindow實(shí)現(xiàn)
我們看一下phoneWindow中的superDispatchTouchEvent
image.png
直接調(diào)用了DecorView的superDispatchTouchEvent的方法躯喇，DecoreView繼承與FrameLayout,是所有界面的父類辫封。而 FrameLayout 作為 ViewGroup 的子類硝枉，所以直接調(diào)用了 ViewGroup 的 dispatchTouchEvent()
我們?cè)诓榭聪耉iewGroup的dispatchTouchEvent()方法
@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
}
// If the event targets the accessibility focused view and this is it, start
// normal event dispatch. Maybe a descendant is what will handle the click.
if (ev.isTargetAccessibilityFocus() && isAccessibilityFocusedViewOrHost()) {
ev.setTargetAccessibilityFocus(false);
}
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;
}
// If intercepted, start normal event dispatch. Also if there is already
// a view that is handling the gesture, do normal event dispatch.
if (intercepted || mFirstTouchTarget != null) {
ev.setTargetAccessibilityFocus(false);
}
// 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 the event is targeting accessibility 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 = 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 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 (!child.canReceivePointerEvents()
|| !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;
}
}
}
// 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;
}
總結(jié)：Android 事件分發(fā)總是遵循 Activity => ViewGroup => View 的傳遞順序
