1、職責(zé)鏈模式概述：
職責(zé)鏈模式(Chain of Responsibility Pattern)：避免請(qǐng)求發(fā)送者與接收者耦合在一起，讓多個(gè)對(duì)象都有可能接收請(qǐng)求，將這些對(duì)象連接成一條鏈，并且沿著這條鏈傳遞請(qǐng)求，直到有對(duì)象處理它為止。職責(zé)鏈模式是一種對(duì)象行為型模式?！对O(shè)計(jì)模式的藝術(shù)》
使用場(chǎng)景：
有企業(yè)OA系統(tǒng)開(kāi)發(fā)經(jīng)驗(yàn)的同學(xué)，會(huì)對(duì)職責(zé)鏈模式有比較深刻的理解。在多數(shù)OA系統(tǒng)中都有請(qǐng)假、物資申請(qǐng)等各種審批流程，這些流程比較復(fù)雜，一個(gè)請(qǐng)求對(duì)應(yīng)著多個(gè)處理者，而且不同角色的處理者對(duì)應(yīng)的層級(jí)不同。因此大多數(shù)設(shè)計(jì)人員都會(huì)采用職責(zé)鏈模式來(lái)進(jìn)行流程設(shè)計(jì)，降低系統(tǒng)耦合性，提升擴(kuò)展性。
2、策略模式UML類(lèi)圖：
image.png
Handler(抽象處理類(lèi))：作為所有處理者的父類(lèi)，定義處理者的公共接口，并且持有一個(gè)后繼節(jié)點(diǎn)處理者Handler的對(duì)象引用；
ConcreteHandler(具體處理類(lèi))：繼承抽象處理類(lèi)，實(shí)現(xiàn)處理請(qǐng)求的方法。
Android事件分發(fā)剖析
image.png
上圖展示了一個(gè)事件從輸入到最終處理的一個(gè)基本流程，其中省略了window這一層，事實(shí)上事件的分發(fā)順序?yàn)椋篈ctivity--->PhoneWindow--->DecorView---->具體布局。圖中出現(xiàn)了3個(gè)方法：dispatchTouchEvent、onInterceptTouchEvent、onTouchEvent，這三個(gè)方法負(fù)責(zé)完成一個(gè)事件的分發(fā)。Android系統(tǒng)通過(guò)職責(zé)鏈模式實(shí)現(xiàn)事件分發(fā)功能，自頂向下遍歷所有的處理者，直至找到一個(gè)處理者處理事件，或者都不處理。Activity、Window、ViewGroup、葉子View存在層次，事件會(huì)按照上述層次進(jìn)行傳遞。當(dāng)然，View體系也運(yùn)用了組合模式進(jìn)行設(shè)計(jì)，本篇文章只分析事件分發(fā)機(jī)制。下面分析上述3個(gè)方法的源碼：
View類(lèi)：
public boolean dispatchTouchEvent(MotionEvent event) {
// If the event should be handled by accessibility focus first.
if (event.isTargetAccessibilityFocus()) {
// We don't have focus or no virtual descendant has it, do not handle the event.
if (!isAccessibilityFocusedViewOrHost()) {
return false;
}
// We have focus and got the event, then use normal event dispatch.
event.setTargetAccessibilityFocus(false);
}
boolean result = false;
if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(event, 0);
}
final int actionMasked = event.getActionMasked();
if (actionMasked == MotionEvent.ACTION_DOWN) {
// Defensive cleanup for new gesture
stopNestedScroll();
}
if (onFilterTouchEventForSecurity(event)) {
//noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
result = true;
}
//View的dispatchTouchEvent方法若是正常流程一定會(huì)調(diào)用onTouchEvent方法
if (!result && onTouchEvent(event)) {
result = true;
}
}
if (!result && mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
}
// Clean up after nested scrolls if this is the end of a gesture;
// also cancel it if we tried an ACTION_DOWN but we didn't want the rest
// of the gesture.
if (actionMasked == MotionEvent.ACTION_UP ||
actionMasked == MotionEvent.ACTION_CANCEL ||
(actionMasked == MotionEvent.ACTION_DOWN && !result)) {
stopNestedScroll();
}
return result;
}
ViewGroup類(lèi)：
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.
//對(duì)于輔助功能的事件處理
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) {
//down事件處理
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
if (!disallowIntercept) {
//disallowIntercept默認(rèn)是false，意味著肯定會(huì)調(diào)用onInterceptTouchEvent方法
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.(檢查是否被取消)
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;
//事件未被取消、攔截
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;
//newTouchTarget為空，并且子View個(gè)數(shù)不為0
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.（由上至下掃描所有能接收該事件的子View）
final ArrayList<View> preorderedList = buildOrderedChildList();
final boolean customOrder = preorderedList == null
&& isChildrenDrawingOrderEnabled();
final View[] children = mChildren;
//遍歷所有的子View，尋找事件處理者
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)) {
//該子View無(wú)法處理，跳出本次循環(huán)繼續(xù)遍歷
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.
//找到事件對(duì)應(yīng)的處理者，推出循環(huán)
newTouchTarget.pointerIdBits |= idBitsToAssign;
break;
}
resetCancelNextUpFlag(child);
//真正分發(fā)事件的方法，若子View為ViewGroup則繼續(xù)則遞歸調(diào)用，重復(fù)此過(guò)程
//若子View就是葉子View那么就調(diào)用該View的dispatchTouchEvent
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();
}
//如果發(fā)現(xiàn)沒(méi)有子元素可以持有該事件
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;
}
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
View child, int desiredPointerIdBits) {
final boolean handled;
//事件被取消
final int oldAction = event.getAction();
if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
event.setAction(MotionEvent.ACTION_CANCEL);
//沒(méi)有子元素
if (child == null) {
//調(diào)用父類(lèi)方法
handled = super.dispatchTouchEvent(event);
} else {
handled = child.dispatchTouchEvent(event);
}
event.setAction(oldAction);
return handled;
}
// Calculate the number of pointers to deliver.（計(jì)算即將傳遞點(diǎn)的數(shù)量）
final int oldPointerIdBits = event.getPointerIdBits();
final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;
// If for some reason we ended up in an inconsistent state where it looks like we
// might produce a motion event with no pointers in it, then drop the event.
//由于某些原因，事件沒(méi)有相應(yīng)的點(diǎn)，那么丟棄該事件
if (newPointerIdBits == 0) {
return false;
}
// If the number of pointers is the same and we don't need to perform any fancy
// irreversible transformations, then we can reuse the motion event for this
// dispatch as long as we are careful to revert any changes we make.
// Otherwise we need to make a copy.
final MotionEvent transformedEvent;
if (newPointerIdBits == oldPointerIdBits) {
if (child == null || child.hasIdentityMatrix()) {
if (child == null) {
//子元素為空，調(diào)用父類(lèi)方法
handled = super.dispatchTouchEvent(event);
} else {
final float offsetX = mScrollX - child.mLeft;
final float offsetY = mScrollY - child.mTop;
event.offsetLocation(offsetX, offsetY);
handled = child.dispatchTouchEvent(event);
event.offsetLocation(-offsetX, -offsetY);
}
//通過(guò)上述判斷，若事件被持有則可以直接返回
return handled;
}
transformedEvent = MotionEvent.obtain(event);
} else {
transformedEvent = event.split(newPointerIdBits);
}
// Perform any necessary transformations and dispatch.
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());
}
handled = child.dispatchTouchEvent(transformedEvent);
}
// Done.
transformedEvent.recycle();
return handled;
}
上述源碼可以看出，View事件分發(fā)機(jī)制，對(duì)于View而言，onTouch、onTouchEvent、onClick優(yōu)先級(jí)依次遞減。
4、優(yōu)缺點(diǎn)分析：
優(yōu)點(diǎn)：
1）將請(qǐng)求與處理者分離，請(qǐng)求事件無(wú)須知道具體由哪個(gè)處理者進(jìn)行處理，降低系統(tǒng)耦合；
2）當(dāng)系統(tǒng)需要增加新的處理者類(lèi)時(shí)，無(wú)須需改原系統(tǒng)代碼，符合開(kāi)閉原則；
缺點(diǎn)：
1）由于請(qǐng)求者并不知道職責(zé)鏈配置規(guī)則，有可能出現(xiàn)請(qǐng)求不被任何處理者處理；
2）職責(zé)鏈建立不當(dāng)，容易出現(xiàn)死循環(huán)；
結(jié)束語(yǔ)
職責(zé)鏈模式在軟件設(shè)計(jì)中經(jīng)常被用到，其使用場(chǎng)景特征比較明顯。在使用職責(zé)鏈模式時(shí)，要?jiǎng)澐趾酶鱾€(gè)處理類(lèi)的條件，切記不要出現(xiàn)死循環(huán)。
