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介紹：責(zé)任鏈模式（Iterator Pattern），是行為型設(shè)計模式之一。

定義：使多個對象都有機(jī)會處理請求，從而避免了請求的發(fā)送者和接受者之間的耦合關(guān)系。將這些對象連成一條鏈，并沿著這條鏈傳遞該請求，直到有對象處理它為止。

使用場景：
多個對象可以處理同一個請求，但具體由哪個對象處理則在運行時動態(tài)決定。
在請求處理者不明確的情況下向多個對象中的一個提交一個請求。
需要動態(tài)指定一組對象處理請求。

UML類圖

抽象處理者

// 抽象處理者
public abstract class Handler {
    protected Handler successor;

    public abstract void handleRequest(String condition);
}

具體的處理者1

// 具體的處理者1
public class ConcreteHandler1 extends Handler{
    @Override
    public void handleRequest(String condition) {
        if ("ConcreteHandler1".equalsIgnoreCase(condition)) {
            System.out.println("ConcreteHandler1 handler");
            return;
        }

        successor.handleRequest(condition);
    }
}

具體的處理者2

// 具體的處理者2
public class ConcreteHandler2 extends Handler{
    @Override
    public void handleRequest(String condition) {
        if ("ConcreteHandler2".equalsIgnoreCase(condition)) {
            System.out.println("ConcreteHandler2 handler");
            return;
        }

        successor.handleRequest(condition);
    }
}

客戶端

// 客戶端
public class Client {
    public static void main(String[] args) {
        Handler handler1 = new ConcreteHandler1();

        Handler handler2 = new ConcreteHandler2();

        // 設(shè)置handler1的下一個節(jié)點
        handler1.successor = handler2;

        // 設(shè)置handler2的下一個節(jié)點
        handler2.successor = handler1;

        // 處理請求
        handler1.handleRequest("ConcreteHandler2");
    }
}

Handler：抽象處理者角色，聲明一個請求處理方法，并在其中保持一個對下一個處理節(jié)點Handler對象的引用。
ConcreteHandler：具體處理者角色，對請求進(jìn)行處理，如果不能處理則將該請求轉(zhuǎn)發(fā)給下一個節(jié)點上的處理對象。

案例簡單實現(xiàn)

下面以小明申請報銷費用為例。

假設(shè)小明向組長申請報銷5萬元的費用，組長一看是一筆不小的數(shù)目，他沒有權(quán)限審批，于是組長就拿著票據(jù)去找部門主管，主管一看要報這么多錢，自己權(quán)限內(nèi)只能批5千以下的費用，完成超出了自己的權(quán)限范圍，于是主管又跑去找經(jīng)理，經(jīng)理一看二話不說就直接拿著票據(jù)奔向了老板辦公室，因為他只能批一萬以下的費用。

這個例子中，小明只與組長產(chǎn)生了關(guān)系，后續(xù)具體由誰處理票據(jù)，小明并不關(guān)心，唯一在乎的是報賬的結(jié)果。上例中每一類人代表這條鏈上的一個節(jié)點，小明是請求的發(fā)起者，而老板是處于鏈條頂端的類。

先聲明一個抽象的領(lǐng)導(dǎo)類

public abstract class Leader {

    protected Leader nextHandler; // 上一級領(lǐng)導(dǎo)處理者

    // 處理報銷請求
    public void handleRequest(int money) {
        if (money < limit()) {
            handle(money);
            return;
        }


        if (nextHandler != null) {
            nextHandler.handleRequest(money);
        } else {
            System.out.println(this.getClass().getCanonicalName() + " 權(quán)限不足");
        }
    }

    // 處理報賬行為
    protected abstract void handle(int money);

    // 自身能批復(fù)的額度權(quán)限
    protected abstract int limit();
}

在這個抽象的領(lǐng)導(dǎo)類中只做了兩件事，一是定義了兩個接口方法來確定一個領(lǐng)導(dǎo)者應(yīng)有的行為和屬性，二是聲明一個處理報賬請求的方法來確定當(dāng)前領(lǐng)導(dǎo)是否有能力處理報賬請求，如果沒有權(quán)限，則將該請求轉(zhuǎn)發(fā)給上級領(lǐng)導(dǎo)處理。

各個具體的領(lǐng)導(dǎo)者：

// 組長
public class GroupLeader extends Leader {
    @Override
    protected void handle(int money) {
        System.out.println("組長批復(fù)報銷：" + money);
    }

    @Override
    protected int limit() {
        return 1000;
    }
}

// 主管
public class Director extends Leader {
    @Override
    protected void handle(int money) {
        System.out.println("主管批復(fù)報銷：" + money);
    }

    @Override
    protected int limit() {
        return 5000;
    }
}

// 經(jīng)理
public class Manager extends Leader {
    @Override
    protected void handle(int money) {
        System.out.println("經(jīng)理批復(fù)報銷：" + money);
    }

    @Override
    protected int limit() {
        return 10000;
    }
}

// 老板
public class Boss extends Leader {
    @Override
    protected void handle(int money) {
        System.out.println("老板批復(fù)報銷：" + money);
    }

    @Override
    protected int limit() {
        return Integer.MAX_VALUE;
    }
}

接下來就是小明同學(xué)發(fā)起報賬申請了

public class XiaoMin {
    public static void main(String[] args) {
        // 構(gòu)造各個領(lǐng)導(dǎo)者
        Leader group = new GroupLeader();
        Leader director = new Director();
        Leader manager = new Manager();
        Leader boss = new Boss();

        // 設(shè)置上一級領(lǐng)導(dǎo)處理者對象
        group.nextHandler = director;
        director.nextHandler = manager;
        manager.nextHandler = boss;

        // 發(fā)起報賬申請
        group.handleRequest(50000);
    }
}

Android源碼中的責(zé)任鏈模式的實現(xiàn)

責(zé)任鏈模式在Android源碼中比較類似的實現(xiàn)莫過于對事件的分發(fā)處理，每當(dāng)用戶接觸屏幕時，Android都會將對應(yīng)的事件包裝成一個事件對象從ViewTree的頂部至上而下地分發(fā)傳遞。

接下來主要來看看ViewGroup中是如何將事件派發(fā)到子View的。ViewGroup中執(zhí)行事件派發(fā)的方法是dispatchTouchEvent，在該方法中對事件進(jìn)行了同一的分發(fā)：

public boolean dispatchTouchEvent(MotionEvent ev) {
    if (mInputEventConsistencyVerifier != null) {
        mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
    }

    // 對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;

        // 處理原始的DOWN事件
        if (actionMasked == MotionEvent.ACTION_DOWN) {
            // 這里主要是在新事件開始時處理完上一個事件
            cancelAndClearTouchTargets(ev);
            resetTouchState();
        }

        // 檢查事件攔截
        final boolean intercepted;
        if (actionMasked == MotionEvent.ACTION_DOWN
                || mFirstTouchTarget != null) {
            final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
            if (!disallowIntercept) {
                intercepted = onInterceptTouchEvent(ev);
                // 恢復(fù)事件防止其改變
                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;
        }

        // 如果事件被攔截了，則進(jìn)行正常的事件分發(fā)
        if (intercepted || mFirstTouchTarget != null) {
            ev.setTargetAccessibilityFocus(false);
        }

        // 檢查事件是否取消
        final boolean canceled = resetCancelNextUpFlag(this)
                || actionMasked == MotionEvent.ACTION_CANCEL;

        // 如果有必要的話，為DOWN事件檢查所有的目標(biāo)對象
        final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
        TouchTarget newTouchTarget = null;
        boolean alreadyDispatchedToNewTouchTarget = false;
        // 如果事件未被取消未被攔截
        if (!canceled && !intercepted) {

            // 如果有輔助功能的參與，則直接將事件投遞到對應(yīng)的View，否則將事件分發(fā)給所有的子View
            View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus()
                    ? findChildWithAccessibilityFocus() : null;

            // 如果事件為起始事件
            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;
                // 如果TouchTarget為空且子View不為0
                if (newTouchTarget == null && childrenCount != 0) {
                    final float x = ev.getX(actionIndex);
                    final float y = ev.getY(actionIndex);
                    // 由上至下去尋找一個可以接收該事件的子View
                    final ArrayList<View> preorderedList = buildTouchDispatchChildList();
                    final boolean customOrder = preorderedList == null
                            && isChildrenDrawingOrderEnabled();
                    final View[] children = mChildren;
                    // 遍歷子View
                    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;
                        }

                        // 如果這個子View無法接收Pointer Event或者這個事件點壓根沒有落在子View的邊界范圍內(nèi)
                        if (!child.canReceivePointerEvents()
                                || !isTransformedTouchPointInView(x, y, child, null)) {
                            ev.setTargetAccessibilityFocus(false);
                            continue;
                        }

                        // 找到Event該由哪個子View持有
                        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);
                        // 投遞事件執(zhí)行觸摸操作
                        // 如果子View還是一個ViewGroup，則遞歸調(diào)用重復(fù)次過程
                        // 如果子元素是一個View，那么則會調(diào)用View的dispatchTouchEvent，最終由onTouchEvent處理
                        if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
                            // 子View在其邊界范圍內(nèi)接收事件
                            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)沒有子View可以持有該次事件
                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;
}

再看看dispatchTransformedTouchEvent方法如何調(diào)度子View的dispatchTouchEvent方法的

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
                                              View child, int desiredPointerIdBits) {
    final boolean handled;

    // 如果事件被取消
    final int oldAction = event.getAction();
    if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
        event.setAction(MotionEvent.ACTION_CANCEL);
        // 如果沒有子View
        if (child == null) {
            // 那么就直接調(diào)用父類的dispatchTouchEvent，注意，這里的父類終會為View類
            handled = super.dispatchTouchEvent(event);
        } else {
            // 如果有子View則傳遞CANCEL事件
            handled = child.dispatchTouchEvent(event);
        }
        event.setAction(oldAction);
        return handled;
    }

    // 計算即將被傳遞的點的數(shù)量
    final int oldPointerIdBits = event.getPointerIdBits();
    final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;

    // 如果事件沒有相應(yīng)的點，那么就丟棄該事件
    if (newPointerIdBits == 0) {
        return false;
    }

    // 如果事件點的數(shù)量一致
    final MotionEvent transformedEvent;
    if (newPointerIdBits == oldPointerIdBits) {
        // 子View為空或者子View有一個單位矩陣
        if (child == null || child.hasIdentityMatrix()) {
            // 如果子View為空的情況
            if (child == null) {
                // 為空則調(diào)用父類dispatchTouchEvent
                handled = super.dispatchTouchEvent(event);
            } else {
                // 否則嘗試獲取xy方向上的偏移量（如果通過scrollTo或scrollBy對子視圖進(jìn)行滾動的話）
                final float offsetX = mScrollX - child.mLeft;
                final float offsetY = mScrollY - child.mTop;
                // 將MotionEvent進(jìn)行坐標(biāo)變換
                event.offsetLocation(offsetX, offsetY);

                // 再將變換后的MotionEvent轉(zhuǎn)遞給子View
                handled = child.dispatchTouchEvent(event);
                // 復(fù)位MotionEvent以便之后再次使用
                event.offsetLocation(-offsetX, -offsetY);
            }
            // 如果通過以上的邏輯判斷，當(dāng)前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;
}

ViewGroup事件投遞的遞歸調(diào)用就類似一條責(zé)任鏈，一旦其尋找到責(zé)任者，那么將由責(zé)任者持有并消費掉該次事件，具體地體現(xiàn)在View的 onTouchEvent 方法中返回值的設(shè)置，如果 onTouchEvent 返回false，那么意味著當(dāng)前View不會是該次事件的責(zé)任人，將不會對其持有，如果為true則相反，此時View會持有該事件并不再向外傳遞。

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