設計模式：責任鏈模式

時間 2019-12-21

標籤設計模式責任简体版

原文原文鏈接

博客主頁java

介紹：責任鏈模式（Iterator Pattern），是行爲型設計模式之一。segmentfault

定義：使多個對象都有機會處理請求，從而避免了請求的發送者和接受者之間的耦合關係。將這些對象連成一條鏈，並沿着這條鏈傳遞該請求，直到有對象處理它爲止。設計模式

使用場景：
多個對象能夠處理同一個請求，但具體由哪一個對象處理則在運行時動態決定。
在請求處理者不明確的狀況下向多個對象中的一個提交一個請求。
須要動態指定一組對象處理請求。ide

UML類圖

抽象處理者ui

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

    public abstract void handleRequest(String condition);
}

具體的處理者1this

// 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2spa

// 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);
    }
}

客戶端設計

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

        Handler handler2 = new ConcreteHandler2();

        // 設置handler1的下一個節點
        handler1.successor = handler2;

        // 設置handler2的下一個節點
        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，並在其中保持一個對下一個處理節點Handler對象的引用。
ConcreteHandler：具體處理者角色，對請求進行處理，若是不能處理則將該請求轉發給下一個節點上的處理對象。rest

案例簡單實現

下面以小明申請報銷費用爲例。code

假設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目，他沒有權限審批，因而組長就拿着票據去找部門主管，主管一看要報這麼多錢，本身權限內只能批5千如下的費用，完成超出了本身的權限範圍，因而主管又跑去找經理，經理一看二話不說就直接拿着票據奔向了老闆辦公室，由於他只能批一萬如下的費用。

這個例子中，小明只與組長產生了關係，後續具體由誰處理票據，小明並不關心，惟一在意的是報帳的結果。上例中每一類人表明這條鏈上的一個節點，小明是請求的發起者，而老闆是處於鏈條頂端的類。

先聲明一個抽象的領導類

public abstract class Leader {

    protected Leader nextHandler; // 上一級領導處理者

    // 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() + " 權限不足");
        }
    }

    // 處理報帳行爲
    protected abstract void handle(int money);

    // 自身能批覆的額度權限
    protected abstract int limit();
}

在這個抽象的領導類中只作了兩件事，一是定義了兩個接口方法來肯定一個領導者應有的行爲和屬性，二是聲明一個處理報帳請求的方法來肯定當前領導是否有能力處理報帳請求，若是沒有權限，則將該請求轉發給上級領導處理。

各個具體的領導者：

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

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

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

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

// 經理
public class Manager extends Leader {
    @Override
    protected void handle(int money) {
        System.out.println("經理批覆報銷：" + money);
    }

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

// 老闆
public class Boss extends Leader {
    @Override
    protected void handle(int money) {
        System.out.println("老闆批覆報銷：" + money);
    }

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

接下來就是小明同窗發起報帳申請了

public class XiaoMin {
    public static void main(String[] args) {
        // 構造各個領導者
        Leader group = new GroupLeader();
        Leader director = new Director();
        Leader manager = new Manager();
        Leader boss = new Boss();

        // 設置上一級領導處理者對象
        group.nextHandler = director;
        director.nextHandler = manager;
        manager.nextHandler = boss;

        // 發起報帳申請
        group.handleRequest(50000);
    }
}

Android源碼中的責任鏈模式的實現

責任鏈模式在Android源碼中比較相似的實現莫過於對事件的分發處理，每當用戶接觸屏幕時，Android都會將對應的事件包裝成一個事件對象從ViewTree的頂部至上而下地分發傳遞。

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

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) {
            // 這裏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;
        }

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

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

        // 若是有必要的話，爲DOWN事件檢查全部的目標對象
        final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
        TouchTarget newTouchTarget = null;
        boolean alreadyDispatchedToNewTouchTarget = false;
        // 若是事件未被取消未被攔截
        if (!canceled && !intercepted) {

            // 若是有輔助功能的參與，則直接將事件投遞到對應的View，不然將事件分發給全部的子View
            View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus()
                    ? findChildWithAccessibilityFocus() : null;

            // 若是事件爲起始事件
            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;
                // 若是TouchTarget爲空且子View不爲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能夠接收該事件的子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;
                        }

                        // 若是這個子View沒法接收Pointer Event或者這個事件點壓根沒有落在子View的邊界範圍內
                        if (!child.canReceivePointerEvents()
                                || !isTransformedTouchPointInView(x, y, child, null)) {
                            ev.setTargetAccessibilityFocus(false);
                            continue;
                        }

                        // 找到Event該由哪一個子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);
                        // 投遞事件執行觸摸操做
                        // 若是子View仍是一個ViewGroup，則遞歸調用重複次過程
                        // 若是子元素是一個View，那麼則會調用View的dispatchTouchEvent，最終由onTouchEvent處理
                        if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
                            // 子View在其邊界範圍內接收事件
                            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();
                }

                // 若是發現沒有子View能夠持有該次事件
                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方法如何調度子View的dispatchTouchEvent方法的

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

    // 若是事件被取消
    final int oldAction = event.getAction();
    if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
        event.setAction(MotionEvent.ACTION_CANCEL);
        // 若是沒有子View
        if (child == null) {
            // 那麼就直接調用父類的dispatchTouchEvent，注意，這裏的父類終會爲View類
            handled = super.dispatchTouchEvent(event);
        } else {
            // 若是有子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量
    final int oldPointerIdBits = event.getPointerIdBits();
    final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;

    // 若是事件沒有相應的點，那麼就丟棄該事件
    if (newPointerIdBits == 0) {
        return false;
    }

    // 若是事件點的數量一致
    final MotionEvent transformedEvent;
    if (newPointerIdBits == oldPointerIdBits) {
        // 子View爲空或者子View有一個單位矩陣
        if (child == null || child.hasIdentityMatrix()) {
            // 若是子View爲空的狀況
            if (child == null) {
                // 爲空則調用父類dispatchTouchEvent
                handled = super.dispatchTouchEvent(event);
            } else {
                // 不然嘗試獲取xy方向上的偏移量（若是經過scrollTo或scrollBy對子視圖進行滾動的話）
                final float offsetX = mScrollX - child.mLeft;
                final float offsetY = mScrollY - child.mTop;
                // 將MotionEvent進行座標變換
                event.offsetLocation(offsetX, offsetY);

                // 再將變換後的MotionEvent轉遞給子View
                handled = child.dispatchTouchEvent(event);
                // 復位MotionEvent以便以後再次使用
                event.offsetLocation(-offsetX, -offsetY);
            }
            // 若是經過以上的邏輯判斷，當前事件被持有則能夠直接返回
            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事件投遞的遞歸調用就相似一條責任鏈，一旦其尋找到責任者，那麼將由責任者持有並消費掉該次事件，具體地體如今View的 onTouchEvent 方法中返回值的設置，若是 onTouchEvent 返回false，那麼意味着當前View不會是該次事件的責任人，將不會對其持有，若是爲true則相反，此時View會持有該事件並再也不向外傳遞。

若是個人文章對您有幫助，不妨點個贊鼓勵一下(^_^)