Android TV開發(fā)按鍵與焦點(diǎn)深入分析（三）--按鍵事件轉(zhuǎn)換成焦點(diǎn)移動(dòng)的過(guò)程

上兩篇文章分別單獨(dú)分析了KeyEvent在View樹中分發(fā)和View獲得焦點(diǎn)的過(guò)程炊汤，實(shí)際上這兩個(gè)并不是獨(dú)立的躬贡，當(dāng)我們按下按鍵的時(shí)候會(huì)發(fā)現(xiàn)如果我們不攔截按鍵事件抵拘，按鍵事件就會(huì)轉(zhuǎn)換成焦點(diǎn)View的切換祖凫，現(xiàn)在就開始分析這個(gè)轉(zhuǎn)換的過(guò)程。

1.ViewRootImpl中的整體過(guò)程

第一篇中提到過(guò)KeyEvent在View樹中分發(fā)是有Boolean返回值的蜕着，代碼注解如下：

View中
/**
    * Dispatch a key event to the next view on the focus path. This path runs
    * from the top of the view tree down to the currently focused view. If this
    * view has focus, it will dispatch to itself. Otherwise it will dispatch
    * the next node down the focus path. This method also fires any key
    * listeners.
    *
    * @param event The key event to be dispatched.
    * @return True if the event was handled, false otherwise.
    */
   public boolean dispatchKeyEvent(KeyEvent event)

返回true代表這個(gè)按鍵事件已經(jīng)被消耗掉了谋竖，false代表還沒被消耗。默認(rèn)返回的是fasle承匣，只有我們?cè)谶@個(gè)過(guò)程中想要攔截蓖乘、處理了按鍵事件，才會(huì)返回true韧骗。
最后會(huì)把這個(gè)結(jié)果向上一層一層地反饋到按鍵事件產(chǎn)生的位置嘉抒，這個(gè)位置就是ViewRootImpl的processKeyEvent方法中，如下：

ViewRootImpl中
private int processKeyEvent(QueuedInputEvent q) {
            final KeyEvent event = (KeyEvent)q.mEvent;

            ......

            // Deliver the key to the view hierarchy.
            if (mView.dispatchKeyEvent(event)) {//View樹中分發(fā)按鍵事件
                return FINISH_HANDLED;//被處理了
            }
            //沒被處理  
            .......
            // Handle automatic focus changes.
            if (event.getAction() == KeyEvent.ACTION_DOWN) {
                if (groupNavigationDirection != 0) {
                    if (performKeyboardGroupNavigation(groupNavigationDirection)) {
                        return FINISH_HANDLED;
                    }
                } else {//尋找焦點(diǎn)View
                    if (performFocusNavigation(event)) {
                        return FINISH_HANDLED;
                    }
                }
            }
}

mView就是DecorView袍暴，是否已被消耗的結(jié)果的終點(diǎn)就是這里些侍。如果是被消耗了就直接返回，沒有政模，那就調(diào)用performFocusNavigation方法岗宣，從方法名字就可以看出這個(gè)方法就是要將按鍵事件轉(zhuǎn)換成焦點(diǎn)的移動(dòng)，方法如下：

ViewRootImpl中
private boolean performFocusNavigation(KeyEvent event) {
            int direction = 0;
            //將按鍵事件的上下左右轉(zhuǎn)換成焦點(diǎn)移動(dòng)方向的上下左右
            switch (event.getKeyCode()) {
                case KeyEvent.KEYCODE_DPAD_LEFT:
                    if (event.hasNoModifiers()) {
                        direction = View.FOCUS_LEFT;
                    }
                    break;
                case KeyEvent.KEYCODE_DPAD_RIGHT:
                    if (event.hasNoModifiers()) {
                        direction = View.FOCUS_RIGHT;
                    }
                    break;
                case KeyEvent.KEYCODE_DPAD_UP:
                    if (event.hasNoModifiers()) {
                        direction = View.FOCUS_UP;
                    }
                    break;
                case KeyEvent.KEYCODE_DPAD_DOWN:
                    if (event.hasNoModifiers()) {
                        direction = View.FOCUS_DOWN;
                    }
                    break;
                case KeyEvent.KEYCODE_TAB:
                    if (event.hasNoModifiers()) {
                        direction = View.FOCUS_FORWARD;
                    } else if (event.hasModifiers(KeyEvent.META_SHIFT_ON)) {
                        direction = View.FOCUS_BACKWARD;
                    }
                    break;
            }
            if (direction != 0) {
                View focused = mView.findFocus();//找出此時(shí)這個(gè)有焦點(diǎn)的View
                if (focused != null) {
                    //調(diào)用它的focusSearch方法淋样，顧名思義尋找這個(gè)方向上的下一個(gè)獲得焦點(diǎn)的View
                    View v = focused.focusSearch(direction);
                    if (v != null && v != focused) {
                        // do the math the get the interesting rect
                        // of previous focused into the coord system of
                        // newly focused view
                        focused.getFocusedRect(mTempRect);
                        if (mView instanceof ViewGroup) {
                            ((ViewGroup) mView).offsetDescendantRectToMyCoords(
                                    focused, mTempRect);
                            ((ViewGroup) mView).offsetRectIntoDescendantCoords(
                                    v, mTempRect);
                        }
                        //調(diào)用它的requestFocus耗式，讓它獲得焦點(diǎn)
                        if (v.requestFocus(direction, mTempRect)) {
                            playSoundEffect(SoundEffectConstants
                                    .getContantForFocusDirection(direction));
                            return true;
                        }
                    }

                    // Give the focused view a last chance to handle the dpad key.
                    if (mView.dispatchUnhandledMove(focused, direction)) {
                        return true;
                    }
                } else {
                    if (mView.restoreDefaultFocus()) {
                        return true;
                    }
                }
            }
            return false;
        }

這個(gè)方法中一氣呵成完成了按鍵事件轉(zhuǎn)換成焦點(diǎn)View變化的全部過(guò)程，可以概括為以下4個(gè)步驟：

將按鍵事件的上下左右轉(zhuǎn)換成焦點(diǎn)移動(dòng)方向的上下左右
找出View樹中有焦點(diǎn)的View
調(diào)用焦點(diǎn)View的focusSearch方法尋找下一個(gè)獲得焦點(diǎn)的View
調(diào)用下一個(gè)獲得焦點(diǎn)的View的requestFocus方法习蓬，讓它獲得焦點(diǎn)

下面分別分析第2纽什、3步的過(guò)程措嵌，第4步請(qǐng)看上一篇的分析

2.尋找有焦點(diǎn)的View

調(diào)用的是View的findFocus方法躲叼，ViewGroup和View的findFoucs方法分別如下：

ViewGroup中
@Override
    public View findFocus() {
        if (isFocused()) {
            return this;
        }
        if (mFocused != null) {
            return mFocused.findFocus();
        }
        return null;
    }    
View中
    public boolean isFocused() {
        return (mPrivateFlags & PFLAG_FOCUSED) != 0;
    }

View中
    public View findFocus() {
        return (mPrivateFlags & PFLAG_FOCUSED) != 0 ? this : null;
    }

尋找的依據(jù)就是上一篇中分析的PFLAG_FOCUSED標(biāo)志位以及ViewGroup的mFocused成員變量，首先是ViewGroup判斷自己是不是有焦點(diǎn)企巢，然后再判斷自己是不是包含了有焦點(diǎn)的子View枫慷，多次按照焦點(diǎn)的路徑遍歷就找出了焦點(diǎn)View。

3.尋找下一個(gè)獲得焦點(diǎn)的View

View的focusSearch方法

此刻已經(jīng)找出了焦點(diǎn)View，需要調(diào)用它的focusSearch去尋找下一個(gè)焦點(diǎn)View或听，View的focusSearch方法如下：

View中
    public View focusSearch(@FocusRealDirection int direction) {
        if (mParent != null) {
            return mParent.focusSearch(this, direction);
        } else {
            return null;
        }
    }

直接調(diào)用了它的父View的方法探孝，如下：

ViewGroup中
@Override
    public View focusSearch(View focused, int direction) {
        if (isRootNamespace()) {
            // root namespace means we should consider ourselves the top of the
            // tree for focus searching; otherwise we could be focus searching
            // into other tabs.  see LocalActivityManager and TabHost for more info.
            return FocusFinder.getInstance().findNextFocus(this, focused, direction);
        } else if (mParent != null) {
            return mParent.focusSearch(focused, direction);
        }
        return null;
    }

ViewGroup重寫了這個(gè)方法，如果自己是RootNamespace那就調(diào)用FocusFinder去尋找View誉裆，但什么時(shí)候isRootNamespace()成立顿颅，我現(xiàn)在還沒遇到過(guò)舔腾，所以一般情況下最后會(huì)調(diào)用到ViewRootImpl的focusSearch方法砰奕，這個(gè)方法如下：

ViewRootImpl中 
@Override
    public View focusSearch(View focused, int direction) {
        checkThread();
        if (!(mView instanceof ViewGroup)) {
            return null;
        }
        return FocusFinder.getInstance().findNextFocus((ViewGroup) mView, focused, direction);
    }

殊途同歸，最后還是調(diào)用了FocusFinder去尋找View尼夺，看來(lái)尋找下一個(gè)焦點(diǎn)View的重任全部都封裝在了這個(gè)FocusFinder類中斩跌。

FocusFinder單例

 private static final ThreadLocal<FocusFinder> tlFocusFinder =
            new ThreadLocal<FocusFinder>() {
                @Override
                protected FocusFinder initialValue() {
                    return new FocusFinder();
                }
            };

    /**
     * Get the focus finder for this thread.
     */
    public static FocusFinder getInstance() {
        return tlFocusFinder.get();
    }

值得注意的是FocusFinder居然是線程單例的绍些，而不是進(jìn)程單例的，這樣做的原因大概是一方面發(fā)揮單例優(yōu)勢(shì)耀鸦，避免頻繁創(chuàng)建FocusFinder柬批，畢竟這是一個(gè)比較基本的功能，節(jié)約資源提高效率袖订；另一方面氮帐，萬(wàn)一其他線程也要調(diào)用FocusFinder去做一些尋找View的事情，如果進(jìn)程單例那不就影響主線程的效率了洛姑？所以線程單例最合適吧揪漩。

添加所有可以獲得焦點(diǎn)的View

言歸正傳，繼續(xù)看它的findNextFocus方法吏口，如下：

FocusFinder中
private View findNextFocus(ViewGroup root, View focused, Rect focusedRect, int direction) {
       View next = null;
       //找出焦點(diǎn)跳轉(zhuǎn)View的范圍
       ViewGroup effectiveRoot = getEffectiveRoot(root, focused);
       if (focused != null) {//找出在xml中指定的該方向的下一個(gè)獲得焦點(diǎn)的View
           next = findNextUserSpecifiedFocus(effectiveRoot, focused, direction);
       }
       if (next != null) {//指定了直接返回
           return next;
       }
       ArrayList<View> focusables = mTempList;
       try {
           focusables.clear();
           //從最頂點(diǎn)奄容，將所有可以獲得焦點(diǎn)的View添加到focusables中
           effectiveRoot.addFocusables(focusables, direction);
           if (!focusables.isEmpty()) {
               //從中找出下一個(gè)可以獲得焦點(diǎn)的View
               next = findNextFocus(effectiveRoot, focused, focusedRect, direction, focusables);
           }
       } finally {
           focusables.clear();
       }
       return next;
   }

首先找出焦點(diǎn)跳轉(zhuǎn)的View的范圍，我這里測(cè)試時(shí)effectiveRoot就是DecorView产徊，也就是整個(gè)View樹中View都在考慮的范圍內(nèi)昂勒。然后調(diào)用findNextUserSpecifiedFocus方法去獲取我們手動(dòng)為這個(gè)View設(shè)置的上下左右的焦點(diǎn)View，對(duì)應(yīng)xml布局中的android:nextFocusXX

            android:nextFocusDown="@id/textView"
            android:nextFocusLeft="@id/textView"
            android:nextFocusRight="@id/textView"
            android:nextFocusUp="@id/textView"

如果設(shè)置了舟铜，那就直接返回設(shè)置的View戈盈，沒有則繼續(xù)尋找，addFocusables方法把View樹中所有可能獲得焦點(diǎn)的View都放進(jìn)了focusables這個(gè)list中谆刨。
ViewGroup中的addFocusables方法如下：

ViewGroup中
    @Override
    public void addFocusables(ArrayList<View> views, int direction, int focusableMode) {
        final int focusableCount = views.size();

        final int descendantFocusability = getDescendantFocusability();
        final boolean blockFocusForTouchscreen = shouldBlockFocusForTouchscreen();
        final boolean focusSelf = (isFocusableInTouchMode() || !blockFocusForTouchscreen);

        if (descendantFocusability == FOCUS_BLOCK_DESCENDANTS) {//攔截了焦點(diǎn)塘娶，只判斷、添加自己
            if (focusSelf) {
                super.addFocusables(views, direction, focusableMode);
            }
            return;
        }

        if (blockFocusForTouchscreen) {
            focusableMode |= FOCUSABLES_TOUCH_MODE;
        }
        //在所有子View之前添加自己到views
        if ((descendantFocusability == FOCUS_BEFORE_DESCENDANTS) && focusSelf) {
            super.addFocusables(views, direction, focusableMode);
        }

        int count = 0;
        final View[] children = new View[mChildrenCount];
        //挑出可見的View
        for (int i = 0; i < mChildrenCount; ++i) {
            View child = mChildren[i];
            if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE) {
                children[count++] = child;
            }
        }
        //對(duì)所有子View排序
        FocusFinder.sort(children, 0, count, this, isLayoutRtl());
        for (int i = 0; i < count; ++i) {//把所有子View按順序添加到views
            children[i].addFocusables(views, direction, focusableMode);
        }

        // When set to FOCUS_AFTER_DESCENDANTS, we only add ourselves if
        // there aren't any focusable descendants.  this is
        // to avoid the focus search finding layouts when a more precise search
        // among the focusable children would be more interesting.
        if ((descendantFocusability == FOCUS_AFTER_DESCENDANTS) && focusSelf
                && focusableCount == views.size()) {
            super.addFocusables(views, direction, focusableMode);
        }
    }

如果ViewGroup攔截焦點(diǎn)痊夭，那就不用再考慮子View了刁岸；如果ViewGroup在子View之前獲得焦點(diǎn)，那就先添加她我，反之后添加虹曙；對(duì)于兄弟View迫横，在添加之前還要對(duì)它們進(jìn)行排序，排序的依據(jù)是從上到下酝碳、從左到右矾踱。

View的addFocusables方法

    public void addFocusables(ArrayList<View> views, @FocusDirection int direction,
            @FocusableMode int focusableMode) {
        if (views == null) {
            return;
        }
        if (!canTakeFocus()) {
            return;
        }
        if ((focusableMode & FOCUSABLES_TOUCH_MODE) == FOCUSABLES_TOUCH_MODE
                && !isFocusableInTouchMode()) {
            return;
        }
        views.add(this);
    }

直接判斷自己是否可以獲得焦點(diǎn)，可以的話就把自己加到views中去疏哗。

到這里所有的可以獲得焦點(diǎn)的View 都被添加到了focusables中去了呛讲，在這個(gè)過(guò)程中與方向還沒有關(guān)系，只是枚舉添加了所有可能的View返奉。

尋找最優(yōu)View

有了focusables列表圣蝎，這時(shí)調(diào)用同名方法findNextFocus在focusables找出最合適的那個(gè)View，方法如下：

private View findNextFocus(ViewGroup root, View focused, Rect focusedRect,
            int direction, ArrayList<View> focusables) {
        if (focused != null) {
            if (focusedRect == null) {
                focusedRect = mFocusedRect;
            }
            // fill in interesting rect from focused
            focused.getFocusedRect(focusedRect);
            //將focused的坐標(biāo)變成rootView下的坐標(biāo)
            root.offsetDescendantRectToMyCoords(focused, focusedRect);
        } else {
           ......
        }

        switch (direction) {
            case View.FOCUS_FORWARD:
            case View.FOCUS_BACKWARD:
                return findNextFocusInRelativeDirection(focusables, root, focused, focusedRect,
                        direction);
            case View.FOCUS_UP:
            case View.FOCUS_DOWN:
            case View.FOCUS_LEFT:
            case View.FOCUS_RIGHT:
     
                return findNextFocusInAbsoluteDirection(focusables, root, focused,
                        focusedRect, direction);
            default:
                throw new IllegalArgumentException("Unknown direction: " + direction);
        }
    }

到這里衡瓶，尋找的才與方向有關(guān)徘公，下面的分析以按右鍵為例，對(duì)應(yīng)的是View.FOCUS_RIGHT哮针，調(diào)用了findNextFocusInAbsoluteDirection方法：

View findNextFocusInAbsoluteDirection(ArrayList<View> focusables, ViewGroup root, View focused,
            Rect focusedRect, int direction) {
        // initialize the best candidate to something impossible
        // (so the first plausible view will become the best choice)
        mBestCandidateRect.set(focusedRect);
        switch(direction) {
            case View.FOCUS_LEFT:
                mBestCandidateRect.offset(focusedRect.width() + 1, 0);
                break;
            case View.FOCUS_RIGHT://向右尋找時(shí)关面，將初始的位置設(shè)為當(dāng)前View的最左邊
                mBestCandidateRect.offset(-(focusedRect.width() + 1), 0);
                break;
            case View.FOCUS_UP:
                mBestCandidateRect.offset(0, focusedRect.height() + 1);
                break;
            case View.FOCUS_DOWN:
                mBestCandidateRect.offset(0, -(focusedRect.height() + 1));
        }

        View closest = null;

        int numFocusables = focusables.size();
        //遍歷所有的可獲得焦點(diǎn)的View
        for (int i = 0; i < numFocusables; i++) {
            View focusable = focusables.get(i);
            //排除自己
            // only interested in other non-root views
            if (focusable == focused || focusable == root) continue;
            //獲得這個(gè)View的rect，并把它調(diào)整到和focusedRect一致
            // get focus bounds of other view in same coordinate system
            focusable.getFocusedRect(mOtherRect);
            root.offsetDescendantRectToMyCoords(focusable, mOtherRect);
            //判斷這個(gè)View是不是比mBestCandidateRect更優(yōu)
            if (isBetterCandidate(direction, focusedRect, mOtherRect, mBestCandidateRect)) {
                //更優(yōu)十厢，那么將它設(shè)置成mBestCandidateRect等太，并將closest賦值
                mBestCandidateRect.set(mOtherRect);
                closest = focusable;
            }
        }
        return closest;
    }

實(shí)際上尋找的過(guò)程就是在比較各個(gè)View的占用區(qū)域的相對(duì)關(guān)系，這里首先設(shè)置了一個(gè)mBestCandidateRect代表最合適的View的區(qū)域蛮放，對(duì)于向右缩抡，是焦點(diǎn)View左邊間距一像素的同等大小的一個(gè)區(qū)域，顯然這是最不合適的區(qū)域包颁，這只是一個(gè)初始值瞻想。然后就開始遍歷所有可能的View，調(diào)用isBetterCandidate方法去判斷娩嚼，這里就不展開這個(gè)方法蘑险，太啰嗦了，用下面的圖代替岳悟。
假如下圖中View1此時(shí)有焦點(diǎn)佃迄，按下右鍵時(shí)會(huì)怎么樣呢？

實(shí)際上判斷的依據(jù)和下圖中的畫的各種虛線有關(guān)

foucs_right

按下右鍵贵少，初始的最佳區(qū)域就是紅色虛線框的位置呵俏，顯示是最不適合的，然后再遍歷所有的可能的View滔灶，滿足以下兩點(diǎn)才可以擊敗紅色虛線框的位置成為待選的View：

以紫色虛線作為標(biāo)準(zhǔn)普碎，View的右邊線必須在紫色虛線的右邊
以黑色虛線作為標(biāo)準(zhǔn)，View的左邊線必須在黑色虛線的右邊

顯然View2和View3都滿足宽气，這時(shí)就需要進(jìn)一步的比較了随常，進(jìn)一步比較需要參考View的上下兩邊，滿足以下兩點(diǎn)的最優(yōu)：

View的下邊線在上面的那條藍(lán)色虛線之下
View的上邊線在下面的那條藍(lán)色虛線之上

也就是這個(gè)View的與焦點(diǎn)View在上下兩邊上有重疊的區(qū)域就可以了萄涯，所以View3是最優(yōu)的位置绪氛，View3將獲得焦點(diǎn)。
還有一種情況涝影，要是兩個(gè)View都與焦點(diǎn)View在上下兩邊有重疊的區(qū)域枣察，那誰(shuí)更優(yōu)呢？如下：