前言

現在基本上都是觸控式螢幕手機，導致很多開發者對焦點並沒有深刻認識，但仍然存在一些非觸屏手機，還有Android TV平臺都與焦點息息相關，這裡就總結下自己看過多篇文章和自己的實踐經驗。先看兩張焦點圖。

焦點基礎及原始碼分析通過兩篇來帶大家深入認識。第一篇從最基本的View，ViewGroup獲取焦點說起。

術語

• 父類 extends後的類叫做父類
• 父View View樹中的父級View稱作父View
• 擁有焦點 表示該View獲取到了焦點，focused狀態為true
• 包含焦點 表示該View或者其子View獲取到了焦點，而自己focused狀態有可能為false，但一定有一個子View focused為true。

View樹中焦點

先看下Activity的View樹結構

其頂層就是DecorView,子View可以是View也可以是ViewGroup,但父View一定是ViewGroup.

獲取焦點的前提

1. View#isFocusable返回true, 如果在觸控模式, 則View#isFocusableInTouchMode也要返回true
2. View及父View必須處於可見
3. 父View不能有FOCUS_BLOCK_DESCENDANTS屬性

View

請求焦點流程

呼叫View#requestFocus方法
View.java

public final boolean requestFocus
 
() {
    return requestFocus(View.FOCUS_DOWN);
}
private boolean requestFocusNoSearch(int direction, Rect previouslyFocusedRect) {
    // need to be focusable
    // 1. 判斷View是否可獲取焦點和是否可見
    if ((mViewFlags & FOCUSABLE) != FOCUSABLE
            || (mViewFlags & VISIBILITY_MASK) != VISIBLE) {
        return
 
 false;
    }
    // 2. 如果為觸控模式下，則FOCUSABLE_IN_TOUCH_MODE必須為true才可以去獲得焦點
    // need to be focusable in touch mode if in touch mode
    if (isInTouchMode() &&
        (FOCUSABLE_IN_TOUCH_MODE != (mViewFlags & FOCUSABLE_IN_TOUCH_MODE))) {
           return false;
    }
    // 3. 判斷所有父View是否有BLOCK子類焦點的屬性，如果沒有才可以去獲取焦點
    // need to not have any parents blocking us
    if (hasAncestorThatBlocksDescendantFocus()) {
        return false;
    }
    // 4 下文分析
    handleFocusGainInternal(direction, previouslyFocusedRect);
    return true;
}

註釋1,2,3就是上文提到的獲取焦點的3個前提。
接下來分析註釋4

void handleFocusGainInternal(@FocusRealDirection int direction, Rect previouslyFocusedRect) {
    // 1. 置FOCUSED標記位
    if ((mPrivateFlags & PFLAG_FOCUSED) == 0) {
        mPrivateFlags |= PFLAG_FOCUSED;
        // 呼叫RootView 
        View oldFocus = (mAttachInfo != null) ? getRootView().findFocus() : null;

        if (mParent != null) {
            // 2. 告知父View自己已聚焦，下文詳細分析
            mParent.requestChildFocus(this, this);
            updateFocusedInCluster(oldFocus, direction);
        }
        if (mAttachInfo != null) {
            // 3. 通知OnGlobalFocusChangeListener
            mAttachInfo.mTreeObserver.dispatchOnGlobalFocusChange(oldFocus, this);
        }
        // 4. 觸發OnFocusChangeListener回撥，只能受到自己View焦點狀態的改變
        onFocusChanged(true, direction, previouslyFocusedRect);
        ...
    }
}

繼續分析註釋2的requestChildFocus，該方法作用通知父View自己已獲取焦點，然後父View去更新自己管轄範圍(直接子View)內焦點狀態，將狀態繼續通知給其父View

ViewGroup.java

@Override
public void requestChildFocus(View child, View focused) {
    if (getDescendantFocusability() == FOCUS_BLOCK_DESCENDANTS) {
        return;
    }

    // 1. 清除自己的焦點，如果有的話
    // Unfocus us, if necessary
    super.unFocus(focused);

    // We had a previous notion of who had focus. Clear it.
    // 2. mFocused用指向自己直接子View擁有或者包含焦點的View，如果存在則讓其失焦
    if (mFocused != child) {
        if (mFocused != null) {
            mFocused.unFocus(focused);
        }
        // 3. mFocused重新指向新的請求焦點的View，
        mFocused = child;
    }
    if (mParent != null) {
        // 4. 繼續通知父View自己已有或者已擁有焦點
        mParent.requestChildFocus(this, focused);
    }
}

這裡解釋下mFocused是什麼，直接看下圖

mFocused標識了自己的直接子View的焦點狀態。

mParent.requestChildFocus方法一級一級向上呼叫通知自己焦點狀態直到頂層View，對於Activity，頂層View就是DecorView，然後繼續呼叫到所有View的管理類ViewRootImpl的requestChildFocus的方法，其進行整個介面重繪。

ViewRoolImpl.java

@Override
public void requestChildFocus(View child, View focused) {
    checkThread();
    // 會執行performTraversals
    scheduleTraversals();
}
// 介面重繪
private void performTraversals() {
    ...
    performMeasure(); // 回撥onMeasue()
    ...
    performLayout(); // 回撥onLayout()
    ...
    performDraw(); // 回撥ondraw()
    ...
}

清除焦點

View#clearFocus主動放棄焦點
View.java

public void clearFocus() {
    clearFocusInternal(null, true, true);
}

void clearFocusInternal(View focused, boolean propagate, boolean refocus) {        
    // 1. 改變焦點標誌位
    if ((mPrivateFlags & PFLAG_FOCUSED) != 0) {
        mPrivateFlags &= ~PFLAG_FOCUSED;
        if (propagate && mParent != null) {
        // 2. 通知父view當前View放棄焦點,和requestChildFocus相反
            mParent.clearChildFocus(this);
        }
        // 3. 觸發OnFocusChangeListener回撥
        onFocusChanged(false, 0, null);
        refreshDrawableState();
        // 4. rootViewRequestFocus()呼叫當前View樹的根View(rootView)的requestFocus方法
        if (propagate && (!refocus || !rootViewRequestFocus())) {
            notifyGlobalFocusCleared(this);
        }
    }
}

ViewGroup

請求焦點

也是呼叫requestFocus()方法

ViewGroup.java

@Override
public boolean requestFocus(int direction, Rect previouslyFocusedRect) {
    int descendantFocusability = getDescendantFocusability();
    switch (descendantFocusability) {
        // 1. 如果BLOCK子View焦點，則直接走當前View請求焦點的流程
        case FOCUS_BLOCK_DESCENDANTS:
            return super.requestFocus(direction, previouslyFocusedRect);
        case FOCUS_BEFORE_DESCENDANTS: {
            // 2. 如果為BEFORE子View，則先走View請求焦點流程，如果沒有得到焦點則走下個方法onRequestFocusInDescendants方法，其根據direction查詢所有子View去請求焦點
            final boolean took = super.requestFocus(direction, previouslyFocusedRect);
            return took ? took : onRequestFocusInDescendants(direction, previouslyFocusedRect);
        }
        case FOCUS_AFTER_DESCENDANTS: {
            // 3. AFTER子View，和BEFORE相反，先遍歷所有子View請求焦點，如果沒有請求到，則走當前View請求焦點流程
            final boolean took = onRequestFocusInDescendants(direction, previouslyFocusedRect);
            return took ? took : super.requestFocus(direction, previouslyFocusedRect);
        }
    }
}

清除焦點

ViewGroup.java

@Override
public void clearFocus() {
    if (mFocused == null) {
        // mFocused為null，則子View無焦點，則清除自己焦點
        super.clearFocus();
    } else {
        // mFocused置為null，並使其清除焦點
        View focused = mFocused;
        mFocused = null;
        focused.clearFocus();
    }
}

總結

介面是由View樹組成，父View通過mFocused來標識直接子View的焦點狀態，當子View焦點狀態改變會通知其父View，如果該ViewGroup之前有擁有焦點的View，父View會清除其焦點，並更新mFocused為新View，然後繼續將自己狀態通知其父View，這樣一級一級更新，整個View樹焦點狀態保持最新，最後由ViewRootImpl更新介面

按D-pad(遙控器上下左右就是)鍵盤，焦點如何移動，原始碼分析盡在下篇:)Android8.0 焦點處理流程(二)