View Focus的处理过程及ViewGroup的mFocused字段分析

　　通过上篇的介绍，我们知道在对KeyEvent的处理中有非常重要的一环，那就是KeyEvent在focus view的path上自上而下的分发，

换句话说只有focus的view才有资格参与KeyEvent的处理，所以说focused view在KeyEvent的处理中很重要，我们需要弄清楚明白

focus view是如何设置以及改变的。

　　通过Android官方文档http://developer.android.com/reference/android/view/View.html中关于Focus Handling的介绍，

我们知道framework会根据用户的输入处理常规的focus移动，包括当删除、隐藏或添加新的view时改变focus。一个view有资格获得

focus的前提是isFocusable()方法返回true，你可以通过setFocusable(boolean)方法来设置它。另外当在touch mode下的时候，还

需要isFocusableInTouchMode()也返回true，你也可以通过setFocusableInTouchMode(boolean)来设置它。focus的移动是基于这

样的算法，它尝试在某个给定的方向上找最临近的view，设置它为新的focus。在极个别情况，如果默认的算法不符合你的需求，你也可以

在xml布局文件中通过显式指定nextFocusDown/Left/Right/Up这些属性来表明focus移动的顺序。在运行时刻，你也可以通过调用

View.requestFocus()方法来动态地让某个view获得focus。作为开始，我们先看看这几个具备获得焦点前提的方法，如下：

    /**
     * Returns whether this View is able to take focus.
     *
     * @return True if this view can take focus, or false otherwise.
     * @attr ref android.R.styleable#View_focusable
     */
    @ViewDebug.ExportedProperty(category = "focus")
    public final boolean isFocusable() {
        return FOCUSABLE == (mViewFlags & FOCUSABLE_MASK); // 各种位操作，不熟悉、习惯的同学可以翻本C语言的书看看，
    }                                                      // 这里顺便推荐下《C Primer Plus》,一本足矣，而且里面
                                                           // 有一章是专门介绍bit操作的应用的，非常赞！！！
    /**
     * When a view is focusable, it may not want to take focus when in touch mode.
     * For example, a button would like focus when the user is navigating via a D-pad
     * so that the user can click on it, but once the user starts touching the screen,
     * the button shouldn't take focus
     * @return Whether the view is focusable in touch mode.
     * @attr ref android.R.styleable#View_focusableInTouchMode
     */
    @ViewDebug.ExportedProperty
    public final boolean isFocusableInTouchMode() {
        return FOCUSABLE_IN_TOUCH_MODE == (mViewFlags & FOCUSABLE_IN_TOUCH_MODE);
    }

    /**
     * Set whether this view can receive the focus.
     *
     * Setting this to false will also ensure that this view is not focusable
     * in touch mode.
     *
     * @param focusable If true, this view can receive the focus.
     *
     * @see #setFocusableInTouchMode(boolean)
     * @attr ref android.R.styleable#View_focusable
     */
    public void setFocusable(boolean focusable) {
        if (!focusable) { // 注意：是false的时候，会顺便保证在touch mode下也不能获得focus
            setFlags(0, FOCUSABLE_IN_TOUCH_MODE);
        }
        setFlags(focusable ? FOCUSABLE : NOT_FOCUSABLE, FOCUSABLE_MASK); // 设置FOCUSABLEB位
    }

    /**
     * Set whether this view can receive focus while in touch mode.
     *
     * Setting this to true will also ensure that this view is focusable.
     *
     * @param focusableInTouchMode If true, this view can receive the focus while
     *   in touch mode.
     *
     * @see #setFocusable(boolean)
     * @attr ref android.R.styleable#View_focusableInTouchMode
     */
    public void setFocusableInTouchMode(boolean focusableInTouchMode) {
        // Focusable in touch mode should always be set before the focusable flag
        // otherwise, setting the focusable flag will trigger a focusableViewAvailable()
        // which, in touch mode, will not successfully request focus on this view
        // because the focusable in touch mode flag is not set
        setFlags(focusableInTouchMode ? FOCUSABLE_IN_TOUCH_MODE : 0, FOCUSABLE_IN_TOUCH_MODE);
        if (focusableInTouchMode) { // 如果是true顺便打开FOCUSABLE位
            setFlags(FOCUSABLE, FOCUSABLE_MASK);
        }
    }

    接下来我们就看看本文的重点View.requestFocus()等相关方法，代码如下：

/**
     * Call this to try to give focus to a specific view or to one of its
     * descendants.
     *
     * A view will not actually take focus if it is not focusable ({@link #isFocusable} returns
     * false), or if it is focusable and it is not focusable in touch mode
     * ({@link #isFocusableInTouchMode}) while the device is in touch mode.
     *
     * See also {@link #focusSearch(int)}, which is what you call to say that you
     * have focus, and you want your parent to look for the next one.
     *
     * This is equivalent to calling {@link #requestFocus(int, Rect)} with arguments
     * {@link #FOCUS_DOWN} and <code>null</code>.
     *
     * @return Whether this view or one of its descendants actually took focus.
     */
    public final boolean requestFocus() {
        return requestFocus(View.FOCUS_DOWN);
    }

    /**
     * Call this to try to give focus to a specific view or to one of its
     * descendants and give it a hint about what direction focus is heading.
     *
     * A view will not actually take focus if it is not focusable ({@link #isFocusable} returns
     * false), or if it is focusable and it is not focusable in touch mode
     * ({@link #isFocusableInTouchMode}) while the device is in touch mode.
     *
     * See also {@link #focusSearch(int)}, which is what you call to say that you
     * have focus, and you want your parent to look for the next one.
     *
     * This is equivalent to calling {@link #requestFocus(int, Rect)} with
     * <code>null</code> set for the previously focused rectangle.
     *
     * @param direction One of FOCUS_UP, FOCUS_DOWN, FOCUS_LEFT, and FOCUS_RIGHT
     * @return Whether this view or one of its descendants actually took focus.
     */
    public final boolean requestFocus(int direction) {
        return requestFocus(direction, null);
    }

    /**
     * Call this to try to give focus to a specific view or to one of its descendants
     * and give it hints about the direction and a specific rectangle that the focus
     * is coming from.  The rectangle can help give larger views a finer grained hint
     * about where focus is coming from, and therefore, where to show selection, or
     * forward focus change internally.
     *
     * A view will not actually take focus if it is not focusable ({@link #isFocusable} returns
     * false), or if it is focusable and it is not focusable in touch mode
     * ({@link #isFocusableInTouchMode}) while the device is in touch mode.
     *
     * A View will not take focus if it is not visible.
     *
     * A View will not take focus if one of its parents has
     * {@link android.view.ViewGroup#getDescendantFocusability()} equal to
     * {@link ViewGroup#FOCUS_BLOCK_DESCENDANTS}.
     *
     * See also {@link #focusSearch(int)}, which is what you call to say that you
     * have focus, and you want your parent to look for the next one.
     *
     * You may wish to override this method if your custom {@link View} has an internal
     * {@link View} that it wishes to forward the request to.
     *
     * @param direction One of FOCUS_UP, FOCUS_DOWN, FOCUS_LEFT, and FOCUS_RIGHT
     * @param previouslyFocusedRect The rectangle (in this View's coordinate system)
     *        to give a finer grained hint about where focus is coming from.  May be null
     *        if there is no hint.
     * @return Whether this view or one of its descendants actually took focus.
     */
    public boolean requestFocus(int direction, Rect previouslyFocusedRect) {
        return requestFocusNoSearch(direction, previouslyFocusedRect);
    }

    private boolean requestFocusNoSearch(int direction, Rect previouslyFocusedRect) { // 此方法就是最终被调用的版本
        // need to be focusable
        if ((mViewFlags & FOCUSABLE_MASK) != FOCUSABLE || // 不是FOCUSABLE，即没资格获取焦点
                (mViewFlags & VISIBILITY_MASK) != VISIBLE) { // 或者不是VISIBLE的，都直接返回false，表示请求获取焦点失败
            return false; // 所以除了上文提到的2个获取focus的前提，其实这里的VISIBLE也应该算是第3个前提吧！
        }

        // need to be focusable in touch mode if in touch mode
        if (isInTouchMode() && // 同样在touch mode下，也要检测FOCUSABLE_IN_TOUCH_MODE标志
            (FOCUSABLE_IN_TOUCH_MODE != (mViewFlags & FOCUSABLE_IN_TOUCH_MODE))) {
               return false; // 不满足也直接返回false，表示失败
        }

        // need to not have any parents blocking us
        if (hasAncestorThatBlocksDescendantFocus()) { // parents阻止我们获得焦点的话，我们也只能以失败告终
            return false;
        }
        // 以上重重关卡都通过了，才会走到这里，真正设置focus
        handleFocusGainInternal(direction, previouslyFocusedRect);
        return true;
    }

    /**
     * Give this view focus. This will cause
     * {@link #onFocusChanged(boolean, int, android.graphics.Rect)} to be called.
     *
     * Note: this does not check whether this {@link View} should get focus, it just
     * gives it focus no matter what.  It should only be called internally by framework
     * code that knows what it is doing, namely {@link #requestFocus(int, Rect)}.
     *
     * @param direction values are {@link View#FOCUS_UP}, {@link View#FOCUS_DOWN},
     *        {@link View#FOCUS_LEFT} or {@link View#FOCUS_RIGHT}. This is the direction which
     *        focus moved when requestFocus() is called. It may not always
     *        apply, in which case use the default View.FOCUS_DOWN.
     * @param previouslyFocusedRect The rectangle of the view that had focus
     *        prior in this View's coordinate system.
     */
    void handleFocusGainInternal(int direction, Rect previouslyFocusedRect) {
        if (DBG) {
            System.out.println(this + " requestFocus()");
        }

        if ((mPrivateFlags & PFLAG_FOCUSED) == 0) { // 只有当前View不是focused view时才会发生一系列操作，否则do nothing
            mPrivateFlags |= PFLAG_FOCUSED; // 如果没focus的话，先设置此view的focused标志，isFocused，hasFocus等方法会检测此标志

            View oldFocus = (mAttachInfo != null) ? getRootView().findFocus() : null; // 找到之前的focus

            if (mParent != null) {
                mParent.requestChildFocus(this, this); // 如果有mParent，则将此新focus请求向上传递
            }

            if (mAttachInfo != null) { // callback接口，将focus change事件notify出去
                mAttachInfo.mTreeObserver.dispatchOnGlobalFocusChange(oldFocus, this);
            }

            onFocusChanged(true, direction, previouslyFocusedRect);
            refreshDrawableState();
        }
    }

接着我们看下关于PFLAG_FOCUSED标志位相关的几个方法，如下：

/**
     * Returns true if this view has focus
     *
     * @return True if this view has focus, false otherwise.
     */
    @ViewDebug.ExportedProperty(category = "focus")
    public boolean isFocused() {
        return (mPrivateFlags & PFLAG_FOCUSED) != 0;
    }

    /**
     * Find the view in the hierarchy rooted at this view that currently has
     * focus.
     *
     * @return The view that currently has focus, or null if no focused view can
     *         be found.
     */
    public View findFocus() {
        return (mPrivateFlags & PFLAG_FOCUSED) != 0 ? this : null;
    }

    /**
     * Returns true if this view has focus iteself, or is the ancestor of the
     * view that has focus.
     *
     * @return True if this view has or contains focus, false otherwise.
     */
    @ViewDebug.ExportedProperty(category = "focus")
    public boolean hasFocus() { // 对View来说，hasFocus和isFocus是相同的，ViewGroup类重载了此方法
        return (mPrivateFlags & PFLAG_FOCUSED) != 0;
    }

　　最后，我们看看ViewParent接口(以及其实现ViewGroup)的requestChildFocus()实现，代码如下：

    /**
     * Called when a child of this parent wants focus
     * 
     * @param child The child of this ViewParent that wants focus. This view
     *        will contain the focused view. It is not necessarily the view that
     *        actually has focus.
     * @param focused The view that is a descendant of child that actually has
     *        focus
     */
    public void requestChildFocus(View child, View focused); // parent中的某个child请求获得focus，child要么是focused，
                                                             // 要么是focused的parent
    // 我们可以看到其实现类有ViewGroup、ScrollView等，这里我们看下ViewGroup类的，其他的有兴趣的同学可以自行研究

    @Override
    public void requestChildFocus(View child, View focused) {
        if (DBG) {
            System.out.println(this + " requestChildFocus()");
        }
        if (getDescendantFocusability() == FOCUS_BLOCK_DESCENDANTS) {
            return; // 如果此ViewGroup被设置为阻止任何children获得focus，则直接返回
        }

        // Unfocus us, if necessary
        super.unFocus(); // 先unFocus 此ViewGroup

        // We had a previous notion of who had focus. Clear it.
        if (mFocused != child) { // 如果mFocused不同于传递进来的child，则更新mFocused
            if (mFocused != null) {
                mFocused.unFocus(); // 让旧的放弃focus
            }

            mFocused = child; // 更新mFocused
        }
        if (mParent != null) { // 接着沿着focus path往上传递（递归调用）
            mParent.requestChildFocus(this, focused); // 注意这里的第2个参数，一直是传递进来的focused不变
        }
    }

我们注意到只有ViewGroup才有mFocused字段，表示focus path上的一个节点。我们看看与之相关的代码：

    // The view contained within this ViewGroup that has or contains focus.
    private View mFocused; // 此ViewGroup中的child view，它要么是focused view本身要么包含focused view

    @Override
    void handleFocusGainInternal(int direction, Rect previouslyFocusedRect) { // 此方法重载了View中的
        if (mFocused != null) { // 添加了对mFocused的处理
            mFocused.unFocus(); // 让mFocused unFocus在这种情况下
            mFocused = null;
        }
        super.handleFocusGainInternal(direction, previouslyFocusedRect);
    }

    /**
     * {@inheritDoc}
     */
    public void clearChildFocus(View child) {
        if (DBG) {
            System.out.println(this + " clearChildFocus()");
        }

        mFocused = null; // 清空
        if (mParent != null) { // 将事件告诉parent
            mParent.clearChildFocus(this);
        }
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public void clearFocus() {
        if (DBG) {
            System.out.println(this + " clearFocus()");
        }
        if (mFocused == null) { // 如果没有mFocused，则ViewGroup自身clearFocus
            super.clearFocus();
        } else { // 否则，让mFocused clearFocus，并且重置为null
            View focused = mFocused;
            mFocused = null;
            focused.clearFocus();
        }
    }

    /**
     * {@inheritDoc}
     */
    @Override
    void unFocus() { // 大体同clearFocus，只是调的是unFocus方法
        if (DBG) {
            System.out.println(this + " unFocus()");
        }
        if (mFocused == null) {
            super.unFocus();
        } else {
            mFocused.unFocus();
            mFocused = null;
        }
    }

    /**
     * Returns the focused child of this view, if any. The child may have focus
     * or contain focus.
     *
     * @return the focused child or null.
     */
    public View getFocusedChild() { // 返回这个字段，供客户端代码使用
        return mFocused;
    }

    /**
     * Returns true if this view has or contains focus
     *
     * @return true if this view has or contains focus
     */
    @Override
    public boolean hasFocus() { // ViewGroup自己是focused或者其子、孙后代包含focused view
        return (mPrivateFlags & PFLAG_FOCUSED) != 0 || mFocused != null;
    }

    /*
     * (non-Javadoc)
     *
     * @see android.view.View#findFocus()
     */
    @Override
    public View findFocus() {
        if (DBG) {
            System.out.println("Find focus in " + this + ": flags="
                    + isFocused() + ", child=" + mFocused);
        }

        if (isFocused()) { // 自己是focused，直接返回this
            return this;
        }

        if (mFocused != null) { // 否则，mFocused不为空，则沿着这条线往下继续找
            return mFocused.findFocus();
        }
        return null;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public boolean hasFocusable() {
        if ((mViewFlags & VISIBILITY_MASK) != VISIBLE) {
            return false;
        }

        if (isFocusable()) {
            return true;
        }

        final int descendantFocusability = getDescendantFocusability();
        if (descendantFocusability != FOCUS_BLOCK_DESCENDANTS) {
            final int count = mChildrenCount;
            final View[] children = mChildren;

            for (int i = 0; i < count; i++) {
                final View child = children[i];
                if (child.hasFocusable()) {
                    return true;
                }
            }
        }

        return false;
    }

接着我们看下View自己的unFocus、clearFocus实现，代码如下：

    /**
     * Called internally by the view system when a new view is getting focus.
     * This is what clears the old focus.
     * <p>
     * <b>NOTE:</b> The parent view's focused child must be updated manually
     * after calling this method. Otherwise, the view hierarchy may be left in
     * an inconstent state.
     */
    void unFocus() {
        if (DBG) {
            System.out.println(this + " unFocus()");
        }

        clearFocusInternal(false, false);
    }

    /**
     * Called when this view wants to give up focus. If focus is cleared
     * {@link #onFocusChanged(boolean, int, android.graphics.Rect)} is called.
     * <p>
     * <strong>Note:</strong> When a View clears focus the framework is trying
     * to give focus to the first focusable View from the top. Hence, if this
     * View is the first from the top that can take focus, then all callbacks
     * related to clearing focus will be invoked after wich the framework will
     * give focus to this view.
     * </p>
     */
    public void clearFocus() {
        if (DBG) {
            System.out.println(this + " clearFocus()");
        }

        clearFocusInternal(true, true);
    }

    /**
     * Clears focus from the view, optionally propagating the change up through
     * the parent hierarchy and requesting that the root view place new focus.
     *
     * @param propagate whether to propagate the change up through the parent
     *            hierarchy
     * @param refocus when propagate is true, specifies whether to request the
     *            root view place new focus
     */
    void clearFocusInternal(boolean propagate, boolean refocus) {
        if ((mPrivateFlags & PFLAG_FOCUSED) != 0) { // 如果当前是focused，先清掉PFLAG_FOCUSED位
            mPrivateFlags &= ~PFLAG_FOCUSED;

            if (propagate && mParent != null) {
                mParent.clearChildFocus(this); // 如果向上传播的话，调用parent.clearChildFocus方法
            }

            onFocusChanged(false, 0, null); // 调用callback方法

            refreshDrawableState(); // 刷新drawable状态

            if (propagate && (!refocus || !rootViewRequestFocus())) {
                notifyGlobalFocusCleared(this);
            }
        }
    }

　　通过上一篇的介绍，我们知道KeyEvent的派发就是在view层次结构的focus path上自上而下发生的，具体参见View.dispatchKeyEvent

的方法doc。刚开始我一直不明白这里的focus path是怎么形成的，怎么按着这个链传递的。这里为了帮助大家理解，我举一个典型的例子，

通过例子可以很清楚的看到传递过程。比方说我们的view层次结构是这样的，C是个Button，B是C的parent，LinearLayout，A是B的parent，

FrameLayout。这里我们先假设C、B、A都是有资格且其parent都不阻止它获得焦点，当我们在代码里调用C.requestFocus()时发生的调用

序列如下：

1. --> B.requestChildFocus(C, C); 当此方法发生后产生的结果是：B.mFocused = C;接着产生2调用；

2. --> A.requestChildFocus(B, C); 同样的，当此方法发生后，A.mFocused = B; 接着往上传递直到parent为空时停止。

当C.requestFocus()调用结束时，如果没有各种失败的case发生，那么C就是当前view层次结构中的focus了，也就是C.isFocused()方法

此时会返回true。看到了吗？通过这个递归调用，focus path的链就形成了，从最顶层的A能通过其mFocused字段找到B，从找到的B能通过

其mFocused字段找到C，以此类推。为了加深这个印象，我们最后再看眼ViewGroup.dispatchKeyEvent()方法:

    @Override
    public boolean dispatchKeyEvent(KeyEvent event) {
        /// 2.2.1.1...
        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onKeyEvent(event, 1);
        }

        if ((mPrivateFlags & (PFLAG_FOCUSED | PFLAG_HAS_BOUNDS))
                == (PFLAG_FOCUSED | PFLAG_HAS_BOUNDS)) { // ViewGroup是focused，则优先交给它自己处理
            /// 2.2.1.2. 
            if (super.dispatchKeyEvent(event)) {
                return true;
            }
        } else if (mFocused != null && (mFocused.mPrivateFlags & PFLAG_HAS_BOUNDS)
                == PFLAG_HAS_BOUNDS) { // 否则就沿着mFocused形成的focus path向下传递
            /// 2.2.1.3. 
            if (mFocused.dispatchKeyEvent(event)) {
                return true;
            }
        }

        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onUnhandledEvent(event, 1);
        }
        /// 2.2.1.4. 
        return false;
    }

　　现在再回过头来看这里的逻辑，是不是感觉特别简单呢？那是因为你已经完全弄清楚了mFocused的由来以及各种变化过程。至此view层次

结构中关于focus的变化过程已经全部分析完毕了，enjoy。