Android焦点事件分发与传递机制

如果您对TouchEvent事件分发机制不太了解的，可以参考我的这篇文章——安卓TounchEvent事件分发机制。

问题：TV端焦点满天飞，如何解决和处理？

记得初入TV开发，以为很简单。TV的这些界面与布局太简单了，分分钟就可以把页面搭建出来，处理好，然后就没有然后了。。。。

下面我们就从源码来带大家进行安卓TV焦点事件的传递

这里先给出Android系统View的绘制流程：

依次执行View类里面的如下三个方法：

• measure(int ,int) :测量View的大小
• layout(int ,int ,int ,int) ：设置子View的位置
• draw(Canvas) ：绘制View内容到Canvas画布上

ViewRootImpl的主要作用如下（此处不多讲，如有意图，看源码）：

• A：链接WindowManager和DecorView的纽带，更广一点可以说是Window和View之间的纽带。
• B：完成View的绘制过程，包括measure、layout、draw过程。
• C：向DecorView分发收到的用户发起的event事件，如按键，触屏等事件。

ViewRootImpl不再多余叙述，进入正题：

Android焦点分发的主要方法以及拦截方法的讲解。

在RootViewImpl中的函数通道是各种策略（InputStage）的组合，各策略负责的任务不同，如SyntheticInputStage、ViewPostImeInputStage、NativePostImeInputStage等等，这些策略以链表结构结构起来，当一个策略者没有消费事件时，就传递个下一个策略者。其中触摸和按键事件由ViewPostImeInputStage处理。

 @Override
        protected int onProcess(QueuedInputEvent q) {
            if (q.mEvent instanceof KeyEvent) {
                return processKeyEvent(q);//如果是按键事件走此处，处理按键和焦点问题了
            } else {
                final int source = q.mEvent.getSource();
                if ((source & InputDevice.SOURCE_CLASS_POINTER) != 0) {
                    return processPointerEvent(q);//如果是触摸事件走此处
                } else if ((source & InputDevice.SOURCE_CLASS_TRACKBALL) != 0) {
                    return processTrackballEvent(q);
                } else {
                    return processGenericMotionEvent(q);
                }
            }
        }

processKeyEvent(QueuedInputEvent q)源码如下：

   @Override
        protected void onDeliverToNext(QueuedInputEvent q) {
            if (mUnbufferedInputDispatch
                    && q.mEvent instanceof MotionEvent
                    && ((MotionEvent)q.mEvent).isTouchEvent()
                    && isTerminalInputEvent(q.mEvent)) {
                mUnbufferedInputDispatch = false;
                scheduleConsumeBatchedInput();
            }
            super.onDeliverToNext(q);
        }

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

            // Deliver the key to the view hierarchy.
            if (mView.dispatchKeyEvent(event)) {
                return FINISH_HANDLED;
            }

            if (shouldDropInputEvent(q)) {
                return FINISH_NOT_HANDLED;
            }

            // If the Control modifier is held, try to interpret the key as a shortcut.
            if (event.getAction() == KeyEvent.ACTION_DOWN
                    && event.isCtrlPressed()
                    && event.getRepeatCount() == 0
                    && !KeyEvent.isModifierKey(event.getKeyCode())) {
                if (mView.dispatchKeyShortcutEvent(event)) {
                    return FINISH_HANDLED;
                }
                if (shouldDropInputEvent(q)) {
                    return FINISH_NOT_HANDLED;
                }
            }

            // Apply the fallback event policy.
            if (mFallbackEventHandler.dispatchKeyEvent(event)) {
                return FINISH_HANDLED;
            }
            if (shouldDropInputEvent(q)) {
                return FINISH_NOT_HANDLED;
            }

            // Handle automatic focus changes.
            if (event.getAction() == KeyEvent.ACTION_DOWN) {
                int direction = 0;
                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();
                    if (focused != null) {
                        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);
                            }
                            if (v.requestFocus(direction, mTempRect)) {
                                playSoundEffect(SoundEffectConstants
                                        .getContantForFocusDirection(direction));
                                return FINISH_HANDLED;
                            }
                        }

                        // Give the focused view a last chance to handle the dpad key.
                        if (mView.dispatchUnhandledMove(focused, direction)) {
                            return FINISH_HANDLED;
                        }
                    } else {
                        // find the best view to give focus to in this non-touch-mode with no-focus
                        View v = focusSearch(null, direction);
                        if (v != null && v.requestFocus(direction)) {
                            return FINISH_HANDLED;
                        }
                    }
                }
            }
            return FORWARD;
        }

进入源码讲解：

(1) 首先由dispatchKeyEvent进行焦点的分发

如果dispatchKeyEvent方法返回true，那么下面的焦点查找步骤就不会继续了。

dispatchKeyEvent方法返回true代表事件（包括焦点和按键）被消费了。

dispatchKeyEvent(event)如果不了解，看我上一篇文章安卓TounchEvent事件分发机制。

mView的dispatchKeyEvent方法，

mView是是Activity的顶层容器DecorView，它是一FrameLayout。

所以这里的dispatchKeyEvent方法应该执行的是ViewGroup的dispatchKeyEvent()方法，而不是View的dispatchKeyEvent方法。

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

        if ((mPrivateFlags & (PFLAG_FOCUSED | PFLAG_HAS_BOUNDS))
                == (PFLAG_FOCUSED | PFLAG_HAS_BOUNDS)) {
            if (super.dispatchKeyEvent(event)) {
                return true;
            }
        } else if (mFocused != null && (mFocused.mPrivateFlags & PFLAG_HAS_BOUNDS)
                == PFLAG_HAS_BOUNDS) {
            if (mFocused.dispatchKeyEvent(event)) {
                return true;
            }
        }

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

ViewGroup的dispatchKeyEvent简略执行流程

首先ViewGroup会执行父类的dispatchKeyEvent方法，如果返回true那么父类的dispatchKeyEvent方法就会返回true，也就代表父类消费了该焦点事件，那么焦点事件自然就不会往下进行分发。

然后ViewGroup会判断mFocused这个view是否为空，如果为空就会****return false，焦点继续往下传递；如果不为空，那就会return mFocused的dispatchKeyEvent方法返回的结果。这个mFocused是ViewGroup中当前获取焦点的子View，这个可以从requestChildFocus方法中得到答案。

requestChildFocus()的源码如下：

   @Override
    public void requestChildFocus(View child, View focused) {
        if (DBG) {
            System.out.println(this + " requestChildFocus()");
        }
        if (getDescendantFocusability() == FOCUS_BLOCK_DESCENDANTS) {
            return;
        }

        // Unfocus us, if necessary
        super.unFocus(focused);

        // We had a previous notion of who had focus. Clear it.
        if (mFocused != child) {
            if (mFocused != null) {
                mFocused.unFocus(focused);
            }

            mFocused = child;
        }
        if (mParent != null) {
            mParent.requestChildFocus(this, focused);
        }
    }

居然有这个彩蛋？

View的dispatchKeyEvent简略执行流程

public boolean dispatchKeyEvent(KeyEvent event) {
        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onKeyEvent(event, 0);
        }

        // Give any attached key listener a first crack at the event.
        //noinspection SimplifiableIfStatement
        ListenerInfo li = mListenerInfo;
        if (li != null && li.mOnKeyListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
                && li.mOnKeyListener.onKey(this, event.getKeyCode(), event)) {
            return true;
        }

        if (event.dispatch(this, mAttachInfo != null
                ? mAttachInfo.mKeyDispatchState : null, this)) {
            return true;
        }

        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
        }
        return false;
    }

要修改ViewGroup焦点事件的分发：

• 重写view的dispatchKeyEvent方法
• 给某个子view设置onKeyListener监听

焦点没有被dispatchKeyEvent拦截的情况下的继续代码中的处理过程，还是进入ViewRootImpl源码

            // Handle automatic focus changes.
            if (event.getAction() == KeyEvent.ACTION_DOWN) {
                int direction = 0;
                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();
                    if (focused != null) {
                        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);
                            }
                            if (v.requestFocus(direction, mTempRect)) {
                                playSoundEffect(SoundEffectConstants
                                        .getContantForFocusDirection(direction));
                                return FINISH_HANDLED;
                            }
                        }

                        // Give the focused view a last chance to handle the dpad key.
                        if (mView.dispatchUnhandledMove(focused, direction)) {
                            return FINISH_HANDLED;
                        }
                    } else {
                        // find the best view to give focus to in this non-touch-mode with no-focus
                        View v = focusSearch(null, direction);
                        if (v != null && v.requestFocus(direction)) {
                            return FINISH_HANDLED;
                        }
                    }
                }
            }

dispatchKeyEvent方法返回false后，先得到按键的方向direction一个int值。direction值是后面来进行焦点查找的。

接着会调用DecorView的findFocus()方法一层一层往下查找已经获取焦点的子View。

DecorView则是PhoneWindow类的一个内部类，继承于FrameLayout，由此可知它是一个ViewGroup。

那么，DecroView到底充当了什么样的角色呢？

其实，DecorView是整个ViewTree的最顶层View，它是一个FrameLayout布局，代表了整个应用的界面。在该布局下面，有标题view和内容view这两个子元素。

 @Override
    public View findFocus() {
        if (DBG) {
            System.out.println("Find focus in " + this + ": flags="
                    + isFocused() + ", child=" + mFocused);
        }

        if (isFocused()) {
            return this;
        }

        if (mFocused != null) {
            return mFocused.findFocus();
        }
        return null;
    }

View的findFocus方法

 /**
     * 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;
    }

View的hasFocus()方法和isFocused()方法对比

Stackoverflow解释来了：

hasFocus() is different from isFocused(). hasFocus() == true means that the View or one of its descendants is focused. If you look closely, there’s a chain of hasFocused Views till you reach the View that isFocused.

 /**
     * Returns true if this view has focus itself, 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() {
        return (mPrivateFlags & PFLAG_FOCUSED) != 0;
    }

  /**
     * 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;
    }

接着，如果mView.findFocus()方法返回的mFocused不为空，说明找到了当前获取焦点的view（mFocused），接着focusSearch会把direction（遥控器按键按下的方向）作为参数，找到特定方向下一个将要获取焦点的view，最后如果该view不为空，那么就让该view获取焦点。

我们来看一下focusSearch方法的源码以及具体实现。

 @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;
    }

focusSearch其实是一层一层地网上调用父View的focusSearch方法，直到当前view是根布局（isRootNamespace()方法），通过注释可以知道focusSearch最终会调用DecorView的focusSearch方法。而DecorView的focusSearch方法找到的焦点view是通过FocusFinder来找到的。

FocusFinder是什么？

根据给定的按键方向，通过当前的获取焦点的View，查找下一个获取焦点的view这样算法的类。焦点没有被拦截的情况下，Android焦点的查找最终都是通过FocusFinder类来实现的。

FocusFinder是如何通过findNextFocus方法寻找焦点的？

 public final View findNextFocus(ViewGroup root, View focused, int direction) {
        return findNextFocus(root, focused, null, direction);
    }

     private View findNextFocus(ViewGroup root, View focused, Rect focusedRect, int direction) {
        View next = null;
        if (focused != null) {
            next = findNextUserSpecifiedFocus(root, focused, direction);
        }
        if (next != null) {
            return next;
        }
        ArrayList<View> focusables = mTempList;
        try {
            focusables.clear();
            root.addFocusables(focusables, direction);
            if (!focusables.isEmpty()) {
                next = findNextFocus(root, focused, focusedRect, direction, focusables);
            }
        } finally {
            focusables.clear();
        }
        return next;
    }

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);
            root.offsetDescendantRectToMyCoords(focused, focusedRect);
        } else {
            if (focusedRect == null) {
                focusedRect = mFocusedRect;
                // make up a rect at top left or bottom right of root
                switch (direction) {
                    case View.FOCUS_RIGHT:
                    case View.FOCUS_DOWN:
                        setFocusTopLeft(root, focusedRect);
                        break;
                    case View.FOCUS_FORWARD:
                        if (root.isLayoutRtl()) {
                            setFocusBottomRight(root, focusedRect);
                        } else {
                            setFocusTopLeft(root, focusedRect);
                        }
                        break;

                    case View.FOCUS_LEFT:
                    case View.FOCUS_UP:
                        setFocusBottomRight(root, focusedRect);
                        break;
                    case View.FOCUS_BACKWARD:
                        if (root.isLayoutRtl()) {
                            setFocusTopLeft(root, focusedRect);
                        } else {
                            setFocusBottomRight(root, focusedRect);
                        break;
                    }
                }
            }
        }

    private View findNextUserSpecifiedFocus(ViewGroup root, View focused, int direction) {
        // check for user specified next focus
        View userSetNextFocus = focused.findUserSetNextFocus(root, direction);
        if (userSetNextFocus != null && userSetNextFocus.isFocusable()
                && (!userSetNextFocus.isInTouchMode()
                        || userSetNextFocus.isFocusableInTouchMode())) {
            return userSetNextFocus;
        }
        return null;
    }

FocusFinder类通过findNextFocus来找焦点的。一层一层往寻找，后面会执行findNextUserSpecifiedFocus()方法，这个方法会执行focused(即当前获取焦点的View)的findUserSetNextFocus方法，如果该方法返回的View不为空，

且isFocusable = true && isInTouchMode() = true的话。

FocusFinder找到的焦点就是findNextUserSpecifiedFocus()返回的View。

findNextFocus会优先根据XML里设置的下一个将获取焦点的View的ID值来寻找将要获取焦点的View。

 View findUserSetNextFocus(View root, @FocusDirection int direction) {
        switch (direction) {
            case FOCUS_LEFT:
                if (mNextFocusLeftId == View.NO_ID) return null;
                return findViewInsideOutShouldExist(root, mNextFocusLeftId);
            case FOCUS_RIGHT:
                if (mNextFocusRightId == View.NO_ID) return null;
                return findViewInsideOutShouldExist(root, mNextFocusRightId);
            case FOCUS_UP:
                if (mNextFocusUpId == View.NO_ID) return null;
                return findViewInsideOutShouldExist(root, mNextFocusUpId);
            case FOCUS_DOWN:
                if (mNextFocusDownId == View.NO_ID) return null;
                return findViewInsideOutShouldExist(root, mNextFocusDownId);
            case FOCUS_FORWARD:
                if (mNextFocusForwardId == View.NO_ID) return null;
                return findViewInsideOutShouldExist(root, mNextFocusForwardId);
            case FOCUS_BACKWARD: {
                if (mID == View.NO_ID) return null;
                final int id = mID;
                return root.findViewByPredicateInsideOut(this, new Predicate<View>() {
                    @Override
                    public boolean apply(View t) {
                        return t.mNextFocusForwardId == id;
                    }
                });
            }
        }
        return null;
    }

焦点事件分发步骤：

• DecorView会调用dispatchKey一层一层进行焦点的分发，如果dispatchKeyEvent方法返回true的话，那么焦点或者按键事件就不会往下分发了。
• 如果你想拦截某个子View，对其设置OnKeyListener进行焦点的拦截。
• 如果焦点没有被拦截的话，那么焦点就会交给系统来处理,还是会继续分发，直到找到那个获取焦点的View
• Android底层先会记录按键的方向，后面DecorView会一层一层往下调用findFocus方法找到当前获取焦点的View
• 后面系统又会根据按键的方向，执行focusSearch方法来寻找下一个将要获取焦点的View
• focusSearch内部其实是通过FocusFinder来查找焦点的。FocusFinder会优先通过View在XML布局设置的下一个焦点的ID来查找焦点。
• 最终如果找到将要获取焦点的View，就让其requestFocus。如果请求无效，将其放在onWindowFocusChanged（）这个方法中去请求。这是在Activity寻找到焦点的时候。

我的前一篇文章，主要是介绍了TouchEvent事件分发机制，省略了焦点分发传递机制的代码，这篇文章与此相反。如果将两个结合起来，太繁杂，冗长了。分开反而有利于您的理解。至此，事件分发机制，你也了解的差不多了，給个粉吧！