Android按键事件处理流程 -- 从事件被派发到View层次结构的根节点DecorView开始分析

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http://www.cxyclub.cn/n/48237/

        刚接触Android开发的时候，对touch、key事件的处理总是一知半解，一会是Activity里的方法，一会是各种View中的，自己始终不清楚到底哪个在先哪个在后，总之对整个处理流程没能很好的把握。每次写这部分代码的时候都有些心虚，因为我不是很清楚什么时候、以什么样的顺序被调用，大都是打下log看看，没问题就算ok了。但随着时间流逝，这种感觉一直折磨着我。期间也在网上搜索了相关资料，但总感觉不是那么令人满意。自打开始研究Android源码起，这部分内容的分析早就被列在我的TODO list上了。因为弄懂这部分处理逻辑对明明白白地写android程序实在是太重要了，所以今天我就带领大家看看这部分的处理逻辑。touch事件的处理我将放在另一篇博客中介绍（相比KeyEvent，大体都一样，只是稍微复杂些）。

　　为了突出本文的重点，我们直接从事件被派发到View层次结构的根节点DecorView开始分析，这里我们先来看看DecorView#

dispatchKeyEvent方法，代码如下：

    @Override
    public boolean dispatchKeyEvent(KeyEvent event) {
        final int keyCode = event.getKeyCode();
        final int action = event.getAction();
        final boolean isDown = action == KeyEvent.ACTION_DOWN;

        /// 1. 第一次down事件的时候，处理panel的快捷键
        if (isDown && (event.getRepeatCount() == 0)) {
            // First handle chording of panel key: if a panel key is held
            // but not released, try to execute a shortcut in it.
            if ((mPanelChordingKey > 0) && (mPanelChordingKey != keyCode)) {
                boolean handled = dispatchKeyShortcutEvent(event);
                if (handled) {
                    return true;
                }
            }

            // If a panel is open, perform a shortcut on it without the
            // chorded panel key
            if ((mPreparedPanel != null) && mPreparedPanel.isOpen) {
                if (performPanelShortcut(mPreparedPanel, keyCode, event, 0)) {
                    return true;
                }
            }
        }

        /// 2. 这里是我们本文的重点，当window没destroy且其Callback非空的话，交给其Callback处理
        if (!isDestroyed()) { // Activity、Dialog都是Callback接口的实现
            final Callback cb = getCallback(); // mFeatureId < 0 表示是application的DecorView，比如Activity、Dialog
            final boolean handled = cb != null && mFeatureId < 0 ? cb.dispatchKeyEvent(event) // 派发给callback的方法
                    : super.dispatchKeyEvent(event); // 否则直接派发到ViewGroup#dispatchKeyEvent（View层次结构）
            if (handled) {
                return true; // 如果被上面的步骤处理了则直接返回true，不再往下传递
            }
        }

        /// 3. 这是key事件的最后一步，如果到这一步还没处理掉，则派发到PhoneWindow对应的onKeyDown, onKeyUp方法
        return isDown ? PhoneWindow.this.onKeyDown(mFeatureId, event.getKeyCode(), event)
                : PhoneWindow.this.onKeyUp(mFeatureId, event.getKeyCode(), event);
    }

　　接下来我们按照这个派发顺序依次来看看相关方法的实现，这里先看看Activity(Callback)的dispatchKeyEvent实现：

    /**
     * Called to process key events.  You can override this to intercept all 
     * key events before they are dispatched to the window.  Be sure to call 
     * this implementation for key events that should be handled normally.
     * 
     * @param event The key event.
     * 
     * @return boolean Return true if this event was consumed.
     */
    @Override
    public boolean dispatchKeyEvent(KeyEvent event) {
        /// 2.1. 回调接口，实际开发中用处不大，你感兴趣可以参看其方法doc
        onUserInteraction();
        Window win = getWindow();
        /// 2.2. 从这里事件的处理交给了与之相关的window对象，实质是派发到了view层次结构
        if (win.superDispatchKeyEvent(event)) {
            return true;
        }
        View decor = mDecor;
        if (decor == null) decor = win.getDecorView();
        /// 2.3. 到这里如果view层次结构没处理则交给KeyEvent本身的dispatch方法，Activity的各种回调方法会被触发
        return event.dispatch(this, decor != null
                ? decor.getKeyDispatcherState() : null, this);
    }

紧接着我们看看，Window#superDispatchKeyEvent方法，相关代码如下：

    <!-- Window.java -->
    /**
     * Used by custom windows, such as Dialog, to pass the key press event
     * further down the view hierarchy. Application developers should
     * not need to implement or call this.
     *
     */
    public abstract boolean superDispatchKeyEvent(KeyEvent event);

    <!-- PhoneWindow.java -->


    @Override
    public boolean superDispatchKeyEvent(KeyEvent event) {
        return mDecor.superDispatchKeyEvent(event);
    }

      <!-- DecorView.superDispatchKeyEvent -->

　　public boolean superDispatchKeyEvent(KeyEvent event) {
            /// 2.2.1. 进入view层次结构了，即调用ViewGroup的对应实现了。。。
            if (super.dispatchKeyEvent(event)) {
                return true; // 如果被view层次结构处理了则直接返回true。
            }

            // Not handled by the view hierarchy, does the action bar want it
            // to cancel out of something special?
            /// 2.2.2. ActionBar对BACK key的特殊处理
            if (event.getKeyCode() == KeyEvent.KEYCODE_BACK) {
                final int action = event.getAction();
                // Back cancels action modes first.
                if (mActionMode != null) {
                    if (action == KeyEvent.ACTION_UP) {
                        mActionMode.finish();
                    }
                    return true;
                }

                // Next collapse any expanded action views.
                if (mActionBar != null && mActionBar.hasExpandedActionView()) {
                    if (action == KeyEvent.ACTION_UP) {
                        mActionBar.collapseActionView();
                    }
                    return true;
                }
            }
            /// 2.2.3. 最后返回false表示没处理掉，会接着2.3.步骤处理
            return false;
        }

然后我们接着看看2.2.1.包括的小步骤，即ViewGroup#dispatchKeyEvent的实现，代码如下：

    @Override
    public boolean dispatchKeyEvent(KeyEvent event) {
        /// 2.2.1.1. keyevent一致性检测用的，可忽略。。。
        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onKeyEvent(event, 1);
        }

        if ((mPrivateFlags & (PFLAG_FOCUSED | PFLAG_HAS_BOUNDS))
                == (PFLAG_FOCUSED | PFLAG_HAS_BOUNDS)) {
            /// 2.2.1.2. 如果此ViewGroup是focused或者具体的大小被设置了，则交给他处理，即调用View的实现
            if (super.dispatchKeyEvent(event)) {
                return true;
            }
        } else if (mFocused != null && (mFocused.mPrivateFlags & PFLAG_HAS_BOUNDS)
                == PFLAG_HAS_BOUNDS) {
            /// 2.2.1.3. 否则，如果此ViewGroup中有focused的child，且child有具体的大小，则交给mFocused处理
            if (mFocused.dispatchKeyEvent(event)) { // 注意这里可能是个递归调用
                return true; // 我们可以看到并不是每个child都能响应key事件，前提必须是focused child才有机会响应
            }
        }

        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onUnhandledEvent(event, 1);
        }
        /// 2.2.1.4. 最后都没被处理返回false，2.2.2.步骤会接着执行。。。
        return false;
    }

这里我们可以看出对KeyEvent来说在View层次结构中，如果ViewGroup条件满足则会优先处理事件而不是先派发给其孩子view，

这一点和touch事件有所不同。这里我们看看View的dispatchKeyEvent实现：

    /**
     * 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) {
        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onKeyEvent(event, 0);
        }

        // Give any attached key listener a first crack at the event.
        //noinspection SimplifiableIfStatement
        ListenerInfo li = mListenerInfo;
        /// 2.2.1.2(3).1. 调用onKeyListener，如果它非空且view是ENABLED状态，监听器优先触发
        if (li != null && li.mOnKeyListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
                && li.mOnKeyListener.onKey(this, event.getKeyCode(), event)) {
            return true;
        }

        /// 2.2.1.2(3).2. 调用KeyEvent.dispatch方法，并将view对象本身作为参数传递进去，view的各种callback方法在这里被触发
        if (event.dispatch(this, mAttachInfo != null
                ? mAttachInfo.mKeyDispatchState : null, this)) {
            return true;
        }

        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
        }
        /// 2.2.1.2(3).3. 还没处理掉返回false，接着2.2.1.4.执行
        return false;
    }

不管是这里的2.2.1.2(3).2.步骤还是前面Activity里的2.3.步骤，都调到了KeyEvent.dispatch方法，不过在看其代码之前我们

先来看看这里用到的mAttachInfo.mKeyDispatchState对象是咋来的，代码如下：

// 这句代码位于View.AttachInfo类里
final KeyEvent.DispatcherState mKeyDispatchState
                = new KeyEvent.DispatcherState();

/**
     * Return the global {@link KeyEvent.DispatcherState KeyEvent.DispatcherState}
     * for this view's window.  Returns null if the view is not currently attached
     * to the window.  Normally you will not need to use this directly, but
     * just use the standard high-level event callbacks like
     * {@link #onKeyDown(int, KeyEvent)}.
     */
    public KeyEvent.DispatcherState getKeyDispatcherState() {
        return mAttachInfo != null ? mAttachInfo.mKeyDispatchState : null;
    }

// KeyEvent.DispatcherState类
    /**
     * Use with {@link KeyEvent#dispatch(Callback, DispatcherState, Object)}
     * for more advanced key dispatching, such as long presses.
     */
    public static class DispatcherState {
        int mDownKeyCode;
        Object mDownTarget;
        SparseIntArray mActiveLongPresses = new SparseIntArray();
        
        /**
         * Reset back to initial state.
         */
        public void reset() { // 清空内部状态
            if (DEBUG) Log.v(TAG, "Reset: " + this);
            mDownKeyCode = 0;
            mDownTarget = null;
            mActiveLongPresses.clear();
        }
        
        /**
         * Stop any tracking associated with this target.
         */
        public void reset(Object target) { // 清空target对应的内部状态
            if (mDownTarget == target) { // 只有相同时才清空，否则啥也不做
                if (DEBUG) Log.v(TAG, "Reset in " + target + ": " + this);
                mDownKeyCode = 0;
                mDownTarget = null;
            }
        }
        
        /**
         * Start tracking the key code associated with the given event.  This
         * can only be called on a key down.  It will allow you to see any
         * long press associated with the key, and will result in
         * {@link KeyEvent#isTracking} return true on the long press and up
         * events.
         * 
         * <p>This is only needed if you are directly dispatching events, rather
         * than handling them in {@link Callback#onKeyDown}.
         */
        public void startTracking(KeyEvent event, Object target) {
            if (event.getAction() != ACTION_DOWN) { // 状态检测
                throw new IllegalArgumentException(
                        "Can only start tracking on a down event");
            }
            if (DEBUG) Log.v(TAG, "Start trackingt in " + target + ": " + this);
            mDownKeyCode = event.getKeyCode(); // 赋值，表示正在track某个keycode
            mDownTarget = target;
        }
        
        /**
         * Return true if the key event is for a key code that is currently
         * being tracked by the dispatcher.
         */
        public boolean isTracking(KeyEvent event) {
            return mDownKeyCode == event.getKeyCode();
        }
        
        /**
         * Keep track of the given event's key code as having performed an
         * action with a long press, so no action should occur on the up.
         * <p>This is only needed if you are directly dispatching events, rather
         * than handling them in {@link Callback#onKeyLongPress}.
         */
        public void performedLongPress(KeyEvent event) {// 用来记录发生了生理长按事件
            mActiveLongPresses.put(event.getKeyCode(), 1);
        }
        
        /**
         * Handle key up event to stop tracking.  This resets the dispatcher state,
         * and updates the key event state based on it.
         * <p>This is only needed if you are directly dispatching events, rather
         * than handling them in {@link Callback#onKeyUp}.
         */
        public void handleUpEvent(KeyEvent event) {
            final int keyCode = event.getKeyCode();
            if (DEBUG) Log.v(TAG, "Handle key up " + event + ": " + this);
            int index = mActiveLongPresses.indexOfKey(keyCode);
            if (index >= 0) { // 如果发生过生理长按则设置event.mFlags为CACELED，这样在接下来的receiver.onKeyUp中有些处理就不会发生了
                if (DEBUG) Log.v(TAG, "  Index: " + index); // 因为事件被标记为CANCELED了
                event.mFlags |= FLAG_CANCELED | FLAG_CANCELED_LONG_PRESS;
                mActiveLongPresses.removeAt(index);
            }
            if (mDownKeyCode == keyCode) {
                if (DEBUG) Log.v(TAG, "  Tracking!");
                event.mFlags |= FLAG_TRACKING; // 设置event正确的mFlags，接下来的receiver.onKeyUp可能会检测此状态
                mDownKeyCode = 0; // reset，表示此keycode的tracking到此结束了
                mDownTarget = null;
            }
        }
    }

大概了解了KeyEvent.DispatcherState类，我们就可以来看看KeyEvent.dispatch方法了，代码如下：

    /**
     * Deliver this key event to a {@link Callback} interface.  If this is
     * an ACTION_MULTIPLE event and it is not handled, then an attempt will
     * be made to deliver a single normal event.
     * 
     * @param receiver The Callback that will be given the event.
     * @param state State information retained across events.
     * @param target The target of the dispatch, for use in tracking.
     * 
     * @return The return value from the Callback method that was called.
     */
    public final boolean dispatch(Callback receiver, DispatcherState state,
            Object target) {
        switch (mAction) {
            case ACTION_DOWN: { // DOWN事件
                mFlags &= ~FLAG_START_TRACKING; //先清掉START_TRACKING标记
                if (DEBUG) Log.v(TAG, "Key down to " + target + " in " + state
                        + ": " + this);
                boolean res = receiver.onKeyDown(mKeyCode, this); // 回调Callback接口的onKeyDown方法，View和Activity都是此接口的实现者
                if (state != null) { // 一般都成立
                    if (res && mRepeatCount == 0 && (mFlags&FLAG_START_TRACKING) != 0) {
                        if (DEBUG) Log.v(TAG, "  Start tracking!"); // receiver.onKeyDown返回true了且不是repeated
                        state.startTracking(this, target); // 并且也没有开始tracking，则开始tracking当前的KeyEvent和target
                    } else if (isLongPress() && state.isTracking(this)) { // 处理生理长按
                        try { // 检测到生理长按则调用receiver.onKeyLongPress方法
                            if (receiver.onKeyLongPress(mKeyCode, this)) {
                                if (DEBUG) Log.v(TAG, "  Clear from long press!");
                                state.performedLongPress(this); // 记录此event已经有生理long press发生了。。。
                                res = true; // 设置为处理了
                            }
                        } catch (AbstractMethodError e) {
                        }
                    }
                }
                return res; // 返回down事件处理的结果
            }
            case ACTION_UP: // UP事件
                if (DEBUG) Log.v(TAG, "Key up to " + target + " in " + state
                        + ": " + this);
                if (state != null) {
                    state.handleUpEvent(this); // reset state的内部状态，也改变了KeyEvent的某些状态
                }
                return receiver.onKeyUp(mKeyCode, this); // 最后调用receiver.onKeyUp方法
            case ACTION_MULTIPLE: // 这里可以忽略掉
                final int count = mRepeatCount;
                final int code = mKeyCode;
                if (receiver.onKeyMultiple(code, count, this)) {
                    return true;
                }
                if (code != KeyEvent.KEYCODE_UNKNOWN) {
                    mAction = ACTION_DOWN;
                    mRepeatCount = 0;
                    boolean handled = receiver.onKeyDown(code, this);
                    if (handled) {
                        mAction = ACTION_UP;
                        receiver.onKeyUp(code, this);
                    }
                    mAction = ACTION_MULTIPLE;
                    mRepeatCount = count;
                    return handled;
                }
                return false;
        }
        return false;
    }

　　看完了KeyEvent的具体实现，我们接着看看receiver（Callback接口）的onKeyDown、onKeyUp实现，先来看View相关的，代码如下：

/**
     * Default implementation of {@link KeyEvent.Callback#onKeyDown(int, KeyEvent)
     * KeyEvent.Callback.onKeyDown()}: perform press of the view
     * when {@link KeyEvent#KEYCODE_DPAD_CENTER} or {@link KeyEvent#KEYCODE_ENTER}
     * is released, if the view is enabled and clickable.
     *
     * <p>Key presses in software keyboards will generally NOT trigger this listener,
     * although some may elect to do so in some situations. Do not rely on this to
     * catch software key presses.
     *
     * @param keyCode A key code that represents the button pressed, from
     *                {@link android.view.KeyEvent}.
     * @param event   The KeyEvent object that defines the button action.
     */
public boolean onKeyDown(int keyCode, KeyEvent event) {
        boolean result = false;

        if (KeyEvent.isConfirmKey(keyCode)) { // 只处理KEYCODE_DPAD_CENTER、KEYCODE_ENTER这2个按键
            if ((mViewFlags & ENABLED_MASK) == DISABLED) {
                return true; // 针对disabled View直接返回true表示处理过了
            }
            // Long clickable items don't necessarily have to be clickable
            if (((mViewFlags & CLICKABLE) == CLICKABLE ||
                    (mViewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) &&
                    (event.getRepeatCount() == 0)) { // clickable或者long_clickable且是第一次down事件
                setPressed(true); // 标记pressed，你可能设置了View不同的background，这时候就会有所体现（比如高亮效果）
                checkForLongClick(0); // 启动View的long click检测
                return true; // 到达这一步就表示KeyEvent被处理掉了
            }
        }
        return result;
    }

/**
     * Default implementation of {@link KeyEvent.Callback#onKeyUp(int, KeyEvent)
     * KeyEvent.Callback.onKeyUp()}: perform clicking of the view
     * when {@link KeyEvent#KEYCODE_DPAD_CENTER} or
     * {@link KeyEvent#KEYCODE_ENTER} is released.
     * <p>Key presses in software keyboards will generally NOT trigger this listener,
     * although some may elect to do so in some situations. Do not rely on this to
     * catch software key presses.
     *
     * @param keyCode A key code that represents the button pressed, from
     *                {@link android.view.KeyEvent}.
     * @param event   The KeyEvent object that defines the button action.
     */
    public boolean onKeyUp(int keyCode, KeyEvent event) {
        if (KeyEvent.isConfirmKey(keyCode)) { // 同onKeyDown，默认也只处理confirm key
            if ((mViewFlags & ENABLED_MASK) == DISABLED) {
                return true; // 同样的逻辑，如果是DISABLED view，直接返回true表示处理过了
            }
            if ((mViewFlags & CLICKABLE) == CLICKABLE && isPressed()) {
                setPressed(false); // 重置pressed状态

                if (!mHasPerformedLongPress) { // 长按没发生的话，
                    // This is a tap, so remove the longpress check
                    removeLongPressCallback(); // 当up事件发生的时候，移除这些已经没用的callback
                    return performClick(); // 调用单击onClick监听器
                }
            }
        }
        return false; // 其他所有的Key默认不处理
    }

/**
     * Sets the pressed state for this view.
     *
     * @see #isClickable()
     * @see #setClickable(boolean)
     *
     * @param pressed Pass true to set the View's internal state to "pressed", or false to reverts
     *        the View's internal state from a previously set "pressed" state.
     */
    public void setPressed(boolean pressed) {
        final boolean needsRefresh = pressed != ((mPrivateFlags & PFLAG_PRESSED) == PFLAG_PRESSED);

        if (pressed) {
            mPrivateFlags |= PFLAG_PRESSED;
        } else {
            mPrivateFlags &= ~PFLAG_PRESSED;
        }

        if (needsRefresh) {
            refreshDrawableState(); // 这行代码会刷新View的显示状态
        }
        dispatchSetPressed(pressed);
    }

private void checkForLongClick(int delayOffset) {
        if ((mViewFlags & LONG_CLICKABLE) == LONG_CLICKABLE) { // 必须得是LONG_CLICKABLE的View
            mHasPerformedLongPress = false; // 设置初始值

            if (mPendingCheckForLongPress == null) { // 只非空的时候才new一个
                mPendingCheckForLongPress = new CheckForLongPress();
            }
            mPendingCheckForLongPress.rememberWindowAttachCount();
            postDelayed(mPendingCheckForLongPress, // post一个Runnable，注意延迟是个差值，而不是delayOffset
                    ViewConfiguration.getLongPressTimeout() - delayOffset);
        }
    }

class CheckForLongPress implements Runnable {

        private int mOriginalWindowAttachCount;

        public void run() {
            if (isPressed() && (mParent != null) // 当时间到了，此Runnable没被移除掉的话，并且这些条件都满足的时候，
                    && mOriginalWindowAttachCount == mWindowAttachCount) {
                if (performLongClick()) { // 客户端定义的onLongClickListener监听器被触发
                    mHasPerformedLongPress = true; // 只有当被上面的方法处理掉了，才表示LongPress发生过了
                }
            }
        }

        public void rememberWindowAttachCount() {
            mOriginalWindowAttachCount = mWindowAttachCount;
        }
    }

/**
     * Call this view's OnLongClickListener, if it is defined. Invokes the context menu if the
     * OnLongClickListener did not consume the event.
     *
     * @return True if one of the above receivers consumed the event, false otherwise.
     */
    public boolean performLongClick() {
        sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_LONG_CLICKED);

        boolean handled = false;
        ListenerInfo li = mListenerInfo;
        if (li != null && li.mOnLongClickListener != null) { // 优先触发监听器
            handled = li.mOnLongClickListener.onLongClick(View.this);
        }
        if (!handled) { // 如果还没处理，显示ContextMenu如果定义了的话
            handled = showContextMenu();
        }
        if (handled) {
            performHapticFeedback(HapticFeedbackConstants.LONG_PRESS);
        }
        return handled; // 返回处理结果
    }

　　接下来，看看Activity对应的onKeyDown，onKeyUp方法：

    /**
     * Called when a key was pressed down and not handled by any of the views
     * inside of the activity. So, for example, key presses while the cursor
     * is inside a TextView will not trigger the event (unless it is a navigation
     * to another object) because TextView handles its own key presses.
     *
     * <p>If the focused view didn't want this event, this method is called.
     *
     * <p>The default implementation takes care of {@link KeyEvent#KEYCODE_BACK}
     * by calling {@link #onBackPressed()}, though the behavior varies based
     * on the application compatibility mode: for
     * {@link android.os.Build.VERSION_CODES#ECLAIR} or later applications,
     * it will set up the dispatch to call {@link #onKeyUp} where the action
     * will be performed; for earlier applications, it will perform the
     * action immediately in on-down, as those versions of the platform
     * behaved.
     *
     * <p>Other additional default key handling may be performed
     * if configured with {@link #setDefaultKeyMode}.
     *
     * @return Return <code>true</code> to prevent this event from being propagated
     * further, or <code>false</code> to indicate that you have not handled
     * this event and it should continue to be propagated.
     * @see #onKeyUp
     * @see android.view.KeyEvent
     */
    public boolean onKeyDown(int keyCode, KeyEvent event)  {
        if (keyCode == KeyEvent.KEYCODE_BACK) {
            if (getApplicationInfo().targetSdkVersion
                    >= Build.VERSION_CODES.ECLAIR) {
                event.startTracking();
            } else {
                onBackPressed();
            }
            return true;
        }

        if (mDefaultKeyMode == DEFAULT_KEYS_DISABLE) {
            return false;
        } else if (mDefaultKeyMode == DEFAULT_KEYS_SHORTCUT) {
            if (getWindow().performPanelShortcut(Window.FEATURE_OPTIONS_PANEL,
                    keyCode, event, Menu.FLAG_ALWAYS_PERFORM_CLOSE)) {
                return true;
            }
            return false;
        } else {
            // Common code for DEFAULT_KEYS_DIALER & DEFAULT_KEYS_SEARCH_*
            boolean clearSpannable = false;
            boolean handled;
            if ((event.getRepeatCount() != 0) || event.isSystem()) {
                clearSpannable = true;
                handled = false;
            } else {
                handled = TextKeyListener.getInstance().onKeyDown(
                        null, mDefaultKeySsb, keyCode, event);
                if (handled && mDefaultKeySsb.length() > 0) {
                    // something useable has been typed - dispatch it now.

                    final String str = mDefaultKeySsb.toString();
                    clearSpannable = true;

                    switch (mDefaultKeyMode) {
                    case DEFAULT_KEYS_DIALER:
                        Intent intent = new Intent(Intent.ACTION_DIAL,  Uri.parse("tel:" + str));
                        intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                        startActivity(intent);
                        break;
                    case DEFAULT_KEYS_SEARCH_LOCAL:
                        startSearch(str, false, null, false);
                        break;
                    case DEFAULT_KEYS_SEARCH_GLOBAL:
                        startSearch(str, false, null, true);
                        break;
                    }
                }
            }
            if (clearSpannable) {
                mDefaultKeySsb.clear();
                mDefaultKeySsb.clearSpans();
                Selection.setSelection(mDefaultKeySsb,0);
            }
            return handled;
        }
    }

/**
     * Called when a key was released and not handled by any of the views
     * inside of the activity. So, for example, key presses while the cursor
     * is inside a TextView will not trigger the event (unless it is a navigation
     * to another object) because TextView handles its own key presses.
     *
     * <p>The default implementation handles KEYCODE_BACK to stop the activity
     * and go back.
     *
     * @return Return <code>true</code> to prevent this event from being propagated
     * further, or <code>false</code> to indicate that you have not handled
     * this event and it should continue to be propagated.
     * @see #onKeyDown
     * @see KeyEvent
     */
    public boolean onKeyUp(int keyCode, KeyEvent event) {
        if (getApplicationInfo().targetSdkVersion
                >= Build.VERSION_CODES.ECLAIR) {
            if (keyCode == KeyEvent.KEYCODE_BACK && event.isTracking()
                    && !event.isCanceled()) {
                onBackPressed();
                return true;
            }
        }
        return false;
    }

　　最后是3.步骤，回到一开始DecorView.dispatchKeyEvent的最后几行代码，我们来看看PhoneWindow对应的onKeyDown，onKeyUp方法：

/**
     * A key was pressed down and not handled by anything else in the window.
     *
     * @see #onKeyUp
     * @see android.view.KeyEvent
     */
    protected boolean onKeyDown(int featureId, int keyCode, KeyEvent event) {
        /* ****************************************************************************
         * HOW TO DECIDE WHERE YOUR KEY HANDLING GOES.
         *
         * If your key handling must happen before the app gets a crack at the event,
         * it goes in PhoneWindowManager.
         *
         * If your key handling should happen in all windows, and does not depend on
         * the state of the current application, other than that the current
         * application can override the behavior by handling the event itself, it
         * should go in PhoneFallbackEventHandler.
         *
         * Only if your handling depends on the window, and the fact that it has
         * a DecorView, should it go here.
         * ****************************************************************************/

        final KeyEvent.DispatcherState dispatcher =
                mDecor != null ? mDecor.getKeyDispatcherState() : null;
        //Log.i(TAG, "Key down: repeat=" + event.getRepeatCount()
        //        + " flags=0x" + Integer.toHexString(event.getFlags()));
        
        switch (keyCode) {
            case KeyEvent.KEYCODE_VOLUME_UP: // key event处理中的最后一步，
            case KeyEvent.KEYCODE_VOLUME_DOWN:
            case KeyEvent.KEYCODE_VOLUME_MUTE: { // 处理音量调节键
                // Similar code is in PhoneFallbackEventHandler in case the window
                // doesn't have one of these.  In this case, we execute it here and
                // eat the event instead, because we have mVolumeControlStreamType
                // and they don't.
                getAudioManager().handleKeyDown(event, mVolumeControlStreamType);
                return true;
            }

            case KeyEvent.KEYCODE_MENU: {
                onKeyDownPanel((featureId < 0) ? FEATURE_OPTIONS_PANEL : featureId, event);
                return true;
            }

            case KeyEvent.KEYCODE_BACK: {
                if (event.getRepeatCount() > 0) break;
                if (featureId < 0) break;
                // Currently don't do anything with long press.
                if (dispatcher != null) {
                    dispatcher.startTracking(event, this);
                }
                return true;
            }

        }

        return false;
    }

/**
     * A key was released and not handled by anything else in the window.
     *
     * @see #onKeyDown
     * @see android.view.KeyEvent
     */
    protected boolean onKeyUp(int featureId, int keyCode, KeyEvent event) {
        final KeyEvent.DispatcherState dispatcher =
                mDecor != null ? mDecor.getKeyDispatcherState() : null;
        if (dispatcher != null) {
            dispatcher.handleUpEvent(event);
        }
        //Log.i(TAG, "Key up: repeat=" + event.getRepeatCount()
        //        + " flags=0x" + Integer.toHexString(event.getFlags()));
        
        switch (keyCode) {
            case KeyEvent.KEYCODE_VOLUME_UP:
            case KeyEvent.KEYCODE_VOLUME_DOWN:
            case KeyEvent.KEYCODE_VOLUME_MUTE: {
                // Similar code is in PhoneFallbackEventHandler in case the window
                // doesn't have one of these.  In this case, we execute it here and
                // eat the event instead, because we have mVolumeControlStreamType
                // and they don't.
                getAudioManager().handleKeyUp(event, mVolumeControlStreamType);
                return true;
            }

            case KeyEvent.KEYCODE_MENU: {
                onKeyUpPanel(featureId < 0 ? FEATURE_OPTIONS_PANEL : featureId,
                        event);
                return true;
            }

            case KeyEvent.KEYCODE_BACK: {
                if (featureId < 0) break;
                if (event.isTracking() && !event.isCanceled()) {
                    if (featureId == FEATURE_OPTIONS_PANEL) {
                        PanelFeatureState st = getPanelState(featureId, false);
                        if (st != null && st.isInExpandedMode) {
                            // If the user is in an expanded menu and hits back, it
                            // should go back to the icon menu
                            reopenMenu(true);
                            return true;
                        }
                    }
                    closePanel(featureId);
                    return true;
                }
                break;
            }

            case KeyEvent.KEYCODE_SEARCH: {
                /*
                 * Do this in onKeyUp since the Search key is also used for
                 * chording quick launch shortcuts.
                 */
                if (getKeyguardManager().inKeyguardRestrictedInputMode()) {
                    break;
                }
                if (event.isTracking() && !event.isCanceled()) {
                    launchDefaultSearch();
                }
                return true;
            }
        }

        return false;
    }

　　至此所有按键事件的处理就分析完毕了，鉴于篇幅略长，我们最后稍微总结下。主要有这么几点：

1. View的各种KeyEvent.Callback接口早于Activity的对应接口被调用；

2. 整个处理环节中只要有一处表明处理掉了，则处理结束，不在往下传递；

3. 各种Callback接口的处理优先级低于监听器，也就是说各种onXXXListener的方法优先被调用。