Android中按键消息的派发过程及源码分析

Android中消息的整体派发过程：接收消息——消息处理前端——窗口管理系统派发消息——窗口进行消息处理

以上过程中前三步都在WmS中完成，按键消息直接发送给当前窗口，而触摸消息则根据触摸坐标位置来匹配所有窗口，并判断坐标落到哪个窗口区域中，然后把消息发送给相应的窗口。对于按键消息还会涉及到“生理长按”的检测，比如一直按住某个键，那么会产生一些列的按键消息，然而第1个和第2个消息之间往往会间隔较长的时间，这种设计是人类本身的生理特点决定的，因为从按下到弹起的过程中，如果CPU处理太快，会导致产生多次该消息，这往往不是用户所期望的，因此Android把这种消息处理延迟加入到了消息处理前端中，应用程序不需要关心第一次的延迟，只需按普通的DOWN消息处理。

下面具体分析Android中按键消息的派发流程：

每个窗口定义了一个ViewRoot（4.0中是ViewRootImpl）对象，而ViewRoot对象中定义了一个inputHandler，窗口管理系统（WmS）派发消息的过程中会调用inputHandler的handlekey()，该函数再调用ViewRoot中的dispatchKey()函数

private final InputHandler mInputHandler = new InputHandler() {        public void handleKey(KeyEvent event, InputQueue.FinishedCallback finishedCallback) {            startInputEvent(finishedCallback);            dispatchKey(event, true);        }        public void handleMotion(MotionEvent event, InputQueue.FinishedCallback finishedCallback) {            startInputEvent(finishedCallback);            dispatchMotion(event, true);        }    };

dispatchKey()函数内部发送一个DISPATCH_KEY消息，消息的处理函数为deliverKeyEvent():

    private void dispatchKey(KeyEvent event, boolean sendDone) {        //noinspection ConstantConditions        if (false && event.getAction() == KeyEvent.ACTION_DOWN) {            if (event.getKeyCode() == KeyEvent.KEYCODE_CAMERA) {                if (DBG) Log.d("keydisp", "===================================================");                if (DBG) Log.d("keydisp", "Focused view Hierarchy is:");                debug();                if (DBG) Log.d("keydisp", "===================================================");            }        }        Message msg = obtainMessage(DISPATCH_KEY);        msg.obj = event;        msg.arg1 = sendDone ? 1 : 0;        if (LOCAL_LOGV) Log.v(            TAG, "sending key " + event + " to " + mView);        enqueueInputEvent(msg, event.getEventTime());    }    @Override    public void handleMessage(Message msg) {        switch (msg.what) {        ...        case FINISHED_EVENT:            handleFinishedEvent(msg.arg1, msg.arg2 != 0);            break;        case DISPATCH_KEY:            deliverKeyEvent((KeyEvent)msg.obj, msg.arg1 != 0);            break;        case DISPATCH_POINTER:            deliverPointerEvent((MotionEvent) msg.obj, msg.arg1 != 0);            break;

deliverKeyEvent()函数的执行流程如下：

1、调用mView.dispatchKeyEventPreIme()，如果有输入法存在，那么按键消息首先会被派发到输入法窗口，如果想在输入法截获消息之前处理该消息，那么可以重载该函数。

2、imm.dispatchKeyEvent()将消息派发到输入法窗口

3、调用deliverKeyEventPostIme()继而调用到mView.dispatchKeyEvent()

private void deliverKeyEvent(KeyEvent event, boolean sendDone) {        if (ViewDebug.DEBUG_LATENCY) {            mInputEventDeliverTimeNanos = System.nanoTime();        }        if (mInputEventConsistencyVerifier != null) {            mInputEventConsistencyVerifier.onKeyEvent(event, 0);        }        // If there is no view, then the event will not be handled.        if (mView == null || !mAdded) {            finishKeyEvent(event, sendDone, false);            return;        }        if (LOCAL_LOGV) Log.v(TAG, "Dispatching key " + event + " to " + mView);        // Perform predispatching before the IME.        if (mView.dispatchKeyEventPreIme(event)) {            finishKeyEvent(event, sendDone, true);            return;        }        // Dispatch to the IME before propagating down the view hierarchy.        // The IME will eventually call back into handleFinishedEvent.        if (mLastWasImTarget) {            InputMethodManager imm = InputMethodManager.peekInstance();            if (imm != null) {                int seq = enqueuePendingEvent(event, sendDone);                if (DEBUG_IMF) Log.v(TAG, "Sending key event to IME: seq="                        + seq + " event=" + event);                imm.dispatchKeyEvent(mView.getContext(), seq, event, mInputMethodCallback);                return;            }        }        // Not dispatching to IME, continue with post IME actions.        deliverKeyEventPostIme(event, sendDone);    }

private void deliverKeyEventPostIme(KeyEvent event, boolean sendDone) {        ...        // Deliver the key to the view hierarchy.        if (mView.dispatchKeyEvent(event)) {            finishKeyEvent(event, sendDone, true);            return;        }        ...}

mView对于应用窗口而言就是PhoneWindow.DecorView，否则就是普通的ViewGroup，我们只讨论DecorView中dispatchKeyEvent的实现：

1、处理系统快捷键

2、调用View中Callback对象的dispatchKeyEvent()，即调用Activity的dispatchKeyEvent()

2、如果Activity没有消耗该消息，则调用PhoneWindow的OnKeyEvent()对消息做最后的处理

        @Override        public boolean dispatchKeyEvent(KeyEvent event) {            final int keyCode = event.getKeyCode();            final int action = event.getAction();            final boolean isDown = action == KeyEvent.ACTION_DOWN;            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;                    }                }            }            if (!isDestroyed()) {                final Callback cb = getCallback();                final boolean handled = cb != null && mFeatureId < 0 ? cb.dispatchKeyEvent(event)                        : super.dispatchKeyEvent(event);                if (handled) {                    return true;                }            }            return isDown ? PhoneWindow.this.onKeyDown(mFeatureId, event.getKeyCode(), event)                    : PhoneWindow.this.onKeyUp(mFeatureId, event.getKeyCode(), event);        }

下面来具体看下Activity中dispatchKeyEvent的执行过程，首先来看源码：

    public boolean dispatchKeyEvent(KeyEvent event) {        onUserInteraction();        Window win = getWindow();        if (win.superDispatchKeyEvent(event)) {            return true;        }        View decor = mDecor;        if (decor == null) decor = win.getDecorView();        return event.dispatch(this, decor != null                ? decor.getKeyDispatcherState() : null, this);    }

主要过程如下：

1、调用onUserInteraction()，可重载该函数在消息派发前做一些处理

2、回调Activity包含的Window对象的superDispatchKeyEvent，该函数继而调用mDecor.superDispatchKveyEent，该函数继而又调用super.dispatchKeyEvent，DecorView的父类是FrameLayout，而FrameLayout未重载dispatchKeyEvent，因此最终调用ViewGroup的dispatchKeyEvent

3、如果DecorView未消耗消息，则调用event的dispatch()函数，这里的第一个参数receiver是Activity对象

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

         public boolean superDispatchKeyEvent(KeyEvent event) {            if (super.dispatchKeyEvent(event)) {                return true;            }            // Not handled by the view hierarchy, does the action bar want it            // to cancel out of something special?            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;                }            }            return false;        }

下面分析ViewGroup中dispatchKeyEvent的执行流程：如果ViewGroup本身拥有焦点，则调用super.dispatchKeyEvent把该消息派发到ViewGroup自身，如果其子视图拥有焦点，则调用mFocused.dispatchKeyEvent将消息派发给子视图，假如子视图也是ViewGroup，并且焦点是其子视图，则继续递归调用ViewGroup的dispatchKeyEvent

    @Override    public boolean dispatchKeyEvent(KeyEvent event) {        if (mInputEventConsistencyVerifier != null) {            mInputEventConsistencyVerifier.onKeyEvent(event, 1);        }        if ((mPrivateFlags & (FOCUSED | HAS_BOUNDS)) == (FOCUSED | HAS_BOUNDS)) {            if (super.dispatchKeyEvent(event)) {                return true;            }        } else if (mFocused != null && (mFocused.mPrivateFlags & HAS_BOUNDS) == HAS_BOUNDS) {            if (mFocused.dispatchKeyEvent(event)) {                return true;            }        }        if (mInputEventConsistencyVerifier != null) {            mInputEventConsistencyVerifier.onUnhandledEvent(event, 1);        }        return false;    }

在View类的dispatchKeyEvent中首先回调onKey()函数，应用程序可重载该函数以实现自定义消息处理，如果onKey函数未消耗该消息，则调用event的dispatch函数，在调用该函数是，第一个参数receiver是View对象本身

    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;    }

如果拥有焦点的View没有处理该按键消息，则继续调用event.dispatch()函数：

/**     * 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: {                mFlags &= ~FLAG_START_TRACKING;                if (DEBUG) Log.v(TAG, "Key down to " + target + " in " + state                        + ": " + this);                boolean res = receiver.onKeyDown(mKeyCode, this);                if (state != null) {                    if (res && mRepeatCount == 0 && (mFlags&FLAG_START_TRACKING) != 0) {                        if (DEBUG) Log.v(TAG, "  Start tracking!");                        state.startTracking(this, target);                    } else if (isLongPress() && state.isTracking(this)) {                        try {                            if (receiver.onKeyLongPress(mKeyCode, this)) {                                if (DEBUG) Log.v(TAG, "  Clear from long press!");                                state.performedLongPress(this);                                res = true;                            }                        } catch (AbstractMethodError e) {                        }                    }                }                return res;            }            case ACTION_UP:                if (DEBUG) Log.v(TAG, "Key up to " + target + " in " + state                        + ": " + this);                if (state != null) {                    state.handleUpEvent(this);                }                return receiver.onKeyUp(mKeyCode, this);            case ACTION_MULTIPLE:                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;    }

该函数中主要根据相应的逻辑回调了receiver中的onKeyDown, onKeyUp,OnKeyLongPress, OnKeyMultiple函数。View中onKeyDown和onKeyUp有自己默认的处理，主要处理presse状态，长按检测，onCick回调。而OnKeyLongPress和OnKeyMultiple为空实现。对于Activity的OnKeyDown和onKeyUp函数主要实现按数字启动打电话程序（ onKeyDown）以及back键的onBackPressed回调(onKeyUp)

如果按键消息在View树内部和Activity中没有被处理，就会调用到PhoneWindow的OnKeyDown和OnKeyUp函数，这是按键消息的最后处理机会，在PhoneWindow的OnKeyDown和OnKeyUp函数中主要处理了一些系统按键，例如音量键、音乐播放控制按键、照相机键、菜单键、拨号键、Search键等，具体代码就不再贴了。

以上博文主要根据《Android内核剖析》第13章以及Android源码总结而成