[置顶] Android WebKit消息处理(二)Touch事件的分发处理

Android WebKit消息处理(二)Touch事件的分发处理

上一章《Android WebKit消息处理》讲到了Android WebKit主体初始化流程以及消息处理框架的搭建。这一章主要讲讲Android WebKit touch事件的处理与分发。

在WebViewCore初始化过程：

    /* Initialize private data within the WebCore thread.
     */
    private void initialize() {
        /* Initialize our private BrowserFrame class to handle all
         * frame-related functions. We need to create a new view which
         * in turn creates a C level FrameView and attaches it to the frame.
         */
        mBrowserFrame = new BrowserFrame(mContext, this, mCallbackProxy,
                mSettings, mJavascriptInterfaces);
        mJavascriptInterfaces = null;
        // Sync the native settings and also create the WebCore thread handler.
        mSettings.syncSettingsAndCreateHandler(mBrowserFrame);
        // Create the handler and transfer messages for the IconDatabase
        WebIconDatabaseClassic.getInstance().createHandler();
        // Create the handler for WebStorageClassic
        WebStorageClassic.getInstance().createHandler();
        // Create the handler for GeolocationPermissions.
        GeolocationPermissionsClassic.getInstance().createHandler();

        // ...
        // ...   

        // The transferMessages call will transfer all pending messages to the
        // WebCore thread handler.
        mEventHub.transferMessages();

        // Send a message back to WebView to tell it that we have set up the
        // WebCore thread.
        if (mWebViewClassic != null) {
            Message.obtain(mWebViewClassic.mPrivateHandler,
                    WebViewClassic.WEBCORE_INITIALIZED_MSG_ID,
                    mNativeClass, 0).sendToTarget();
        }    

    }    

在WebViewCore初始化结束之后，会给WebViewClassic发送WEBCORE_INITIALIZED_MSG_ID消息。

                case WEBCORE_INITIALIZED_MSG_ID:
                    // nativeCreate sets mNativeClass to a non-zero value
                    String drawableDir = BrowserFrame.getRawResFilename(
                            BrowserFrame.DRAWABLEDIR, mContext);
                    nativeCreate(msg.arg1, drawableDir, ActivityManager.isHighEndGfx());
                    if (mDelaySetPicture != null) {
                        setNewPicture(mDelaySetPicture, true);
                        mDelaySetPicture = null;
                    }    
                    if (mIsPaused) {
                        nativeSetPauseDrawing(mNativeClass, true);
                    }    
                    mInputDispatcher = new WebViewInputDispatcher(this,
                            mWebViewCore.getInputDispatcherCallbacks());
                    break;

在WebViewClassic收到来自WebViewCore的初始化完毕的消息之后，会创建WebViewInputDispatcher对象：

                    mInputDispatcher = new WebViewInputDispatcher(this,
                            mWebViewCore.getInputDispatcherCallbacks());

WebViewClassic的mInputDispatcher对象是touch事件分发处理的核心。下面讲的Touch事件的分发处理绝大部分是在WebViewInputDispatcher中进行的。

WebViewInputDispatcher构造函数的原型如下：

public WebViewInputDispatcher(UiCallbacks uiCallbacks, WebKitCallbacks webKitCallbacks)

uiCallbacks: WebViewClassic的mPrivateHandler

webKitCallbacks: WebViewCore的mEventHub.

关于mPrivateHandler和mEventHub的关系图，详见《Android WebKit消息处理》。从构造函数就可以看出，Touch事件的Ui消息将会派遣给WebViwClassic处理，WebCore消息将会派遣给WebViewCore去处理。

WebViewInputDispatcher消息处理

WebViewInputDispatcher把touch输入事件分为Ui事件和WebKit事件（就是上面WebCore事件），更具WebViewClassic在创建WebViewInputDispatcher的时候传入的参数，会分别创建自己的mUiHandler和mWebKitHandler。同时保存mUiCallbacks和mWebKitCallbacks以便于分别向WebViewClassic和WebViewCore发送消息：

    public WebViewInputDispatcher(UiCallbacks uiCallbacks, WebKitCallbacks webKitCallbacks) {
        this.mUiCallbacks = uiCallbacks;
        mUiHandler = new UiHandler(uiCallbacks.getUiLooper());

        this.mWebKitCallbacks = webKitCallbacks;
        mWebKitHandler = new WebKitHandler(webKitCallbacks.getWebKitLooper());

        ViewConfiguration config = ViewConfiguration.get(mUiCallbacks.getContext());
        mDoubleTapSlopSquared = config.getScaledDoubleTapSlop();
        mDoubleTapSlopSquared = (mDoubleTapSlopSquared * mDoubleTapSlopSquared);
        mTouchSlopSquared = config.getScaledTouchSlop();
        mTouchSlopSquared = (mTouchSlopSquared * mTouchSlopSquared);
    } 

其中mUiHandler是依附在UI线程上，mWebKitHandler是依附在WebCore线程上。
由于WebView是继承于View，所以当点击页面的时候，WebView的onTouchEvent会被调用，从而触发WebViewClassic的onTouchEvent：

    @Override
    public boolean onTouchEvent(MotionEvent ev) {
        if (mNativeClass == 0 || (!mWebView.isClickable() && !mWebView.isLongClickable())) {
            return false;
        }    

        if (mInputDispatcher == null) {
            return false;
        }    

        if (mWebView.isFocusable() && mWebView.isFocusableInTouchMode()
                && !mWebView.isFocused()) {
            mWebView.requestFocus();
        }    

        if (mInputDispatcher.postPointerEvent(ev, getScrollX(),
                getScrollY() - getTitleHeight(), mZoomManager.getInvScale())) {
            mInputDispatcher.dispatchUiEvents();
            return true;
        } else {
            Log.w(LOGTAG, "mInputDispatcher rejected the event!");
            return false;
        }    
    }    

这是Android WebKit的所有touch输入事件的入口。touch事件通过postPointerEvent进入，然后对UI事件和WebKit事件进行分拣，派遣到对应的消息队列。

Ui事件和WebKit事件的分拣

在消息分拣过程中，会判断touch输入事件是否真的需要派发给WebKit.

   private void enqueueEventLocked(DispatchEvent d) {
        if (!shouldSkipWebKit(d)) {
            enqueueWebKitEventLocked(d);
        } else {
            enqueueUiEventLocked(d);
        }
    }

    private boolean shouldSkipWebKit(DispatchEvent d) {
        switch (d.mEventType) {
            case EVENT_TYPE_CLICK:
            case EVENT_TYPE_HOVER:
            case EVENT_TYPE_SCROLL:
            case EVENT_TYPE_HIT_TEST:
                return false;
            case EVENT_TYPE_TOUCH:
                // TODO: This should be cleaned up. We now have WebViewInputDispatcher
                // and WebViewClassic both checking for slop and doing their own
                // thing - they should be consolidated. And by consolidated, I mean
                // WebViewClassic's version should just be deleted.
                // The reason this is done is because webpages seem to expect
                // that they only get an ontouchmove if the slop has been exceeded.
                if (mIsTapCandidate && d.mEvent != null
                        && d.mEvent.getActionMasked() == MotionEvent.ACTION_MOVE) {
                    return true;
                }
                return !mPostSendTouchEventsToWebKit
                        || mPostDoNotSendTouchEventsToWebKitUntilNextGesture;
        }
        return true;
    }

通常情况下，几乎所有的touch事件都需要派发给WebKit，除非WebKit不需要：

1. 在Document初始化过程中。
2. 在FrameLoader stopLoad之后。
3. Document被析构。

这两种情况都会导致Document::removeAllEventListerners()被调用：

void Document::removeAllEventListeners()
{
#if ENABLE(TOUCH_EVENTS)
    Page* ownerPage = page();
    if (!m_inPageCache && ownerPage && (m_frame == ownerPage->mainFrame()) && hasListenerType(Document::TOUCH_LISTENER)) {
        // Inform the Chrome Client that it no longer needs to forward touch
        // events to WebCore as the document removes all the event listeners.
        ownerPage->chrome()->client()->needTouchEvents(false);
    }    
#endif

    EventTarget::removeAllEventListeners();

    if (DOMWindow* domWindow = this->domWindow())
        domWindow->removeAllEventListeners();
    for (Node* node = firstChild(); node; node = node->traverseNextNode())
        node->removeAllEventListeners();
}

其中：

ownerPage->chrome()->client()->needTouchEvents(false);

会通过JNI调用WebViewCore的needTouchEvents()，从而就会导致上面所说的shouldSkipWebKIt(d)返回ture，是的事件不会被派遣到WebKit而直接被添加到UI的消息队列。这也就是为什么在网页在Load或者切换的过程中，前面一段时间，点击页面不会响应。当然，在Document初始化完毕之后，会调用needTouchEvents(true)告诉WebViewInputDispatcher ：Come on! I need your touch. 这样，Touch输入事件就被分别派遣到Ui消息队列和WebKit消息队列。

其实到这里，touch消息分拣已经很清晰简单了：

WebKit消息的处理

    private final DispatchEventQueue mWebKitDispatchEventQueue = new DispatchEventQueue();
    private final TouchStream mWebKitTouchStream = new TouchStream();
    private final WebKitCallbacks mWebKitCallbacks;
    private final WebKitHandler mWebKitHandler;

WebViewInputDispatcher定义了专门的mWebKitDispatchEventQueue来处理派发给WebKit(WebCore)的touch输入事件。enequeueWebKitEventLocked最终会将事件添加到mWebKitDispatchEvenetQueue当中，然后调用scheduleWebKitDispatchLocked()发送MSG_DISPATCH_WEBKIT_EVENTS消息给mWebKitHandler去处理事件。

        //mWebKitHandler的handleMessage
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case MSG_DISPATCH_WEBKIT_EVENTS:
                    dispatchWebKitEvents(true);
                    break;
                default:
                    throw new IllegalStateException("Unknown message type: " + msg.what);
            }    
        }    
    } 

dispatchWebKitEvenets()才是真正最终对mWebKitDispatcherEventQueue中消息进行分发和处理的：

   private void dispatchWebKitEvents(boolean calledFromHandler) {
        for (;;) {
            // Get the next event, but leave it in the queue so we can move it to the UI
            // queue if a timeout occurs.
            DispatchEvent d;
            MotionEvent event;
            final int eventType;
            int flags;
            synchronized (mLock) {
                if (!ENABLE_EVENT_BATCHING) {
                    drainStaleWebKitEventsLocked();
                }
                d = mWebKitDispatchEventQueue.mHead;
                if (d == null) {
                    if (mWebKitDispatchScheduled) {
                        mWebKitDispatchScheduled = false;
                        if (!calledFromHandler) {
                            mWebKitHandler.removeMessages(
                                    WebKitHandler.MSG_DISPATCH_WEBKIT_EVENTS);
                        }
                    }
                    return;
                }
        // ...
        // ...
}

内部逻辑其实就是一个for循环，不停的取消息，然后根据当前的状态然后处理。

UI消息的处理：

    // UI state, tracks events observed by the UI.  (guarded by mLock)
    private final DispatchEventQueue mUiDispatchEventQueue = new DispatchEventQueue();
    private final TouchStream mUiTouchStream = new TouchStream();
    private final UiCallbacks mUiCallbacks;
    private final UiHandler mUiHandler;
    private boolean mUiDispatchScheduled;

跟WebKit的消息处理类似，WebViewInputDispatcher也定义了专门处理UI事件的mUiDispatcherQueue，每当有UI的Touch输入事件的时候，enqueueUiEventLocked()的事件最终会被添加到mUiDispatcherQueue中，然后调用scheduleUiDispatchLocked，发送MSG_DISPATCH_UI_EVENTS，然后触发dispatchUiEvents：

    /**
     * Dispatches pending UI events.
     * Must only be called from the UI thread.
     *
     * This method may be used to flush the queue of pending input events
     * immediately.  This method may help to reduce input dispatch latency
     * if called before certain expensive operations such as drawing.
     */
    public void dispatchUiEvents() {
        dispatchUiEvents(false);
    }

    private void dispatchUiEvents(boolean calledFromHandler) {
        for (;;) {
            MotionEvent event;
            final int eventType;
            final int flags;
            synchronized (mLock) {
                DispatchEvent d = mUiDispatchEventQueue.dequeue();
                if (d == null) {
                    if (mUiDispatchScheduled) {
                        mUiDispatchScheduled = false;
                        if (!calledFromHandler) {
                            mUiHandler.removeMessages(UiHandler.MSG_DISPATCH_UI_EVENTS);
                        }
                    }
                    return;
                }
     // ...
     }
     // ...
}

同样，会进入一个for循环，不停的取事件然后处理。

篇幅有点长了，看到这里，可能还不是非常清楚，用下面这图概括下：

在所有的WebKit消息被处理的过程中，有些touch事件是需要给Ui进行反馈的，例如高亮，长按弹出菜单等等。具体这些事件会在后续文章中逐一进行解析的。

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