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 /* -*- Mode: Java; c-basic-offset: 4; tab-width: 20; indent-tabs-mode: nil; -*-

  * This Source Code Form is subject to the terms of the Mozilla Public

  * License, v. 2.0. If a copy of the MPL was not distributed with this file,

  * You can obtain one at http://mozilla.org/MPL/2.0/. */

 package org.mozilla.gecko.gfx;

 import org.mozilla.gecko.Tab;

 import org.mozilla.gecko.Tabs;

 import android.content.Context;

 import android.os.SystemClock;

 import android.util.Log;

 import android.view.GestureDetector;

 import android.view.MotionEvent;

 import android.view.View;

 import java.util.LinkedList;

 import java.util.Queue;

 /**

  * This class handles incoming touch events from the user and sends them to

  * listeners in Gecko and/or performs the "default action" (asynchronous pan/zoom

  * behaviour. EVERYTHING IN THIS CLASS MUST RUN ON THE UI THREAD.

  *

  * In the following code/comments, a "block" of events refers to a contiguous

  * sequence of events that starts with a DOWN or POINTER_DOWN and goes up to

  * but not including the next DOWN or POINTER_DOWN event.

  *

  * "Dispatching" an event refers to performing the default actions for the event,

  * which at our level of abstraction just means sending it off to the gesture

  * detectors and the pan/zoom controller.

  *

  * If an event is "default-prevented" that means one or more listeners in Gecko

  * has called preventDefault() on the event, which means that the default action

  * for that event should not occur. Usually we care about a "block" of events being

  * default-prevented, which means that the DOWN/POINTER_DOWN event that started

  * the block, or the first MOVE event following that, were prevent-defaulted.

  *

  * A "default-prevented notification" is when we here in Java-land receive a notification

  * from gecko as to whether or not a block of events was default-prevented. This happens

  * at some point after the first or second event in the block is processed in Gecko.

  * This code assumes we get EXACTLY ONE default-prevented notification for each block

  * of events.

  *

  * Note that even if all events are default-prevented, we still send specific types

  * of notifications to the pan/zoom controller. The notifications are needed

  * to respond to user actions a timely manner regardless of default-prevention,

  * and fix issues like bug 749384.

  */

 final class TouchEventHandler implements Tabs.OnTabsChangedListener {

     private static final String LOGTAG = "GeckoTouchEventHandler";

     // The time limit for listeners to respond with preventDefault on touchevents

     // before we begin panning the page

     private final int EVENT_LISTENER_TIMEOUT = 200;

     private final View mView;

     private final GestureDetector mGestureDetector;

     private final SimpleScaleGestureDetector mScaleGestureDetector;

     private final JavaPanZoomController mPanZoomController;

     // the queue of events that we are holding on to while waiting for a preventDefault

     // notification

     private final Queue<MotionEvent> mEventQueue;

     private final ListenerTimeoutProcessor mListenerTimeoutProcessor;

     // whether or not we should wait for touch listeners to respond (this state is

     // per-tab and is updated when we switch tabs).

     private boolean mWaitForTouchListeners;

     // true if we should hold incoming events in our queue. this is re-set for every

     // block of events, this is cleared once we find out if the block has been

     // default-prevented or not (or we time out waiting for that).

     private boolean mHoldInQueue;

     // false if the current event block has been default-prevented. In this case,

     // we still pass the event to both Gecko and the pan/zoom controller, but the

     // latter will not use it to scroll content. It may still use the events for

     // other things, such as making the dynamic toolbar visible.

     private boolean mAllowDefaultAction;

     // this next variable requires some explanation. strap yourself in.

     //

     // for each block of events, we do two things: (1) send the events to gecko and expect

     // exactly one default-prevented notification in return, and (2) kick off a delayed

     // ListenerTimeoutProcessor that triggers in case we don't hear from the listener in

     // a timely fashion.

     // since events are constantly coming in, we need to be able to handle more than one

     // block of events in the queue.

     //

     // this means that there are ordering restrictions on these that we can take advantage of,

     // and need to abide by. blocks of events in the queue will always be in the order that

     // the user generated them. default-prevented notifications we get from gecko will be in

     // the same order as the blocks of events in the queue. the ListenerTimeoutProcessors that

     // have been posted will also fire in the same order as the blocks of events in the queue.

     // HOWEVER, we may get multiple default-prevented notifications interleaved with multiple

     // ListenerTimeoutProcessor firings, and that interleaving is not predictable.

     //

     // therefore, we need to make sure that for each block of events, we process the queued

     // events exactly once, either when we get the default-prevented notification, or when the

     // timeout expires (whichever happens first). there is no way to associate the

     // default-prevented notification with a particular block of events other than via ordering,

     //

     // so what we do to accomplish this is to track a "processing balance", which is the number

     // of default-prevented notifications that we have received, minus the number of ListenerTimeoutProcessors

     // that have fired. (think "balance" as in teeter-totter balance). this value is:

     // - zero when we are in a state where the next default-prevented notification we expect

     //   to receive and the next ListenerTimeoutProcessor we expect to fire both correspond to

     //   the next block of events in the queue.

     // - positive when we are in a state where we have received more default-prevented notifications

     //   than ListenerTimeoutProcessors. This means that the next default-prevented notification

     //   does correspond to the block at the head of the queue, but the next n ListenerTimeoutProcessors

     //   need to be ignored as they are for blocks we have already processed. (n is the absolute value

     //   of the balance.)

     // - negative when we are in a state where we have received more ListenerTimeoutProcessors than

     //   default-prevented notifications. This means that the next ListenerTimeoutProcessor that

     //   we receive does correspond to the block at the head of the queue, but the next n

     //   default-prevented notifications need to be ignored as they are for blocks we have already

     //   processed. (n is the absolute value of the balance.)

     private int mProcessingBalance;

     TouchEventHandler(Context context, View view, JavaPanZoomController panZoomController) {

         mView = view;

         mEventQueue = new LinkedList<MotionEvent>();

         mPanZoomController = panZoomController;

         mGestureDetector = new GestureDetector(context, mPanZoomController);

         mScaleGestureDetector = new SimpleScaleGestureDetector(mPanZoomController);

         mListenerTimeoutProcessor = new ListenerTimeoutProcessor();

         mAllowDefaultAction = true;

         mGestureDetector.setOnDoubleTapListener(mPanZoomController);

         Tabs.registerOnTabsChangedListener(this);

     }

     public void destroy() {

         Tabs.unregisterOnTabsChangedListener(this);

     }

     /* This function MUST be called on the UI thread */

     public boolean handleEvent(MotionEvent event) {

         if (isDownEvent(event)) {

             // this is the start of a new block of events! whee!

             mHoldInQueue = mWaitForTouchListeners;

             // Set mAllowDefaultAction to true so that in the event we dispatch events, the

             // PanZoomController doesn't treat them as if they've been prevent-defaulted

             // when they haven't.

             mAllowDefaultAction = true;

             if (mHoldInQueue) {

                 // if the new block we are starting is the current block (i.e. there are no

                 // other blocks waiting in the queue, then we should let the pan/zoom controller

                 // know we are waiting for the touch listeners to run

                 if (mEventQueue.isEmpty()) {

                     mPanZoomController.startingNewEventBlock(event, true);

                 }

             } else {

                 // we're not going to be holding this block of events in the queue, but we need

                 // a marker of some sort so that the processEventBlock loop deals with the blocks

                 // in the right order as notifications come in. we use a single null event in

                 // the queue as a placeholder for a block of events that has already been dispatched.

                 mEventQueue.add(null);

                 mPanZoomController.startingNewEventBlock(event, false);

             }

             // set the timeout so that we dispatch these events and update mProcessingBalance

             // if we don't get a default-prevented notification

             mView.postDelayed(mListenerTimeoutProcessor, EVENT_LISTENER_TIMEOUT);

         }

         // if we need to hold the events, add it to the queue, otherwise dispatch

         // it directly.

         if (mHoldInQueue) {

             mEventQueue.add(MotionEvent.obtain(event));

         } else {

             dispatchEvent(event, mAllowDefaultAction);

         }

         return false;

     }

     /**

      * This function is how gecko sends us a default-prevented notification. It is called

      * once gecko knows definitively whether the block of events has had preventDefault

      * called on it (either on the initial down event that starts the block, or on

      * the first event following that down event).

      *

      * This function MUST be called on the UI thread.

      */

     public void handleEventListenerAction(boolean allowDefaultAction) {

         if (mProcessingBalance > 0) {

             // this event listener that triggered this took too long, and the corresponding

             // ListenerTimeoutProcessor runnable already ran for the event in question. the

             // block of events this is for has already been processed, so we don't need to

             // do anything here.

         } else {

             processEventBlock(allowDefaultAction);

         }

         mProcessingBalance--;

     }

     /* This function MUST be called on the UI thread. */

     public void setWaitForTouchListeners(boolean aValue) {

         mWaitForTouchListeners = aValue;

     }

     private boolean isDownEvent(MotionEvent event) {

         int action = (event.getAction() & MotionEvent.ACTION_MASK);

         return (action == MotionEvent.ACTION_DOWN || action == MotionEvent.ACTION_POINTER_DOWN);

     }

     private boolean touchFinished(MotionEvent event) {

         int action = (event.getAction() & MotionEvent.ACTION_MASK);

         return (action == MotionEvent.ACTION_UP || action == MotionEvent.ACTION_CANCEL);

     }

     /**

      * Dispatch the event to the gesture detectors and the pan/zoom controller.

      */

     private void dispatchEvent(MotionEvent event, boolean allowDefaultAction) {

         if (allowDefaultAction) {

             if (mGestureDetector.onTouchEvent(event)) {

                 return;

             }

             mScaleGestureDetector.onTouchEvent(event);

             if (mScaleGestureDetector.isInProgress()) {

                 return;

             }

         }

         mPanZoomController.handleEvent(event, !allowDefaultAction);

     }

     /**

      * Process the block of events at the head of the queue now that we know

      * whether it has been default-prevented or not.

      */

     private void processEventBlock(boolean allowDefaultAction) {

         if (mEventQueue.isEmpty()) {

             Log.e(LOGTAG, "Unexpected empty event queue in processEventBlock!", new Exception());

             return;

         }

         // the odd loop condition is because the first event in the queue will

         // always be a DOWN or POINTER_DOWN event, and we want to process all

         // the events in the queue starting at that one, up to but not including

         // the next DOWN or POINTER_DOWN event.

         MotionEvent event = mEventQueue.poll();

         while (true) {

             // event being null here is valid and represents a block of events

             // that has already been dispatched.

             if (event != null) {

                 dispatchEvent(event, allowDefaultAction);

                 event.recycle();

                 event = null;

             }

             if (mEventQueue.isEmpty()) {

                 // we have processed the backlog of events, and are all caught up.

                 // now we can set clear the hold flag and set the dispatch flag so

                 // that the handleEvent() function can do the right thing for all

                 // remaining events in this block (which is still ongoing) without

                 // having to put them in the queue.

                 mHoldInQueue = false;

                 mAllowDefaultAction = allowDefaultAction;

                 break;

             }

             event = mEventQueue.peek();

             if (event == null || isDownEvent(event)) {

                 // we have finished processing the block we were interested in.

                 // now we wait for the next call to processEventBlock

                 if (event != null) {

                     mPanZoomController.startingNewEventBlock(event, true);

                 }

                 break;

             }

             // pop the event we peeked above, as it is still part of the block and

             // we want to keep processing

             mEventQueue.remove();

         }

     }

     private class ListenerTimeoutProcessor implements Runnable {

         /* This MUST be run on the UI thread */

         @Override

         public void run() {

             if (mProcessingBalance < 0) {

                 // gecko already responded with default-prevented notification, and so

                 // the block of events this ListenerTimeoutProcessor corresponds to have

                 // already been removed from the queue.

             } else {

                 processEventBlock(true);

             }

             mProcessingBalance++;

         }

     }

     // Tabs.OnTabsChangedListener implementation

     @Override

     public void onTabChanged(Tab tab, Tabs.TabEvents msg, Object data) {

         if ((Tabs.getInstance().isSelectedTab(tab) && msg == Tabs.TabEvents.STOP) || msg == Tabs.TabEvents.SELECTED) {

             mWaitForTouchListeners = tab.getHasTouchListeners();

         }

     }

 }