The Tor Browser: mobile/android/base/gfx/TouchEventHandler.java@b8a032363ba2 (annotated)

mobile/android/base/gfx/TouchEventHandler.java@b8a032363ba2 (annotated)

mobile/android/base/gfx/TouchEventHandler.java

Thu, 22 Jan 2015 13:21:57 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Thu, 22 Jan 2015 13:21:57 +0100
branch: TOR_BUG_9701
changeset 15: b8a032363ba2
permissions: -rw-r--r--

Incorporate requested changes from Mozilla in review:
https://bugzilla.mozilla.org/show_bug.cgi?id=1123480#c6

 /* -*- 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();
         }
     }
 }

The Tor Browser / annotate

mobile/android/base/gfx/TouchEventHandler.java@b8a032363ba2 (annotated)

mobile/android/base/gfx/TouchEventHandler.java