Handle long press in PDF documents.

chrome/browser/resources/pdf/gesture_detector.js

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 'use strict';
 
 /**
  * A class that listens for touch events and produces events when these
  * touches form gestures (e.g. pinching).
  */
 class GestureDetector {
   /**
    * Constructs a GestureDetector.
    * @param {!Element} element The element to monitor for touch gestures.
    */
   constructor(element) {
     this.element_ = element;
 
     this.element_.addEventListener(
         'touchstart', this.onTouchStart_.bind(this), { passive: false });

[Kevin McNee, 2017/05/02 23:18:18]

Since we're no longer calling preventDefault in this case, this should no longer
be non-passive.
This is updated in my CL, so you can just merge that.

[dsinclair, 2017/05/03 14:18:40]

Yea, I'm going to use your Cl for this bit, I'd done this before I saw your
change.

     this.element_.addEventListener(
-        'touchmove', this.onTouch_.bind(this), { passive: true });
+        'touchmove', this.onTouch_.bind(this), { passive: false });
     this.element_.addEventListener(
         'touchend', this.onTouch_.bind(this), { passive: true });
     this.element_.addEventListener(
         'touchcancel', this.onTouch_.bind(this), { passive: true });
 
-    this.pinchStartEvent_ = null;
+    this.touchStartEvent_ = null;
     this.lastEvent_ = null;
 
     this.listeners_ = new Map([
       ['pinchstart', []],
       ['pinchupdate', []],
       ['pinchend', []]
     ]);
   }
 
   /**
    * Add a |listener| to be notified of |type| events.
    * @param {string} type The event type to be notified for.
    * @param {Function} listener The callback.
    */
   addEventListener(type, listener) {
     if (this.listeners_.has(type)) {
       this.listeners_.get(type).push(listener);
     }
   }
 
   /**
+   * Returns true if the previous touch start was a two finger touch.
+   * @return {!bool} True if touch start was two finger.

[Kevin McNee, 2017/05/02 23:18:18]

The ! is redundant since booleans are non-nullable. You can just use {boolean}.

[dsinclair, 2017/05/03 14:18:40]

Done.

+   */
+  isTwoFingerTouch() {
+    return this.touchStartEvent_ && this.touchStartEvent_.touches.length == 2;

[Kevin McNee, 2017/05/02 23:18:19]

I notice that |touchStartEvent_| is never cleared even after the gesture ends. I
take it by the time you call this, the current state of the gesture may have
changed.

But then we get that logic at the beginning of onTouch_ which is a bit
unintuitive (there could be a |touchStartEvent_| without a |lastEvent_|).
Perhaps it would be clearer if we just recorded the most recent number of
fingers separately and just stored the events for the gesture detection logic.

[dsinclair, 2017/05/03 14:18:40]

Done.

+  }
+
+  /**
    * Call the relevant listeners with the given |pinchEvent|.
    * @private
    * @param {!Object} pinchEvent The event to notify the listeners of.
    */
   notify_(pinchEvent) {
     let listeners = this.listeners_.get(pinchEvent.type);
 
     for (let l of listeners)
       l(pinchEvent);
   }
 
   /**
    * The callback for touchstart events on the element.
    * @private
    * @param {!TouchEvent} event Touch event on the element.
    */
   onTouchStart_(event) {
-    // We must preventDefault if there is a two finger touch. By doing so
-    // native pinch-zoom does not interfere with our way of handling the event.
-    if (event.touches.length == 2) {
-      event.preventDefault();
-      this.pinchStartEvent_ = event;
+    this.touchStartEvent_ = event;
+
+    if (this.isTwoFingerTouch()) {
       this.lastEvent_ = event;
       this.notify_({
         type: 'pinchstart',
         center: GestureDetector.center_(event)
       });
     }
   }
 
   /**
    * The callback for touch move, end, and cancel events on the element.
    * @private
    * @param {!TouchEvent} event Touch event on the element.
    */
   onTouch_(event) {
-    if (!this.pinchStartEvent_)
+    // Missing a lastEvent_ means we either haven't got a touchstart or we've
+    // finished handling the pinch already.
+    if (!this.lastEvent_ || !this.isTwoFingerTouch())
       return;
 
+    // Prevent default the touchmove to block the native pinch-zoom.
+    if (event.type == "touchmove")
+      event.preventDefault();

[dsinclair, 2017/05/02 19:44:05]

This is very similar to https://codereview.chromium.org/2847893005/ and will be
updated on top of that CL when it lands.

+
     // Check if the pinch ends with the current event.
     if (event.touches.length < 2 ||
         this.lastEvent_.touches.length !== event.touches.length) {
       let startScaleRatio = GestureDetector.pinchScaleRatio_(
-          this.lastEvent_, this.pinchStartEvent_);
+          this.lastEvent_, this.touchStartEvent_);
       let center = GestureDetector.center_(this.lastEvent_);
       let endEvent = {
         type: 'pinchend',
         startScaleRatio: startScaleRatio,
         center: center
       };
-      this.pinchStartEvent_ = null;
+
       this.lastEvent_ = null;
       this.notify_(endEvent);
       return;
     }
 
     let scaleRatio = GestureDetector.pinchScaleRatio_(event, this.lastEvent_);
     let startScaleRatio = GestureDetector.pinchScaleRatio_(
-        event, this.pinchStartEvent_);
+        event, this.touchStartEvent_);
     let center = GestureDetector.center_(event);
     this.notify_({
       type: 'pinchupdate',
       scaleRatio: scaleRatio,
       direction: scaleRatio > 1.0 ? 'in' : 'out',
       startScaleRatio: startScaleRatio,
       center: center
     });
 
     this.lastEvent_ = event;
@@ skipping 36 matching lines @@
    */
   static center_(event) {
     let touch1 = event.touches[0];
     let touch2 = event.touches[1];
     return {
       x: (touch1.clientX + touch2.clientX) / 2,
       y: (touch1.clientY + touch2.clientY) / 2
     };
   }
 };