Improved Pinch-Zoom for PDF.

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));
+    this.element_.addEventListener('touchmove', this.onTouch_.bind(this));
+    this.element_.addEventListener('touchend', this.onTouch_.bind(this));
+    this.element_.addEventListener('touchcancel', this.onTouch_.bind(this));
+
+    this.pinchStartEvent_ = 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);
+    }
+  }
+
+  /**
+   * 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.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_)
+      return;
+
+    // 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_);
+      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_);
+    let center = GestureDetector.center_(event);
+    this.notify_({
+      type: 'pinchupdate',
+      scaleRatio: scaleRatio,
+      direction: scaleRatio > 1.0 ? 'in' : 'out',
+      startScaleRatio: startScaleRatio,
+      center: center
+    });
+
+    this.lastEvent_ = event;
+  }
+
+  /**
+   * Computes the change in scale between this touch event
+   * and a previous one.
+   * @private
+   * @param {!TouchEvent} event Latest touch event on the element.
+   * @param {!TouchEvent} prevEvent A previous touch event on the element.
+   * @return {?number} The ratio of the scale of this event and the
+   *     scale of the previous one.
+   */
+  static pinchScaleRatio_(event, prevEvent) {
+    let distance1 = GestureDetector.distance_(prevEvent);
+    let distance2 = GestureDetector.distance_(event);
+    return distance1 === 0 ? null : distance2 / distance1;
+  }
+
+  /**
+   * Computes the distance between fingers.
+   * @private
+   * @param {!TouchEvent} event Touch event with at least 2 touch points.
+   * @return {number} Distance between touch[0] and touch[1].
+   */
+  static distance_(event) {
+    let touch1 = event.touches[0];
+    let touch2 = event.touches[1];
+    let dx = touch1.clientX - touch2.clientX;
+    let dy = touch1.clientY - touch2.clientY;
+    return Math.sqrt(dx * dx + dy * dy);
+  }
+
+  /**
+   * Computes the midpoint between fingers.
+   * @private
+   * @param {!TouchEvent} event Touch event with at least 2 touch points.
+   * @return {!Object} Midpoint between touch[0] and touch[1].
+   */
+  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
+    };
+  }
+};