[Android] Perform eager gesture recognition on MotionEvents

content/browser/renderer_host/input/gesture_event_queue.h

+// Copyright 2013 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.
+
+#ifndef CONTENT_BROWSER_RENDERER_HOST_INPUT_GESTURE_EVENT_QUEUE_H_
+#define CONTENT_BROWSER_RENDERER_HOST_INPUT_GESTURE_EVENT_QUEUE_H_
+
+#include <deque>
+#include <queue>
+
+#include "content/browser/renderer_host/input/gesture_event_packet.h"
+#include "content/common/content_export.h"
+#include "content/port/common/input_event_ack_state.h"
+#include "third_party/WebKit/public/web/WebInputEvent.h"
+
+namespace content {
+
+// Interface with which the |GestureEventQueue| forwards gestures for a given
+// touch event.
+class CONTENT_EXPORT GestureEventQueueClient {
+ public:
+  virtual void ForwardGestureEvent(const blink::WebGestureEvent&) = 0;
+};
+
+// Handles dispatch of touch-derived gestures created by the platform.
+// Gestures are forwarded or dropped depending on the ack dispositions of the
+// generating touch sequence.
+class CONTENT_EXPORT GestureEventQueue {
+ public:
+  explicit GestureEventQueue(GestureEventQueueClient* client);
+  ~GestureEventQueue();
+
+  // To be called upon receipt of gesture-related events.  In particular,
+  // |packet| contains [0, n] gestures that correspond to a given event.  That
+  // event may be a touch, or a touch timeout for certain stationary gestures.
+  // It is imperative that a single packet is received for *each* touch event,
+  // even those that did not produce a gesture.
+  void OnGestureEventPacket(const GestureEventPacket& packet);
+
+  // To be called upon receipt of *all* touch event acks.
+  void OnTouchEventAck(InputEventAckState ack_state);
+
+ private:
+  // Utility class for tracking gesture events and dispositions for a single
+  // gesture sequence. A single sequence corresponds to all gestures created
+  // between the first finger down and the last finger up, including gestures
+  // generated by timeouts from a statinoary finger.
+  class GestureSequence {
+   public:
+    GestureSequence();
+    ~GestureSequence();
+
+    void Push(const GestureEventPacket& packet);
+    void Pop();
+    const GestureEventPacket& Front() const;
+    void UpdateState(InputEventAckState ack_state);
+    bool IsGesturePrevented() const;
+    bool IsEmpty() const;
+
+   private:
+    std::queue<GestureEventPacket> packets_;
+    enum GestureHandlingState {
+      PENDING,                 // The sequence has yet to receive an ack.
+      ALLOWED_UNTIL_PREVENTED, // Gestures in the sequence are allowed until
+                               // a source touch is preventDefault'ed.
+      ALWAYS_ALLOWED,          // All remaining sequence gestures are forwarded.
+      ALWAYS_PREVENTED         // All remaining sequence gestures are dropped.
+    };
+    GestureHandlingState state_;
+  };
+  void UpdateAndDispatchPackets(GestureSequence* sequence,
+                                InputEventAckState ack_result);
+  void SendPacket(const GestureEventPacket& packet);
+  void SendGesture(const blink::WebGestureEvent& gesture);
+  void CancelTapIfNecessary();
+  void CancelFlingIfNecessary();
+  GestureSequence& Head();
+  GestureSequence& Tail();
+
+  GestureEventQueueClient* client_;
+  std::queue<GestureSequence> sequences_;
+
+  // Bookkeeping for inserting synthetic Gesture{Tap,Fling}Cancel events
+  // when necessary, e.g., GestureTapCancel when scrolling begins, or
+  // GestureFlingCancel when a user taps following a GestureFlingStart.
+  bool needs_tap_ending_event_;
+  bool needs_fling_ending_event_;
+
+  DISALLOW_COPY_AND_ASSIGN(GestureEventQueue);
+};
+
+}  // namespace content
+
+#endif  // CONTENT_BROWSER_RENDERER_HOST_INPUT_GESTURE_EVENT_QUEUE_H_