ui/events/gesture_detection/motion_event_buffer.h

   1 // Copyright 2014 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #ifndef UI_EVENTS_GESTURE_DETECTION_MOTION_EVENT_BUFFER_H_
   6 #define UI_EVENTS_GESTURE_DETECTION_MOTION_EVENT_BUFFER_H_
   7
   8 #include "base/memory/scoped_ptr.h"
   9 #include "base/memory/scoped_vector.h"
  10 #include "base/time/time.h"
  11 #include "ui/events/gesture_detection/gesture_detection_export.h"
  12
  13 namespace ui {
  14
  15 class MotionEvent;
  16
  17 // Allows event forwarding and flush requests from a |MotionEventBuffer|.
  18 class MotionEventBufferClient {
  19  public:
  20   virtual ~MotionEventBufferClient() {}
  21   virtual void ForwardMotionEvent(const MotionEvent& event) = 0;
  22   virtual void SetNeedsFlush() = 0;
  23 };
  24
  25 // Utility class for buffering streamed MotionEventVector until a given flush.
  26 // Events that can be combined will remain buffered, and depending on the flush
  27 // time and buffered events, a resampled event with history will be synthesized.
  28 // The primary purpose of this class is to ensure a smooth output motion signal
  29 // by resampling a discrete input signal that may run on a different frequency
  30 // or lack alignment with the output display signal.
  31 // Note that this class is largely based on code from Android's existing touch
  32 // pipeline (in particular, logic from ImageTransport, http://goo.gl/Ixsb0D).
  33 // See the design doc at http://goo.gl/MdmpCf for more details.
  34 class GESTURE_DETECTION_EXPORT MotionEventBuffer {
  35  public:
  36   // The provided |client| must not be null, and |enable_resampling| determines
  37   // resampling behavior (see |resample_|).
  38   MotionEventBuffer(MotionEventBufferClient* client, bool enable_resampling);
  39   ~MotionEventBuffer();
  40
  41   // Should be called upon receipt of an event from the platform, prior to event
  42   // dispatch to UI or content components. Events that can be coalesced will
  43   // remain buffered until the next |Flush()|, while other events will be
  44   // forwarded immediately (incidentally flushing currently buffered events).
  45   void OnMotionEvent(const MotionEvent& event);
  46
  47   // Forward any buffered events, resampling if necessary (see |resample_|)
  48   // according to the provided |frame_time|. This should be called in response
  49   // to |SetNeedsFlush()| calls on the client. If the buffer is empty, no
  50   // events will be forwarded, and if another flush is necessary it will be
  51   // requested.
  52   void Flush(base::TimeTicks frame_time);
  53
  54  private:
  55   typedef ScopedVector<MotionEvent> MotionEventVector;
  56
  57   void FlushWithoutResampling(MotionEventVector events);
  58
  59   MotionEventBufferClient* const client_;
  60   MotionEventVector buffered_events_;
  61
  62   // Time of the most recently extrapolated event. This will be 0 if the
  63   // last sent event was not extrapolated. Used internally to guard against
  64   // conflicts between events received from the platfrom that may have an
  65   // earlier timestamp than that synthesized at the latest resample.
  66   base::TimeTicks last_extrapolated_event_time_;
  67
  68   // Whether buffered events should be resampled upon |Flush()|. If true, short
  69   // horizon interpolation/extrapolation will be used to synthesize the
  70   // forwarded event. Otherwise the most recently buffered event will be
  71   // forwarded, with preceding events as historical entries. Defaults to true.
  72   bool resample_;
  73
  74   DISALLOW_COPY_AND_ASSIGN(MotionEventBuffer);
  75 };
  76
  77 }  // namespace ui
  78
  79 #endif  // UI_EVENTS_GESTURE_DETECTION_MOTION_EVENT_BUFFER_H_