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 class MotionEventGeneric;
  17
  18 // Allows event forwarding and flush requests from a |MotionEventBuffer|.
  19 class MotionEventBufferClient {
  20  public:
  21   virtual ~MotionEventBufferClient() {}
  22   virtual void ForwardMotionEvent(const MotionEvent& event) = 0;
  23   virtual void SetNeedsFlush() = 0;
  24 };
  25
  26 // Utility class for buffering streamed MotionEventVector until a given flush.
  27 // Events that can be combined will remain buffered, and depending on the flush
  28 // time and buffered events, a resampled event with history will be synthesized.
  29 // The primary purpose of this class is to ensure a smooth output motion signal
  30 // by resampling a discrete input signal that may run on a different frequency
  31 // or lack alignment with the output display signal.
  32 // Note that this class is largely based on code from Android's existing touch
  33 // pipeline (in particular, logic from ImageTransport, http://goo.gl/Ixsb0D).
  34 // See the design doc at http://goo.gl/MdmpCf for more details.
  35 class GESTURE_DETECTION_EXPORT MotionEventBuffer {
  36  public:
  37   // The provided |client| must not be null, and |enable_resampling| determines
  38   // resampling behavior (see |resample_|).
  39   MotionEventBuffer(MotionEventBufferClient* client, bool enable_resampling);
  40   ~MotionEventBuffer();
  41
  42   // Should be called upon receipt of an event from the platform, prior to event
  43   // dispatch to UI or content components. Events that can be coalesced will
  44   // remain buffered until the next |Flush()|, while other events will be
  45   // forwarded immediately (incidentally flushing currently buffered events).
  46   void OnMotionEvent(const MotionEvent& event);
  47
  48   // Forward any buffered events, resampling if necessary (see |resample_|)
  49   // according to the provided |frame_time|. This should be called in response
  50   // to |SetNeedsFlush()| calls on the client. If the buffer is empty, no
  51   // events will be forwarded, and if another flush is necessary it will be
  52   // requested.
  53   void Flush(base::TimeTicks frame_time);
  54
  55  private:
  56   typedef ScopedVector<MotionEventGeneric> MotionEventVector;
  57
  58   void FlushWithResampling(MotionEventVector events,
  59                            base::TimeTicks resample_time);
  60   void FlushWithoutResampling(MotionEventVector events);
  61
  62   MotionEventBufferClient* const client_;
  63   MotionEventVector buffered_events_;
  64
  65   // Time of the most recently extrapolated event. This will be 0 if the
  66   // last sent event was not extrapolated. Used internally to guard against
  67   // conflicts between events received from the platform that may have an
  68   // earlier timestamp than that synthesized at the latest resample.
  69   base::TimeTicks last_extrapolated_event_time_;
  70
  71   // Whether buffered events should be resampled upon |Flush()|. If true, short
  72   // horizon interpolation/extrapolation will be used to synthesize the
  73   // forwarded event. Otherwise the most recently buffered event will be
  74   // forwarded, with preceding events as historical entries. Defaults to true.
  75   bool resample_;
  76
  77   DISALLOW_COPY_AND_ASSIGN(MotionEventBuffer);
  78 };
  79
  80 }  // namespace ui
  81
  82 #endif  // UI_EVENTS_GESTURE_DETECTION_MOTION_EVENT_BUFFER_H_