Add an initial revision of an audio server.

services/media/audio/platform/generic/throttle_output.cc

+// Copyright 2015 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.
+
+#include "base/bind.h"
+#include "base/callback.h"
+#include "base/location.h"
+#include "base/logging.h"
+#include "base/time/time.h"
+#include "mojo/services/media/common/cpp/local_time.h"
+#include "services/media/audio/audio_output_manager.h"
+#include "services/media/audio/platform/generic/throttle_output.h"
+
+namespace mojo {
+namespace media {
+namespace audio {
+
+static constexpr LocalDuration TRIM_PERIOD = local_time::from_msec(10);
+
+ThrottleOutput::ThrottleOutput(AudioOutputManager* manager)
+  : StandardOutputBase(manager) {}
+
+ThrottleOutput::~ThrottleOutput() {}
+
+AudioOutputPtr ThrottleOutput::New(AudioOutputManager* manager) {
+  return AudioOutputPtr(new ThrottleOutput(manager));
+}
+
+MediaResult ThrottleOutput::Init() {
+  last_sched_time_ = LocalClock::now();
+  return MediaResult::OK;
+}
+
+bool ThrottleOutput::StartMixJob(MixJob* job,
+                                 const LocalTime& process_start) {
+  // Compute our next callback time, and check to see if we are falling behind
+  // in the process.
+  last_sched_time_ += TRIM_PERIOD;
+  if (process_start > last_sched_time_) {
+    // TODO(johngro): We are falling behind on our trimming.  We should
+    // probably tell someone.
+    last_sched_time_ = process_start + TRIM_PERIOD;;
+  }
+
+  // TODO(johngro): We could optimize this trim operation by scheduling our
+  // callback to the time at which the first pending packet in our queue will
+  // end, instead of using this polling style.  This would have the addition
+  // benefit of tighten the timing on returning packets (currently, we could
+  // hold a packet for up to TRIM_PERIOD - episilon past its end pts before
+  // releasing it)
+  //
+  // In order to do this, however, we would to wake up and recompute whenever
+  // the rate transformations for one of our client tracks changes.  For now, we
+  // just poll because its simpler.
+  SetNextSchedTime(last_sched_time_);
+
+  // The throttle output never actually mixes anything, it just provides
+  // backpressure to the pipeline by holding references to AudioPackets until
+  // after their presentation should be finished.  All we need to do here is
+  // schedule our next callback to keep things running, and let the base class
+  // implementation handle trimming the output.
+  return false;
+}
+
+bool ThrottleOutput::FinishMixJob(const MixJob& job) {
+  // Since we never start any jobs, this should never be called.
+  DCHECK(false);
+  return false;
+}
+
+}  // namespace audio
+}  // namespace media
+}  // namespace mojo
+