[WIP] - trying out some state machine ideas.

media/base/state_machine_driver.cc

Index: media/base/state_machine_driver.cc
diff --git a/media/base/state_machine_driver.cc b/media/base/state_machine_driver.cc
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+++ b/media/base/state_machine_driver.cc
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+// 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.
+
+#include "media/base/state_machine_driver.h"
+
+#include "base/auto_reset.h"
+#include "base/bind.h"
+#include "base/threading/thread_task_runner_handle.h"
+
+namespace media {
+
+StateMachineDriver::StateMachineDriver()
+    : currently_running_(false),
+      run_posted_(false),
+      task_runner_(base::ThreadTaskRunnerHandle::Get()),
+      weak_factory_(this) {}
+
+StateMachineDriver::~StateMachineDriver() {}
+
+void StateMachineDriver::RequestRun(StateMachine* machine) {
+  // Note that we might, technically, have this machine in |to_run|, which
+  // would be good enough.  as it is, this may schedule a run after that one,
+  // which is also okay.
+  machines_to_run_.insert(machine);
+
+  // If we're currently running, then RunAfterPost will notice these machines.
+  // Otherwise, post RunAfterPost if we haven't already.
+  if (!currently_running_ && !run_posted_) {
+    task_runner_->PostTask(FROM_HERE, base::Bind(&StateMachineDriver::RunTask,
+                                                 weak_factory_.GetWeakPtr()));
+
+    // I suppose that we could RequestRun directly here too, since it's not
+    // going to be re-entrant.  i'm not sure.  if two machines are run by
+    // different drivers, then they might be interleaved, but it's unclear
+    // if that's okay or not.  probably, it doesn't matter, since i'd expect
+    // every MCVD to have one driver for all its machines, and the state
+    // wouldn't be shared with other MCVD instances.  then again, maybe
+    // codec allocator state would be shared, or similar.  in that case,
+    // we might have threading issues to worry about too, if the MCVDs
+    // don't share a common thread.
+    // It's also not clear if we want RequestRun to ever run things before
+    // returning, separately from all of that.  We might want two variants:
+    // one that may run immediately, and one that must not.  the former
+    // would be useful for polling media codec, where it would be called
+    // from a timer.
+    run_posted_ = true;
+  }
+}
+
+void StateMachineDriver::RunImmediately(StateMachine* machine) {
+  DCHECK(!currently_running_);
+
+  // TODO(liberato): should we cancel any pending post?  It doesn't hurt if
+  // we don't, but we could.
+  machines_to_run_.insert(machine);
+  RunTask();
+}
+
+void StateMachineDriver::AddStateMachine(
+    std::unique_ptr<StateMachine> machine) {
+  machines_.insert(std::move(machine));
+  machine->SetOwner(this);
+}
+
+void StateMachineDriver::RunTask() {
+  run_posted_ = false;
+  base::AutoReset<bool> auto_reset(&currently_running_, true);
+
+  do {
+    std::unordered_set<StateMachine*> to_run(std::move(machines_to_run_));
+    for (StateMachine* machine : to_run)
+      machine->Run();
+    // That might cause more machines to want to run, so keep trying if we
+    // want to.  We can choose to post, too, if we want.
+  } while (!machines_to_run_.empty());
+}
+
+}  // namespace media