EDK: Add a mojo::platform::MessageLoop abstraction.

mojo/edk/platform/message_loop.h

+// 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.
+
+// This file provides an interface for "message loops", which are used within
+// the EDK itself.
+
+#ifndef MOJO_EDK_PLATFORM_MESSAGE_LOOP_H_
+#define MOJO_EDK_PLATFORM_MESSAGE_LOOP_H_
+
+#include "mojo/edk/util/ref_ptr.h"
+#include "mojo/public/cpp/system/macros.h"
+
+namespace mojo {
+namespace platform {
+
+class TaskRunner;
+
+// Interface for "message loops", which receives and executes tasks. In general,
+// a |MessageLoop| need not be thread-safe: except as otherwise noted, its
+// methods may only be called on the thread it was created on (the |MessageLoop|
+// is said to "belong to" that thread).
+//
+// In general, the result of running a |MessageLoop| "inside" a |MessageLoop|
+// (whether the same one -- the classic "nested message loop" case -- or a
+// different one) is implementation-defined.
+class MessageLoop {
+ public:
+  virtual ~MessageLoop() {}
+
+  // Runs the message loop until it is told to quit (via |QuitWhenIdle()|).
+  virtual void Run() = 0;
+
+  // Runs the message loop until there are no more tasks available to execute
+  // immediately (i.e., not including delayed tasks).
+  virtual void RunUntilIdle() = 0;
+
+  // Quits the message loop when there are no more tasks available to execute
+  // immediately. (Note that this includes "future" tasks, i.e., those that are
+  // posted as a result of executing other tasks, so this may never quit.
+  // However, it does not include delayed tasks.)
+  // TODO(vtl): Do we also want a |QuitNow()|?
+  virtual void QuitWhenIdle() = 0;
+
+  // Gets the |TaskRunner| for this message loop, which can be used to post
+  // tasks to it. For a given |MessageLoop| instance, this will always return a
+  // reference to the same |TaskRunner| (and different |MessageLoop| instances
+  // have different |TaskRunner|s). This may be called from any thread (and
+  // returned |TaskRunner| is also thread-safe).
+  //
+  // Note: The returned |TaskRunner| should only claim to run tasks on the
+  // thread which this message loop belongs to.
+  virtual const mojo::util::RefPtr<TaskRunner>& GetTaskRunner() const = 0;
+
+  // Returns true if this message loop belongs to the current thread (i.e., was
+  // created on and executes tasks on this thread) *and* if it is currently
+  // running (i.e., is called from within a task executed "under" |Run()| or
+  // |RunUntilIdle()|). This may be called from any thread.
+  //
+  // If message loops are nested on the current thread, this must return true
+  // for the "innermost" message loop. The result for "outer" message loops is
+  // implementation-defined.
+  virtual bool IsRunningOnCurrentThread() const = 0;
+
+ protected:
+  MessageLoop() {}
+
+ private:
+  MOJO_DISALLOW_COPY_AND_ASSIGN(MessageLoop);
+};
+
+}  // namespace platform
+}  // namespace mojo
+
+#endif  // MOJO_EDK_PLATFORM_MESSAGE_LOOP_H_