EDK: Remove mojo::platform::MessageLoopForIO and add PlatformHandleWatcher.

mojo/edk/platform/platform_handle_watcher.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.
+
+#ifndef MOJO_EDK_PLATFORM_PLATFORM_HANDLE_WATCHER_H_
+#define MOJO_EDK_PLATFORM_PLATFORM_HANDLE_WATCHER_H_
+
+#include <functional>
+#include <memory>
+
+#include "mojo/edk/platform/platform_handle.h"
+#include "mojo/public/cpp/system/macros.h"
+
+namespace mojo {
+namespace platform {
+
+// Interface for things that can watch |PlatformHandle|s to be readable/writable
+// without blocking. Typically, these are tied to a (particular) |MessageLoop|:
+// handles will only be watched while the loop is running and the lifetime of
+// the object is typically tied to the |MessageLoop|'s lifetime.
+class PlatformHandleWatcher {
+ public:
+  // Abstract "token" class returned by the |Watch()| (below). This object
+  // should be destroyed to cancel watching.
+  //
+  // Note: In theory, libevent (e.g.) can be used without a heap allocation on
+  // each "watch". However, in practice, including libevent's event.h pollutes
+  // the global namespace with |struct event|, which is very undesirable, so
+  // this is hidden via an indirection anyway. However, when using Chromium's
+  // |base::MessagePumpLibevent|, this leads to an extra indirection and yet
+  // another heap allocation.
+  class WatchToken {
+   public:
+    virtual ~WatchToken() {}
+
+   protected:
+    WatchToken() {}
+
+   private:
+    MOJO_DISALLOW_COPY_AND_ASSIGN(WatchToken);
+  };
+
+  ~PlatformHandleWatcher() {}
+
+  // Watches |platform_handle| to be readable and/or writable (without blocking)
+  // as indicated by the presence of |read_callback| and/or |write_callback| (at
+  // least one of which must have a valid target), respectively, at which point
+  // the respective callback will be called by the message loop. If |persistent|
+  // is true, the message loop will continue watching and calling the
+  // callback(s) as appropriate.
+  virtual std::unique_ptr<WatchToken> Watch(
+      PlatformHandle platform_handle,
+      bool persistent,
+      std::function<void()>&& read_callback,
+      std::function<void()>&& write_callback) = 0;
+
+ protected:
+  PlatformHandleWatcher() {}
+
+ private:
+  MOJO_DISALLOW_COPY_AND_ASSIGN(PlatformHandleWatcher);
+};
+
+}  // namespace platform
+}  // namespace mojo
+
+#endif  // MOJO_EDK_PLATFORM_PLATFORM_HANDLE_WATCHER_H_