Making Mandoline apps notice when all the objects they vended are gone so that it can terminate.

mojo/application/app_lifetime_helper.h

Index: mojo/application/app_lifetime_helper.h
diff --git a/mojo/application/app_lifetime_helper.h b/mojo/application/app_lifetime_helper.h
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+// 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_APPLICATION_APP_LIFETIME_HELPER_H_
+#define MOJO_APPLICATION_APP_LIFETIME_HELPER_H_
+
+#include "base/basictypes.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/single_thread_task_runner.h"
+
+namespace mojo {
+
+class AppLifetimeHelper;
+
+// A service implementation should keep this object as a member variable to hold
+// a reference on the application.
+// Since services can live on different threads than the app, this class is
+// safe to use on any thread. However, each instance should only be used on one
+// thread at a time (otherwise there'll be races between the addref resulting
+// from cloning and destruction).
+class AppRefCount {
+ public:
+  ~AppRefCount();
+
+  // When a service creates another object that is held by the client, it should
+  // also vend to it a refcount using this method. That way if the caller stops
+  // holding on to the service but keeps the reference to the object, the app is
+  // still alive.
+  scoped_ptr<AppRefCount> Clone();
+
+ private:
+  friend AppLifetimeHelper;
+
+  AppRefCount(AppLifetimeHelper* app_lifetime_helper,
+              scoped_refptr<base::SingleThreadTaskRunner> app_task_runner);
+
+  // Don't need to use weak ptr because if the app thread is alive that means
+  // app is.
+  AppLifetimeHelper* app_lifetime_helper_;
+  scoped_refptr<base::SingleThreadTaskRunner> app_task_runner_;
+
+#ifndef NDEBUG
+  scoped_refptr<base::SingleThreadTaskRunner> clone_task_runner_;
+#endif
+
+  DISALLOW_COPY_AND_ASSIGN(AppRefCount);
+};
+
+// This is a helper class for apps to manage their lifetime, specifically so
+// apps can quit when they're not used anymore.
+// An app can contain an object of this class as a member variable. Each time it
+// creates an instance of a service, it gives it a refcount using
+// CreateAppRefCount. The service implementation then keeps that object as a
+// member variable. When all the service implemenations go away, the app will be
+// quit with a call to mojo::ApplicationImpl::Terminate().
+class AppLifetimeHelper {
+ public:
+  AppLifetimeHelper();
+  ~AppLifetimeHelper();
+
+  scoped_ptr<AppRefCount> CreateAppRefCount();
+
+ private:
+  friend AppRefCount;
+  void AddRef();
+  void Release();
+
+  int ref_count_;
+
+  DISALLOW_COPY_AND_ASSIGN(AppLifetimeHelper);
+};
+
+}  // namespace mojo
+
+#endif  // MOJO_APPLICATION_APP_LIFETIME_HELPER_H_