Factor stuff from ApplicationImpl out to a new class, ApplicationImplBase. ApplicationImpl is now a rather dumb class that just calls its ApplicationDelegate. Using base/implementation classes is preferable to the delegate pattern because: * It's simpler (see below). * With the delgate pattern, the impl and the delegate both need pointers to each other. And so there are lifetime problems/considerations. * The delegate just ends up calling into the impl a lot. Unfortunately, it's rather hard (impossible) to use ApplicationImplBase right now (as intended): I'll need to refactor ApplicationRunner(Chromium) to not just instantiate ApplicationImpl, etc. (I'll also need a story for the crazy that is ApplicationImpl(Base)::Terminate().) R=vardhan@google.com Review URL: https://codereview.chromium.org/1985223003 .

diff --git a/examples/bank_app/customer.cc b/examples/bank_app/customer.cc
index 089de8d..77562c9 100644
--- a/examples/bank_app/customer.cc
+++ b/examples/bank_app/customer.cc

@@ -6,6 +6,7 @@
 
 #include "examples/bank_app/bank.mojom.h"
 #include "mojo/public/c/system/main.h"
+#include "mojo/public/cpp/application/application_delegate.h"
 #include "mojo/public/cpp/application/application_impl.h"
 #include "mojo/public/cpp/application/application_runner.h"
 #include "mojo/public/cpp/application/connect.h"

diff --git a/mojo/application/application_runner_chromium.cc b/mojo/application/application_runner_chromium.cc
index b528034..7a4635e 100644
--- a/mojo/application/application_runner_chromium.cc
+++ b/mojo/application/application_runner_chromium.cc

@@ -11,13 +11,14 @@
 #include "mojo/message_pump/message_pump_mojo.h"
 #include "mojo/public/cpp/application/application_delegate.h"
 #include "mojo/public/cpp/application/application_impl.h"
+#include "mojo/public/cpp/application/application_impl_base.h"
 #include "mojo/public/cpp/system/handle.h"
 #include "mojo/public/cpp/system/message_pipe.h"
 
 namespace mojo {
 
 // static
-void ApplicationImpl::Terminate() {
+void ApplicationImplBase::Terminate() {
   if (base::MessageLoop::current()->is_running())
     base::MessageLoop::current()->Quit();
 }

diff --git a/mojo/public/cpp/application/BUILD.gn b/mojo/public/cpp/application/BUILD.gn
index 543207f..ec0deae 100644
--- a/mojo/public/cpp/application/BUILD.gn
+++ b/mojo/public/cpp/application/BUILD.gn

@@ -9,10 +9,12 @@
   sources = [
     "application_delegate.h",
     "application_impl.h",
+    "application_impl_base.h",
     "connect.h",
     "connection_context.h",
     "lib/application_delegate.cc",
     "lib/application_impl.cc",
+    "lib/application_impl_base.cc",
     "lib/connect.cc",
     "lib/service_provider_impl.cc",
     "service_connector.h",

diff --git a/mojo/public/cpp/application/application_impl.h b/mojo/public/cpp/application/application_impl.h
index 2621daa..fffc890 100644
--- a/mojo/public/cpp/application/application_impl.h
+++ b/mojo/public/cpp/application/application_impl.h

@@ -5,17 +5,12 @@
 #ifndef MOJO_PUBLIC_CPP_APPLICATION_APPLICATION_IMPL_H_
 #define MOJO_PUBLIC_CPP_APPLICATION_APPLICATION_IMPL_H_
 
-#include <memory>
-#include <string>
-#include <vector>
-
-#include "mojo/public/cpp/application/application_delegate.h"
+#include "mojo/public/cpp/application/application_impl_base.h"
 #include "mojo/public/cpp/system/macros.h"
-#include "mojo/public/interfaces/application/application.mojom.h"
-#include "mojo/public/interfaces/application/shell.mojom.h"
 
 namespace mojo {
 
+class ApplicationDelegate;
 class ServiceProviderImpl;
 
 // Implements the Application interface, which the shell uses for basic
@@ -30,7 +25,7 @@
 // ConfigureIncomingConnection() adds services to each connection. Finally, you
 // instantiate your delegate and pass it to an ApplicationRunner, which will
 // create the ApplicationImpl and then run a message (or run) loop.
-class ApplicationImpl : public Application {
+class ApplicationImpl : public ApplicationImplBase {
  public:
   // Does not take ownership of |delegate|, which must remain valid for the
   // lifetime of ApplicationImpl.
@@ -38,38 +33,13 @@
                   InterfaceRequest<Application> request);
   ~ApplicationImpl() override;
 
-  // Quits the main run loop for this application.
-  // TODO(vtl): This is implemented in application_runner.cc (for example). Its
-  // presence here is pretty dubious.
-  static void Terminate();
-
-  // The Mojo shell. This will return a valid pointer after Initialize() has
-  // been invoked. It will remain valid until this object is destroyed.
-  Shell* shell() const { return shell_.get(); }
-
-  const std::string& url() const { return url_; }
-
-  // Returns any initial configuration arguments, passed by the Shell.
-  const std::vector<std::string>& args() const { return args_; }
-  bool HasArg(const std::string& arg) const;
-
  private:
-  // |Application| implementation.
-  void Initialize(InterfaceHandle<Shell> shell,
-                  Array<String> args,
-                  const mojo::String& url) override;
-  void AcceptConnection(const String& requestor_url,
-                        InterfaceRequest<ServiceProvider> services,
-                        InterfaceHandle<ServiceProvider> exposed_services,
-                        const String& url) override;
-  void RequestQuit() override;
+  // |ApplicationImplBase| implementation/overrides:
+  void OnInitialize() final;
+  bool OnAcceptConnection(ServiceProviderImpl* service_provider_impl) final;
+  void OnQuit() final;
 
-  std::vector<std::unique_ptr<ServiceProviderImpl>> service_provider_impls_;
-  ApplicationDelegate* delegate_;
-  Binding<Application> application_binding_;
-  ShellPtr shell_;
-  std::string url_;
-  std::vector<std::string> args_;
+  ApplicationDelegate* const delegate_;
 
   MOJO_DISALLOW_COPY_AND_ASSIGN(ApplicationImpl);
 };

diff --git a/mojo/public/cpp/application/application_impl_base.h b/mojo/public/cpp/application/application_impl_base.h
new file mode 100644
index 0000000..04109d7
--- /dev/null
+++ b/mojo/public/cpp/application/application_impl_base.h

@@ -0,0 +1,98 @@
+// 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.
+
+#ifndef MOJO_PUBLIC_CPP_APPLICATION_APPLICATION_IMPL_BASE_H_
+#define MOJO_PUBLIC_CPP_APPLICATION_APPLICATION_IMPL_BASE_H_
+
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "mojo/public/cpp/bindings/binding.h"
+#include "mojo/public/cpp/bindings/interface_request.h"
+#include "mojo/public/cpp/system/macros.h"
+#include "mojo/public/interfaces/application/application.mojom.h"
+#include "mojo/public/interfaces/application/shell.mojom.h"
+
+namespace mojo {
+
+class ServiceProviderImpl;
+
+// Base helper class for implementing the |Application| interface, which the
+// shell uses for basic communication with an application (e.g., to connect
+// clients to services provided by an application).
+//
+// To use this class, subclass it and implement/override the required methods
+// (see below).
+//
+// TODO(vtl): ApplicationRunners should take this instead of an
+// ApplicationDelegate. Write more here when that's true (it's pretty hard to
+// use this class in the current setup).
+class ApplicationImplBase : public Application {
+ public:
+  explicit ApplicationImplBase(
+      InterfaceRequest<Application> application_request);
+  ~ApplicationImplBase() override;
+
+  // Quits the main run loop for this application.
+  // TODO(vtl): This is implemented in application_runner.cc (for example). Its
+  // presence here is pretty dubious.
+  static void Terminate();
+
+  // This will be valid after |Initialize()| has been received and remain valid
+  // until this object is destroyed.
+  Shell* shell() const { return shell_.get(); }
+
+  // Returns any initial configuration arguments, passed by the shell.
+  const std::vector<std::string>& args() const { return args_; }
+  bool HasArg(const std::string& arg) const;
+
+  const std::string& url() const { return url_; }
+
+  // Methods to be implemented/overridden by subclasses:
+
+  // Called after |Initialize()| has been received (|shell()|, |args()|, and
+  // |url()| will be valid when this is called. The default implementation does
+  // nothing.
+  virtual void OnInitialize() {}
+
+  // Called when another application connects to this application (i.e., we
+  // receive |AcceptConnection()|). This should either configure what services
+  // are "provided" (made available via a |ServiceProvider|) to that application
+  // and return true, or this may return false to reject the connection
+  // entirely.
+  virtual bool OnAcceptConnection(
+      ServiceProviderImpl* service_provider_impl) = 0;
+
+  // Called before quitting the main message (run) loop, i.e., before
+  // |Terminate()|. The default implementation does nothing.
+  virtual void OnQuit() {}
+
+ private:
+  // |Application| implementation. In general, you probably shouldn't call these
+  // directly (but I can't really stop you).
+  void Initialize(InterfaceHandle<Shell> shell,
+                  Array<String> args,
+                  const mojo::String& url) final;
+  void AcceptConnection(const String& requestor_url,
+                        InterfaceRequest<ServiceProvider> services,
+                        InterfaceHandle<ServiceProvider> exposed_services,
+                        const String& url) final;
+  void RequestQuit() final;
+
+  Binding<Application> application_binding_;
+
+  // Set by |Initialize()|.
+  ShellPtr shell_;
+  std::vector<std::string> args_;
+  std::string url_;
+
+  std::vector<std::unique_ptr<ServiceProviderImpl>> service_provider_impls_;
+
+  MOJO_DISALLOW_COPY_AND_ASSIGN(ApplicationImplBase);
+};
+
+}  // namespace mojo
+
+#endif  // MOJO_PUBLIC_CPP_APPLICATION_APPLICATION_IMPL_BASE_H_

diff --git a/mojo/public/cpp/application/lib/application_impl.cc b/mojo/public/cpp/application/lib/application_impl.cc
index 6be42ff..194c942 100644
--- a/mojo/public/cpp/application/lib/application_impl.cc
+++ b/mojo/public/cpp/application/lib/application_impl.cc

@@ -4,64 +4,27 @@
 
 #include "mojo/public/cpp/application/application_impl.h"
 
-#include <utility>
-
 #include "mojo/public/cpp/application/application_delegate.h"
-#include "mojo/public/cpp/application/connection_context.h"
-#include "mojo/public/cpp/application/service_provider_impl.h"
-#include "mojo/public/cpp/bindings/interface_ptr.h"
-#include "mojo/public/cpp/bindings/interface_request.h"
-#include "mojo/public/cpp/environment/logging.h"
 
 namespace mojo {
 
 ApplicationImpl::ApplicationImpl(ApplicationDelegate* delegate,
                                  InterfaceRequest<Application> request)
-    : delegate_(delegate), application_binding_(this, request.Pass()) {}
+    : ApplicationImplBase(request.Pass()), delegate_(delegate) {}
 
 ApplicationImpl::~ApplicationImpl() {}
 
-bool ApplicationImpl::HasArg(const std::string& arg) const {
-  return std::find(args_.begin(), args_.end(), arg) != args_.end();
-}
-
-void ApplicationImpl::Initialize(InterfaceHandle<Shell> shell,
-                                 Array<String> args,
-                                 const mojo::String& url) {
-  shell_ = ShellPtr::Create(std::move(shell));
-  shell_.set_connection_error_handler([this]() {
-    delegate_->Quit();
-    service_provider_impls_.clear();
-    Terminate();
-  });
-  url_ = url;
-  args_ = args.To<std::vector<std::string>>();
+void ApplicationImpl::OnInitialize() {
   delegate_->Initialize(this);
 }
 
-void ApplicationImpl::AcceptConnection(
-    const String& requestor_url,
-    InterfaceRequest<ServiceProvider> services,
-    InterfaceHandle<ServiceProvider> exposed_services,
-    const String& url) {
-  // Note: The shell no longer actually connects |exposed_services|, so a) we
-  // never actually get valid |exposed_services| here, b) it should be OK to
-  // drop it on the floor.
-  MOJO_LOG_IF(ERROR, exposed_services)
-      << "DEPRECATED: exposed_services is going away";
-  std::unique_ptr<ServiceProviderImpl> service_provider_impl(
-      new ServiceProviderImpl(
-          ConnectionContext(ConnectionContext::Type::INCOMING, requestor_url,
-                            url),
-          services.Pass()));
-  if (!delegate_->ConfigureIncomingConnection(service_provider_impl.get()))
-    return;
-  service_provider_impls_.push_back(std::move(service_provider_impl));
+bool ApplicationImpl::OnAcceptConnection(
+    ServiceProviderImpl* service_provider_impl) {
+  return delegate_->ConfigureIncomingConnection(service_provider_impl);
 }
 
-void ApplicationImpl::RequestQuit() {
+void ApplicationImpl::OnQuit() {
   delegate_->Quit();
-  Terminate();
 }
 
 }  // namespace mojo

diff --git a/mojo/public/cpp/application/lib/application_impl_base.cc b/mojo/public/cpp/application/lib/application_impl_base.cc
new file mode 100644
index 0000000..a5e90d6
--- /dev/null
+++ b/mojo/public/cpp/application/lib/application_impl_base.cc

@@ -0,0 +1,65 @@
+// 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 "mojo/public/cpp/application/application_impl_base.h"
+
+#include <algorithm>
+#include <utility>
+
+#include "mojo/public/cpp/application/connection_context.h"
+#include "mojo/public/cpp/application/service_provider_impl.h"
+#include "mojo/public/cpp/environment/logging.h"
+
+namespace mojo {
+
+ApplicationImplBase::ApplicationImplBase(
+    InterfaceRequest<Application> application_request)
+    : application_binding_(this, application_request.Pass()) {}
+
+ApplicationImplBase::~ApplicationImplBase() {}
+
+bool ApplicationImplBase::HasArg(const std::string& arg) const {
+  return std::find(args_.begin(), args_.end(), arg) != args_.end();
+}
+
+void ApplicationImplBase::Initialize(InterfaceHandle<Shell> shell,
+                                     Array<String> args,
+                                     const mojo::String& url) {
+  shell_ = ShellPtr::Create(std::move(shell));
+  shell_.set_connection_error_handler([this]() {
+    OnQuit();
+    service_provider_impls_.clear();
+    Terminate();
+  });
+  url_ = url;
+  args_ = args.To<std::vector<std::string>>();
+  OnInitialize();
+}
+
+void ApplicationImplBase::AcceptConnection(
+    const String& requestor_url,
+    InterfaceRequest<ServiceProvider> services,
+    InterfaceHandle<ServiceProvider> exposed_services,
+    const String& url) {
+  // Note: The shell no longer actually connects |exposed_services|, so a) we
+  // never actually get valid |exposed_services| here, b) it should be OK to
+  // drop it on the floor.
+  MOJO_LOG_IF(ERROR, exposed_services)
+      << "DEPRECATED: exposed_services is going away";
+  std::unique_ptr<ServiceProviderImpl> service_provider_impl(
+      new ServiceProviderImpl(
+          ConnectionContext(ConnectionContext::Type::INCOMING, requestor_url,
+                            url),
+          services.Pass()));
+  if (!OnAcceptConnection(service_provider_impl.get()))
+    return;
+  service_provider_impls_.push_back(std::move(service_provider_impl));
+}
+
+void ApplicationImplBase::RequestQuit() {
+  OnQuit();
+  Terminate();
+}
+
+}  // namespace mojo

diff --git a/mojo/public/cpp/application/lib/application_runner.cc b/mojo/public/cpp/application/lib/application_runner.cc
index 4ef544b..a5c87b7 100644
--- a/mojo/public/cpp/application/lib/application_runner.cc
+++ b/mojo/public/cpp/application/lib/application_runner.cc

@@ -6,6 +6,7 @@
 
 #include "mojo/public/cpp/application/application_delegate.h"
 #include "mojo/public/cpp/application/application_impl.h"
+#include "mojo/public/cpp/application/application_impl_base.h"
 #include "mojo/public/cpp/environment/environment.h"
 #include "mojo/public/cpp/environment/logging.h"
 #include "mojo/public/cpp/utility/run_loop.h"
@@ -16,7 +17,7 @@
 }  // namespace
 
 // static
-void ApplicationImpl::Terminate() {
+void ApplicationImplBase::Terminate() {
   RunLoop::current()->Quit();
 }
 

diff --git a/mojo/ui/view_provider_app.h b/mojo/ui/view_provider_app.h
index 177f419..4f6293b 100644
--- a/mojo/ui/view_provider_app.h
+++ b/mojo/ui/view_provider_app.h

@@ -9,6 +9,7 @@
 
 #include "mojo/common/strong_binding_set.h"
 #include "mojo/public/c/system/main.h"
+#include "mojo/public/cpp/application/application_delegate.h"
 #include "mojo/public/cpp/application/application_impl.h"
 #include "mojo/public/cpp/application/service_provider_impl.h"
 #include "mojo/public/cpp/system/macros.h"

diff --git a/services/nacl/nonsfi/content_handler_main_nexe.cc b/services/nacl/nonsfi/content_handler_main_nexe.cc
index 207ecf0..0ccada1 100644
--- a/services/nacl/nonsfi/content_handler_main_nexe.cc
+++ b/services/nacl/nonsfi/content_handler_main_nexe.cc

@@ -11,6 +11,7 @@
 #include "mojo/message_pump/message_pump_mojo.h"
 #include "mojo/nacl/nonsfi/nexe_launcher_nonsfi.h"
 #include "mojo/public/c/system/main.h"
+#include "mojo/public/cpp/application/application_delegate.h"
 #include "mojo/public/cpp/application/application_impl.h"
 
 namespace nacl {

diff --git a/services/nacl/nonsfi/content_handler_main_pexe.cc b/services/nacl/nonsfi/content_handler_main_pexe.cc
index b992367..f0ecc3b 100644
--- a/services/nacl/nonsfi/content_handler_main_pexe.cc
+++ b/services/nacl/nonsfi/content_handler_main_pexe.cc

@@ -15,6 +15,7 @@
 #include "mojo/nacl/nonsfi/file_util.h"
 #include "mojo/nacl/nonsfi/nexe_launcher_nonsfi.h"
 #include "mojo/public/c/system/main.h"
+#include "mojo/public/cpp/application/application_delegate.h"
 #include "mojo/public/cpp/application/application_impl.h"
 #include "mojo/public/cpp/application/connect.h"
 #include "mojo/public/cpp/bindings/array.h"