Mojo Log service and a thread-safe client library.

mojo/services/log/cpp/log_client.h

Index: mojo/services/log/cpp/log_client.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_SERVICES_LOG_CPP_LOG_CLIENT_H_
+#define MOJO_SERVICES_LOG_CPP_LOG_CLIENT_H_
+
+#include "mojo/public/c/environment/logger.h"
+#include "mojo/services/log/interfaces/log.mojom.h"
+
+namespace mojo {
+namespace log {
+
+// This is a client library that constructs a MojoLogger that talks to a
+// Mojo logging service (see ../interfaces/log.mojo). It provides a |MojoLogger|
+// implementation for logging, which you may use the default logger.
+//
+// LogClient is a singleton, and it must live for duration of its logger's use.
+// The constructor will initialize some internal state and take ownership of the
+// supplied |log_service|. If the pipe to the supplied |log_service| ever fails,
+// |LogClient| will fallback to using the supplied |fallback_logger|.
+//
+// This class itself is not thread-safe, but the MojoLogger returned by
+// LogClient::GetLogger() is thread-safe for the life time of LogClient.
+//
+// An example app that sets up the LogClient and uses it as the default logger:
+//
+//  class MyDelegate : public mojo::ApplicationDelegate {
+//   public:
+//    void Initialize(mojo::ApplicationImpl* app) override {
+//      LogPtr log;
+//      app->ConnectToService("mojo:log", &log);
+//      log_client_.reset(new LogClient(std::move(log),
+//                                      ApplicationRunner::GetDefaultLogger()));
+//      mojo::ApplicationRunner::SetDefaultLogger(LogClient::GetLogger());
+//    }
+//
+//   private:
+//    std::unique_ptr<mojo::log::LogClient> log_client_;
+//  };
+//
+//  MojoResult MojoMain(MojoHandle app_request) {
+//    mojo::ApplicationRunner runner(new MyDelegate);
+//    return runner.Run(app_request);
+//  }
+//
+// Here, we see that the LogClient lives for the duration of its
+// ApplicationDelegate, which lives for the duration of its ApplicationRunner.
+class LogClient {
+ public:
+  LogClient(LogPtr log_service, const MojoLogger* fallback_logger);
+  ~LogClient();
+
+  // You can retrieve the logger before any LogClient instance is created, but
+  // any uses of the logger may assert-fail.
+  static const MojoLogger* GetLogger();
+};
+
+}  // namespace log
+}  // namespace mojo
+
+#endif  // MOJO_SERVICES_LOG_CPP_LOG_CLIENT_H_