Simplify LogImpl a bit.

services/log/log_impl.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 SERVICES_LOG_LOG_IMPL_H_
 #define SERVICES_LOG_LOG_IMPL_H_
 
-#include <stdio.h>
-
+#include <functional>
 #include <string>
 
 #include "base/macros.h"
 #include "mojo/public/cpp/bindings/interface_request.h"
 #include "mojo/public/cpp/bindings/strong_binding.h"
 #include "mojo/services/log/interfaces/entry.mojom.h"
 #include "mojo/services/log/interfaces/log.mojom.h"
 
 namespace mojo {
 
 class ApplicationConnection;
 
 namespace log {
 
 // This is an implementation of the log service
-// (see mojo/services/log/interfaces/log.mojom).  It formats incoming messages
-// and writes them to the supplied FILE stream.  It does not take ownership of
-// the FILE stream.
+// (see mojo/services/log/interfaces/log.mojom). It formats incoming messages
+// and "prints" them using a supplied function.
 //
 // This service implementation binds a new Log implementation for each incoming
 // application connection.
 class LogImpl : public Log {
  public:
-  // LogImpl does not take ownership of |out_file|, so |out_file| must live for
-  // the duration of the incoming connection (which is left undefined).
+  // Function that prints the given (fully-formatted) log message.
+  using PrintLogMessageFunction =
+      std::function<void(const std::string& message)>;
+
+  // Note that |print_log_message_function| may be called many times, for the
+  // lifetime of the created object.
   static void Create(ApplicationConnection* connection,
                      InterfaceRequest<Log> request,
-                     FILE* out_file);
+                     PrintLogMessageFunction print_log_message_function);
 
   // |Log| implementation:
   void AddEntry(EntryPtr entry) override;
 
  private:
   LogImpl(const std::string& remote_url,
           InterfaceRequest<Log> request,
-          FILE* out_file);
+          PrintLogMessageFunction print_log_message_function);
   ~LogImpl() override;
 
   std::string FormatEntry(const EntryPtr& entry);
 
   const std::string remote_url_;
   StrongBinding<Log> binding_;
-  FILE* out_file_;
+  const PrintLogMessageFunction print_log_message_function_;
 
   DISALLOW_COPY_AND_ASSIGN(LogImpl);
 };
 
 }  // namespace log
 }  // namespace mojo
 
 #endif  // SERVICES_LOG_LOG_IMPL_H_