ImageCapture: ScopedResultCallback

media/capture/video/scoped_callback.h

+// 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 MEDIA_CAPTURE_VIDEO_SCOPED_CALLBACK_H_
+#define MEDIA_CAPTURE_VIDEO_SCOPED_CALLBACK_H_
+
+#include "base/callback_forward.h"
+
+namespace media {
+
+// This class wraps a |callback_| and guarantees that it will be called on
+// destruction and according to the provided OnErrorCallback if it hasn't been
+// retrieved before (via PassCallback()). Inspired by ScopedWebCallbacks
+// template.
+template <typename CallbackType>
+class ScopedCallback {

[miu, 2016/05/27 22:36:38]

I think this exists already (in base/callback_helpers.h).

[mcasas, 2016/05/27 23:36:06]

Partially. There is ScopedClosureRunner(), which will 
call the closure it wraps when it gets destroyed, 
whereas ScopedCallback will call the ctor OnErrorCallback 
when it's destructed. 

The rationale here is that I'd like to create a ScopedCallback
as soon as the mojo 'command' is 'serviced' and use 
is as a move-only type so when it's destroyed, the
said OnErrorCallback will be called, unless the mojo
callback has been taken out (with PassCallback()).

It basically implies that there are two ways to
resolve a mojo callback (success/failure), their 
semantics differing by their arguments.

+  MOVE_ONLY_TYPE_FOR_CPP_03(ScopedCallback);
+
+ public:
+  using OnErrorCallback = base::Callback<void(const CallbackType&)>;
+  ScopedCallback(const CallbackType& callback,
+                 const OnErrorCallback& on_error_callback)
+      : callback_(callback), on_error_callback_(on_error_callback) {}
+
+  ~ScopedCallback() {
+    if (!callback_.is_null())
+      on_error_callback_.Run(callback_);
+  }
+
+  ScopedCallback(ScopedCallback&& other) { *this = std::move(other); }
+
+  ScopedCallback& operator=(ScopedCallback&& other) {
+    callback_ = other.PassCallback();
+    on_error_callback_ = other.on_error_callback_;
+    return *this;
+  }
+
+  CallbackType PassCallback() {
+    CallbackType callback = callback_;
+    callback_.reset();
+    return callback;
+  }
+
+ private:
+  CallbackType callback_;
+  OnErrorCallback on_error_callback_;
+};
+
+}  // namespace media
+
+#endif  // MEDIA_CAPTURE_VIDEO_SCOPED_CALLBACK_H_