[Offline pages] Introduces TaskQueue to serialize tasks that asynchronously access SQLStore

components/offline_pages/core/task.cc

+// 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 "components/offline_pages/core/task.h"
+
+#include "base/bind.h"
+#include "base/threading/thread_task_runner_handle.h"
+
+namespace offline_pages {
+
+Task::Task() {}
+
+Task::~Task() {}
+
+void Task::SetTaskCompletionCallback(
+    scoped_refptr<base::SingleThreadTaskRunner> task_completion_runner,
+    const TaskCompletionCallback& task_completion_callback) {
+  // Attempts to enforce that SetTaskCompletionCallback is at most called once
+  // and enforces that reasonable values are set once that happens.
+  DCHECK(!task_completion_runner_);
+  DCHECK(task_completion_runner);
+  DCHECK(task_completion_callback_.is_null());
+  DCHECK(!task_completion_callback.is_null());
+  task_completion_runner_ = task_completion_runner;
+  task_completion_callback_ = task_completion_callback;
+}
+
+void Task::TaskComplete() {
+  if (task_completion_callback_.is_null() || !task_completion_runner_)
+    return;
+
+  task_completion_runner_->PostTask(
+      FROM_HERE, base::Bind(task_completion_callback_, this));
+}
+
+}  // namespace offline_pages