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

components/offline_pages/core/test_task.h

Index: components/offline_pages/core/test_task.h
diff --git a/components/offline_pages/core/test_task.h b/components/offline_pages/core/test_task.h
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+// 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 COMPONENTS_OFFLINE_PAGES_CORE_TEST_TASK_H_
+#define COMPONENTS_OFFLINE_PAGES_CORE_TEST_TASK_H_
+
+#include "components/offline_pages/core/task.h"
+
+namespace offline_pages {
+
+// Sample resource consumed by the task during execution. In this set of tests
+// used to provide the capability to continue the task.
+class ConsumedResource {
+ public:
+  ConsumedResource();
+  ~ConsumedResource();
+
+  void Step(const base::Closure& step_callback);
+  void CompleteStep();
+  bool HasNextStep() const { return !next_step_.is_null(); }
+
+ private:
+  base::Closure next_step_;
+};
+
+// Sample test task. This should not be used as a example of task implementation

[Pete Williamson, 2016/09/29 00:54:44]

Would it be worthwhile to make it a good example?  Folks often turn to tests for
a good sample.  If not, maybe we could point them to a good example to use
instead (the upcoming ChangeState Task).

[fgorski, 2016/09/29 17:37:37]

No point in making this better as this is a test support.
Add reference to ChangeRequqestsStateTask.

+// with respect to callback safety. Otherwise it captures the idea of splitting
+// the work into multiple steps separated by potentially asynchronous calls
+// spanning multiple threads.
+class TestTask : public Task {
+ public:
+  enum class TaskState { NOT_STARTED, STEP_1, STEP_2, COMPLETED };
+
+  explicit TestTask(ConsumedResource* resource);
+  TestTask(ConsumedResource* resource, bool leave_early);
+  ~TestTask() override;
+
+  // Run is Step 1 in our case.
+  void Run() override;
+
+  void Step2();

[dougarnett, 2016/09/29 15:40:05]

Maybe good to promote the "step" terminology in the class definition and then
this example fits it.

[fgorski, 2016/09/29 17:37:37]

No I don't believe that. I actually thought about renaming this to something
better, but that actually fits the test scenario.

What I expect in implementation is a valid name describing what the step is,
like:
UpdateRequestsState, SelectItemsToUpdate or something like that.

See
https://codereview.chromium.org/2372043002/diff/20001/components/offline_page...
for example.

+
+  void LastStep();
+
+  TaskState state() const { return state_; }
+
+ private:
+  ConsumedResource* resource_;
+  TaskState state_;
+  bool leave_early_;
+};
+
+}  // namespace offline_pages
+
+#endif  // COMPONENTS_OFFLINE_PAGES_CORE_TEST_TASK_H_