Add WebTaskRunner::postCancellableTask

third_party/WebKit/Source/platform/scheduler/CancellableTaskFactory.h

Index: third_party/WebKit/Source/platform/scheduler/CancellableTaskFactory.h
diff --git a/third_party/WebKit/Source/platform/scheduler/CancellableTaskFactory.h b/third_party/WebKit/Source/platform/scheduler/CancellableTaskFactory.h
index e37af101b2859ab6c5bfc2b7009360ec4f14d345..9e57554a5ff6285f4cd9d0db7b3df3984599d4dc 100644
--- a/third_party/WebKit/Source/platform/scheduler/CancellableTaskFactory.h
+++ b/third_party/WebKit/Source/platform/scheduler/CancellableTaskFactory.h
@@ -16,6 +16,26 @@
 #include <memory>
 #include <type_traits>
 
+// WebTaskRunner::postCancellableTask will take over CancellableTaskFactory.

[dcheng, 2016/10/21 06:14:46]

Nit: take over => replace

[tzik, 2016/10/25 07:03:52]

Done.

+// Use postCancellableTask in new code.
+// Example: For |task_runner| and |foo| below.
+//   WebTaskRunner* task_runner;
+//   Foo* foo;
+//
+//   CancellableTaskFactory factory(foo, &Foo::bar);
+//   task_runner->postTask(BLINK_FROM_HERE, factory.cancelAndCreate());
+//   factory.cancel();
+//
+// Above is equivalent to below:
+//
+//   std::unique_ptr<WTF::Closure> task =
+//       WTF::bind(wrapPersistent(foo), &Foo::bar);
+//   RefPtr<TaskHandle> handle =
+//       task_runner->postCancellableTask(BLINK_FROM_HERE, std::move(task));
+//   handle->cancel();
+//
+// Note that the task is not automatically cancelled on TaskHandle scope out.

[dcheng, 2016/10/21 06:14:46]

Hmm, that's unfortunate. That means that something using postCancellableTask
that binds a WeakPersistent won't be able to take advantage of the scheduler
fast-path for cancelled callbacks.

[tzik, 2016/10/25 07:03:52]

It hopefully doesn't make big difference. I expect the users of the
postCancellableTask manually clears the TaskHandle on their destructor, and the
destructor call will be not so late after WeakPersistent invalidation.

+
 namespace blink {
 
 class TraceLocation;