Add WTF::makeCancellable

third_party/WebKit/Source/wtf/MakeCancellable.cpp

Index: third_party/WebKit/Source/wtf/MakeCancellable.cpp
diff --git a/third_party/WebKit/Source/wtf/MakeCancellable.cpp b/third_party/WebKit/Source/wtf/MakeCancellable.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..ff597e5fc3695ab9ca38cc34c6a8c5c755c10441
--- /dev/null
+++ b/third_party/WebKit/Source/wtf/MakeCancellable.cpp
@@ -0,0 +1,46 @@
+// 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 "wtf/MakeCancellable.h"
+
+#include "base/logging.h"
+
+namespace WTF {
+namespace internal {
+
+FunctionCancellerImplBase::FunctionCancellerImplBase() = default;
+FunctionCancellerImplBase::~FunctionCancellerImplBase() = default;
+
+} // namespace internal
+
+FunctionCanceller::FunctionCanceller() = default;
+FunctionCanceller::FunctionCanceller(PassRefPtr<internal::FunctionCancellerImplBase> cancellation)
+    : m_cancellation(cancellation)
+{
+    DCHECK(m_cancellation);
+}
+
+FunctionCanceller::FunctionCanceller(FunctionCanceller&&) = default;
+FunctionCanceller& FunctionCanceller::operator=(FunctionCanceller&&) = default;
+
+FunctionCanceller::~FunctionCanceller()
+{
+    cancel();

[Yuta Kitamura, 2016/07/27 09:08:02]

This might be a bit surprising (task getting cancelled without
explicit cancel() call). Do you have any concrete use case
in mind?

At least we should make an emphasis on this cancel-on-
destruction semantics in the comments for makeCancellable().

[tzik, 2016/07/27 10:10:30]

Added a comment and example there.
Yes, this will replace several cancellation classes listed in the CL
description. And all of them have the cancel-on-destruction semantics.

+}
+
+void FunctionCanceller::detach()
+{
+    m_cancellation = nullptr;
+}
+
+void FunctionCanceller::cancel()
+{
+    if (!m_cancellation)
+        return;
+
+    m_cancellation->cancel();
+    m_cancellation = nullptr;
+}
+
+} // namespace WTF