Worker: Introduce an observation mechanism for WorkerThread termination

third_party/WebKit/Source/core/workers/WorkerThreadLifecycleObserver.h

Index: third_party/WebKit/Source/core/workers/WorkerThreadLifecycleObserver.h
diff --git a/third_party/WebKit/Source/core/workers/WorkerThreadLifecycleObserver.h b/third_party/WebKit/Source/core/workers/WorkerThreadLifecycleObserver.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 WorkerThreadLifecycleObserver_h
+#define WorkerThreadLifecycleObserver_h
+
+#include "core/CoreExport.h"
+#include "platform/LifecycleObserver.h"
+
+namespace blink {
+
+class WorkerThreadContext;
+
+// An interface for observing worker thread termination from the main thread.
+// This may be useful, for example, when an object living on the main thread
+// needs to release references to objects on the worker thread before it gets
+// terminated.
+//
+// A class that inherits this interface should override
+// LifecycleObserver::contextDestroyed() that is called on the main thread when
+// the worker thread is about to terminate. While contextDestroyed() is called,
+// it is guaranteed that the worker thread is still alive.
+//
+// A newly created observer should firstly check whether the worker thread is
+// alive by wasContextDestroyedBeforeObserverCreation(). If this return true,
+// the worker thread has already been terminated before the observer is created,
+// and contextDestroyed() is never notified.
+class CORE_EXPORT WorkerThreadLifecycleObserver : public LifecycleObserver<WorkerThreadContext, WorkerThreadLifecycleObserver> {
+protected:
+    explicit WorkerThreadLifecycleObserver(WorkerThreadContext*);
+
+    bool wasContextDestroyedBeforeObserverCreation() const
+    {
+        return m_wasContextDestroyedBeforeObserverCreation;
+    }
+
+private:
+    const bool m_wasContextDestroyedBeforeObserverCreation;
+};
+
+} // namespace blink
+
+#endif // WorkerThreadLifecycleObserver_h