Introduce ContextObserver

third_party/WebKit/Source/core/dom/ContextLifecycleObserver.h

 /*
  * Copyright (C) 2008 Apple Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 17 matching lines @@
 #define ContextLifecycleObserver_h
 
 #include "core/CoreExport.h"
 #include "core/dom/ExecutionContext.h"
 #include "platform/LifecycleObserver.h"
 
 namespace blink {
 
 class LocalFrame;
 
+// ContextObserver is a lightweight version of ContextLifecycleObserver.
+// If you just want to use getExecutionContext() and frame(), use
+// ContextObserver. If you need to listen contextDestroyed() notifications,
+// use ContextLifecycleObserver.
+class CORE_EXPORT ContextObserver : public GarbageCollectedMixin {
+ public:
+  // Returns null after the observing context is detached.
+  ExecutionContext* getExecutionContext() const;
+
+  // Returns null after the observing context is detached or if the context
+  // doesn't have a frame (i.e., if the context is not a Document).
+  LocalFrame* frame() const;
+
+  DECLARE_VIRTUAL_TRACE();
+
+ protected:
+  explicit ContextObserver(ExecutionContext* executionContext)
+      : m_executionContext(executionContext) {}
+
+  explicit ContextObserver(LocalFrame*);
+
+ private:
+  WeakMember<ExecutionContext> m_executionContext;

[sof, 2016/12/19 09:14:03]

For clarity, what motivates the weakness?

[haraken, 2016/12/19 09:58:15]

For example, this CL (https://codereview.chromium.org/2555873004/) caused memory
leaks.

Currently it's not guaranteed that contextDestroyed is called for all contexts
e.g., documents created by DOMImplementation::createDocument. So we cannot
assume that m_executionConntext is cleared before the context is detached.

+};
+
 class CORE_EXPORT ContextLifecycleObserver
     : public LifecycleObserver<ExecutionContext, ContextLifecycleObserver> {
  public:
   // Returns null after the observing context is detached.
   ExecutionContext* getExecutionContext() const { return lifecycleContext(); }
 
   // Returns null after the observing context is detached or if the context
   // doesn't have a frame (i.e., if the context is not a Document).
   LocalFrame* frame() const;
 
   enum Type {
     GenericType,
     SuspendableObjectType,
   };
 
   Type observerType() const { return m_observerType; }
 
  protected:
   explicit ContextLifecycleObserver(ExecutionContext* executionContext,
                                     Type type = GenericType)
       : LifecycleObserver(executionContext), m_observerType(type) {}
 
  private:
   Type m_observerType;
 };
 
 }  // namespace blink
 
 #endif  // ContextLifecycleObserver_h