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The ActiveDOMObject is used to implement DOM objects that can involve asynchronous operations such as loading data from network (e.g. XMLHttpRequest, WebSocket) to
  • keep them alive while async op is active
  • hold back actions resulting from async op while the document is suspended
The ActiveDOMObject classes can override its hasPendingActivity() method to keep the object alive while some async operation is in progress. As long as hasPendingActivity() returns true, V8 prolongs its life so that it doesn't get garbage collected even when it becomes unreachable. Note that there must be a V8 wrapper for the object in order to this to work. 

While hasPendingActivity() returns true, the life of the parent Document object is also prolonged (explanation TBA). You rarely need to override contextDestroyed() for ActiveDOMObject subclasses.

ActiveDOMObjects are notified of detach of the parent Document object (or shutdown of the parent WorkerThread) as the stop() method called on it. Commonly, they start shutdown of the asynchronous operation in stop() if any. This detach occurs regardless of hasPendingActivity().

suspend() is called when dialogs such as alert(), prompt(), etc. are going to be shown or the WebInspector's pause is going to be active. resume() is called when the dialog is closed or the WebInspector resumes script execution. Between suspend() and resume(), it's recommended that operations on the ActiveDOMObjects are suspended. Since resource loading is automatically suspended by Chrome's resource dispatched code, you may not need to manually hold back async method calls.

When implementing, it is critical that you also add ActiveDOMObject to the interface flags in your IDL file, or GC will pay no attention to hasPendingActivity()!

As of Nov 12 2013 by tyoshino@. Updated July 17, 2014 by kouhei@
Thanks haraken@, abarth@