From Facebook Developer Wiki

public static void requireSession (Callback callback, Callback callback, bool isUserActionHint)

Description

Call this function when you want to enforce that the current user is logged into Facebook.

Parameters

Required	Name	Type	Description
optional	callback	Callback	Callback function to be invoked when session is ready. This parameter may be null.
	callback	Callback	Callback function to be invoked when user has canceled out of the login dialog/window. This parameter may be null.
	isUserActionHint	bool	Provide hint on whether the call is made from user action (onclick, onkeydown, etc.). This hint is generally necessary unless the function is initialized from a Flash object.

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