Closures are used to implement events. A closure is a special pointer type that points to a specific method in a specific class instance. As a component writer, you can treat the closure as a place holder: your code detects that an event occurs, so you call the method (if any) specified by the user for that event. 

Closures maintain a hidden pointer to a class instance. When the user assigns a handler to a component's event, the assignment is not just to a method with a particular name, but rather to a specific method of a specific class instance. That instance is usually the form that contains the component, but it need not be.