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The responder
chain is a series of linked responder objects. It starts with the first responder and ends with the application object. If the first responder
cannot handle an event, it forwards the event to the next responder in the responder chain.

Note: Make
sure that your app has established its object graph before assigning an object to be the first responder. For example, you typically call the

becomeFirstResponder

method
in an override of the

viewDidAppear:

method.
If you try to assign the first responder in

viewWillAppear:

,
your object graph is not yet established, so the

becomeFirstResponder

method
returns

NO

.