Subclassing UIView and UIScrollView - iOS Programming: The Big Nerd Ranch Guide - page 205

Motion Events
In Chapter 5 , we briefly talked about UIView 's superclass, UIResponder . Instances of
UIResponder can become the first responder of the window and will receive events
when the device is shaken or a key is pressed on the keyboard. You will make the instance
of HypnosisView the first responder of Hypnosister 's window. Shaking the device will
send a message to the HypnosisView , and that method will change its circleColor .
To give a UIResponder first responder status, you send it the message be-
comeFirstResponder . In HypnosisterAppDelegate.m , tell the Hypnos-
isView instance to become the first responder. The becomeFirstResponder method
returns a Boolean value indicating whether the receiving object successfully became the
first responder of the window.
[[self window] addSubview:view];
BOOL success = [view becomeFirstResponder];
if (success) {
NSLog(@"HypnosisView became the first responder");
} else {
NSLog(@"Could not become first responder");
}
self.window.backgroundColor = [UIColor whiteColor];
Build and run the application. Notice that the console tells you that the HypnosisView
failed to become the first responder. Most UIResponder objects return NO in be-
comeFirstResponder . This is because most views, by default, only care about events
connected with themselves, and they (almost) always get a chance to handle these events.
For example, a tapped UIButton gets sent a message regardless of who the first respon-
der is. So, a responder object must explicitly state that it is willing to become the first re-
sponder. In HypnosisView.m , override the UIResponder method canBe-
comeFirstResponder to return YES .
- (BOOL)canBecomeFirstResponder
{
return YES;
}
Build and run the application again. The console will report that the HypnosisView is
now the first responder.