Objective-C Fundamentals

148 CHAPTER 7 Protocols
In this animation, you take the boxView and move it down the screen by 100 pixels.
Calling the commitAnimations method executes the animation. An important configuration
step of this animation was setting animationWillStartSelector and
animationDidStopSelector. These tell UIView to call certain classes when the animation
starts and ends. These method calls are forwarded to the delegate and the protocol
methods are called. The only thing left to do is create the animationStarted and
animationStopped methods that alert the delegate (see the following listing).
- (void)animationStarted {
if ([delegate
respondsToSelector:@selector(animationStartedWithView:)])
{
[delegate animationStartedWithView:self];
}
}
- (void)animationStopped {
if ([delegate
respondsToSelector:@selector(animationHasFinishedWithView:)])
{
[delegate animationHasFinishedWithView:self];
}
}
- (void)dealloc {
[boxView release];
[super dealloc];
}
@end
Listing 7.4 Filling in myView.m, part 4
Here is where the protocol methods are called. You tell the delegate to fire off the
appropriate method when the animation has started and stopped. You don’t want to
assume that a delegate is set or that the delegate has implemented the protocol methods.
To check this, you use the respondsToSelector method. NSObject implements
this method, so you can call it on nearly any object. It’s a great check to include in
your code to avoid crashing and generally a good coding practice. If the delegate is
assigned and it does implement the protocol methods, you call the method and pass it
the view in which the protocols are defined. Now that you’ve defined your protocol,
let’s create a class that will conform to it.
7.2.2 Making a class conform to a protocol
To make your application delegate conform to the animationNotification protocol,
you implement both methods from the protocol in your application delegate. First
you signal that the application delegate does conform to the animationNotification
protocol. This is done in the myProtocolAppDelegate.h file. The following listing
shows the new code for your App Delegate header file.