Warning for unused variable 'completion' - objective-c

Question

Have this implementation file which compiles and runs fine, but gives me an unused variable warning in a block. Am new to blocks, so am not quite sure what is wrong. Have commented the line where the warning is.
Can someone please explain this? Thanks in advance!
#import "RTViewController.h"
#import <AVFoundation/AVFoundation.h>
#import <AudioToolbox/AudioToolbox.h>
#interface RTViewController () {
AVAudioPlayer *backgroundAudioPlayer;
SystemSoundID burnRubberSoundID;
}
#end
#implementation RTViewController
#synthesize car;
#synthesize testDriveButton;
#synthesize backgroundImage;
- (void)viewDidLoad
{
[super viewDidLoad];
self.title = #"Road Trip";
NSURL* backgroundURL = [NSURL fileURLWithPath:[[NSBundle mainBundle]
pathForResource:#"CarRunning"
ofType:#"aif"]];
backgroundAudioPlayer = [[AVAudioPlayer alloc]
initWithContentsOfURL:backgroundURL error:nil];
backgroundAudioPlayer.numberOfLoops = -1;
[backgroundAudioPlayer prepareToPlay];
NSURL* burnRubberURL = [NSURL fileURLWithPath:[[NSBundle mainBundle] pathForResource:#"BurnRubber" ofType:#"aif"]];
AudioServicesCreateSystemSoundID((__bridge CFURLRef)burnRubberURL, &burnRubberSoundID);
}
- (void)viewDidUnload
{
[self setCar:nil];
[self setTestDriveButton:nil];
[self setBackgroundImage:nil];
[super viewDidUnload];
// Release any retained subviews of the main view.
}
- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
{
//return (interfaceOrientation != UIInterfaceOrientationPortraitUpsideDown);
return (interfaceOrientation == UIInterfaceOrientationPortrait);
}
- (IBAction)TestDrive:(id)sender {
AudioServicesPlaySystemSound(burnRubberSoundID);
[self performSelector:#selector(playCarSound) withObject:self afterDelay:.2];
CGPoint center = CGPointMake(car.center.x, self.view.frame.origin.y + car.frame.size.height/2);
[UIView animateWithDuration:3 animations:^ {
car.center = center;
}
completion:^(BOOL finished) {
[self rotate];
}];
}
-(void)playCarSound {
[backgroundAudioPlayer play];
}
- (void)rotate {
CGAffineTransform transform = CGAffineTransformMakeRotation(M_PI);
void (^animation) () = ^() {
car.transform = transform;
};
void (^completion) (BOOL) = ^ (BOOL finished) {
[self returnCar];
};
[UIView animateWithDuration:3 animations:animation completion:completion];
}
- (void)returnCar {
CGPoint center = CGPointMake(car.center.x, self.view.frame.origin.y + self.view.frame.size.height - car.frame.size.height/2);
void (^animation)() = ^() {
car.center = center;
};
void (^completion)(BOOL) = ^(BOOL finished) {
[self continueRotation];
};
[UIView animateWithDuration:3 animations:animation completion:completion];
}
- (void)continueRotation {
CGAffineTransform transform = CGAffineTransformMakeRotation(0);
void (^animation)() = ^() {
car.transform = transform;
};
void (^completion)(BOOL) = ^(BOOL finished) { //Unused variable 'completion'
[backgroundAudioPlayer stop];
[backgroundAudioPlayer prepareToPlay];
};
[UIView animateWithDuration:3 animations:animation completion:nil];
}
#end

Answer 1

The problem is that you forgot to use the variable completion in the line:
[UIView animateWithDuration:3 animations:animation completion:nil];
^^^
Here
So change that line to this:
[UIView animateWithDuration:3 animations:animation completion:completion];
Also, note that you can write your code like this:
- (void)continueRotation
{
[UIView animateWithDuration:3 animations:^{
car.transform = CGAffineTransformMakeRotation(0);
} completion:^(BOOL finished) {
[backgroundAudioPlayer stop];
[backgroundAudioPlayer prepareToPlay];
}];
}

Answer 2

That warning simply means that you've instantiated a variable but are not using it anywhere in the code.
But it's a warning, your code will still compile and should work fine. It's just telling you there's no reason to instantiate something if it's never gonna be used.