Objective C block property is always nil

Question

I'm trying to learn more about Objective C blocks and how they work. I've set up a simple project with two UIViewControllers embedded in a UINavigationController in Storyboard. I'm attempting to change the background color of the first ViewController's view from the second view controller. Here's some code:

ViewController.m

@implementation ViewController{
    ColorBlock _colorBlock;
}

-(void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender{
    if([segue.identifier isEqualToString:@"theSegue"]){
        SecondViewController *vc = [self.storyboard instantiateViewControllerWithIdentifier:@"SecondViewController"];
        vc.colorBlock = _colorBlock;
    }
}

- (IBAction)moveToSecondViewController:(id)sender {
    __weak id weakSelf = self;
    _colorBlock = ^{
        [[weakSelf view] setBackgroundColor:[UIColor redColor]];
    };
}

SecondViewController.h

typedef void (^ColorBlock)(void);

@interface SecondViewController : UIViewController

@property (readwrite, copy) ColorBlock colorBlock;

@end

SecondViewController.m

- (IBAction)buttonTapped:(id)sender {
    if(self.colorBlock){
        self.colorBlock();
    }
}

The first ViewController's background color isn't being changed because in the buttonTapped: method of SecondViewController.m, self.colorBlock is nil, causing the block invocation not to be called. I thought I had successfully set the block in prepareForSegue:sender:. Why is my block property nil?

are you sure moveToSecondViewController: is called (and called before prepare for segue?) pls double check with a breakpoint in both. — danh
– danh, Commented Sep 12, 2013 at 21:21
Yes moveToSecondViewController: is called and my _colorBlock ivar is successfully set. Then, prepareForSegue:sender: is called and the ivar is successfully set onto SecondViewController. — hgwhittle
– hgwhittle, Commented Sep 12, 2013 at 21:24

Rob · Accepted Answer · 2013-09-12 21:25:07Z

3

In your prepareForSegue, the destination has already been instantiated. So assuming that SecondViewController is the destination, you can do:

-(void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender{
    if([segue.identifier isEqualToString:@"theSegue"]){
        SecondViewController *vc = segue.destinationViewController;
        NSAssert([vc isKindOfClass:[SecondViewController class]], @"destination is not SecondViewController class");
        vc.colorBlock = _colorBlock;
    }
}

answered Sep 12, 2013 at 21:25

Rob

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3 Comments

Rob Over a year ago

@danh No, he's instantiating a new copy of SecondViewController. Thus, he's setting a property in some phantom VC that he's not segueing to.

danh Over a year ago

oh. good catch. yes, this should do it. i was seeing what i was expecting in the code, not what was there. (though I think the assert is a little too defensive)

hgwhittle Over a year ago

Ah yes, I forgot about that destinationViewController property. That solved it!

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