Scope between methods - objective-c

Question

Another newbie question. I am trying to understand how to use scope effectively to organise my projects using classes to hold the data instead of having everything on the view controller. So, I am working on a couple of versions of a simple project to understand how scope works.
I have a view controller hooked to a view. In that view there are buttons that when clicked show images. I want to add another button that randomizes the images. I also have a class called "Cards" to hold the cards and the methods for creating and shuffling the cards. I have duplicated the project, so I have one that works and one that doesn't.
First project. These are the files:
view controller h file:
#import <UIKit/UIKit.h>
#import "Cards.h"
#interface ViewController : UIViewController
- (IBAction)buttonPressed:(id)sender;
#end
view contoller m file:
#import "ViewController.h"
#interface ViewController ()
#end
#implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
Cards *instance = [[Cards alloc] init];
instance.images = [instance createImages];
NSLog(#"I've got %lu Images", (unsigned long)instance.images.count);
instance.shuffled = [instance shuffleImages];
NSLog(#"Image numbers shuffled: %#", instance.shuffled);
}
- (IBAction)buttonPressed:(id)sender {
//Nothing hooked to this yet
}
- (void)didReceiveMemoryWarning {
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
#end
Cards h file:
#import <UIKit/UIKit.h>
#import <Foundation/Foundation.h>
#interface Cards : NSObject
// Creating Images
#property NSMutableArray *images;
- (NSMutableArray*) createImages;
//Shuffling Images
#property NSMutableArray *shuffled;
- (NSMutableArray*) shuffleImages;
#end
Cards m file:
#import "Cards.h"
#implementation Cards
- (NSMutableArray*) createImages{
self.images = [[NSMutableArray alloc] initWithObjects:
[UIImage imageNamed:#"Image1.png"],
[UIImage imageNamed:#"Image2.png"],
[UIImage imageNamed:#"Image3.png"],
[UIImage imageNamed:#"Image4.png"], nil];
return self.images;
}
- (NSMutableArray*) shuffleImages{
NSUInteger imageCount = [self.images count];
NSMutableArray *localvar = [[NSMutableArray alloc]init];
for (int tileID = 0; tileID < imageCount; tileID++){
[localvar addObject:[NSNumber
numberWithInt:tileID]];
}
for (NSUInteger i = 0; i < imageCount; ++i) {
NSInteger nElements = imageCount - i;
NSInteger n = (arc4random() % nElements) + i;
[localvar exchangeObjectAtIndex:i
withObjectAtIndex:n];
}
return localvar;
}
#end
This works and I get the expected output on the console:
2015-12-31 23:43:44.885 VCScope[2138:533369] I've got 4 Images
2015-12-31 23:43:44.886 VCScope[2138:533369] Image numbers shuffled: (
0,
2,
3,
1
)
Second project:
What I want to do, is put a button to randomize the images only when the button is pressed and not as part of viewDidLoad. So, in my second project, I have the same files for the view controller.h and for both the Cards.h and Cards.m, but on the view controller.m I move the calling of the method for the shuffling of the cards to a UIButton method, like so:
new View controller m file:
#import "ViewController.h"
#interface ViewController ()
#end
#implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
Cards *instance = [[Cards alloc] init];
instance.images = [instance createImages];
NSLog(#"I've got %lu Images", (unsigned long)instance.images.count);
}
- (IBAction)buttonPressed:(id)sender {
Cards *instance = [[Cards alloc] init];
instance.shuffled = [instance shuffleImages];
NSLog(#"Image numbers shuffled: %#", instance.shuffled);
}
- (void)didReceiveMemoryWarning {
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
#end
This outputs to the console the following:
2015-12-31 23:32:07.495 4StackVCScope[2029:486608] I've got 4 Images
2015-12-31 23:32:11.924 4StackVCScope[2029:486608] Image numbers: (
)
So it's not working and I am guessing it's to do with scope. Can someone throw some light into this? thanks

Answer 1

Welcome to Stack Overflow. You mention you're a "newbie", but it would be helpful to know what background you have so I know how much detail is needed here.
Cards *instance = [[Cards alloc] init];
creates a fresh Cards instance in a local variable. You are doing this separately inside -viewDidLoad and in -buttonPressed:.
If you want one Cards object per ViewController, then the view controller needs to have per-instance storage for it. There are several possible ways to do this. Which one you pick is a question of code style and API design.
If the Cards instance is for internal use only, you can declare an ivar in your #implementation block:
#implementation ViewController {
Cards *_cards;
}
- (void)viewDidLoad { _cards = ... }
- (IBAction)buttonPressed:(id)sender { access _cards }
#end
(Ivars can be declared in the public #interface as well, but I wouldn't recommend that as it leaks implementation details.)
Or you can use a property in the public interface:
// in your .h file:
#interface ViewController
#property (nonatomic) Cards *cards;
#end
// in your #implementation:
- (void)viewDidLoad { self.cards = ... }
- (IBAction)buttonPressed:(id)sender { access self.cards }
A property can also be privately declared in a class extension:
// in your .m file:
#interface ViewController ()
#property (nonatomic) Cards *cards;
#end