Question

I’m really struggling with with, and I know how easy it is but I’m just not getting it to work.

I’m very new to Obj-C…

Basically, this app is very simple.

It piles on modal view controllers one on top of the other until the end where the user gets a result and then they are all dismissed and the user can start again.

In each of the modal view controllers, I want to, for example, add 1 to an int I have declared in the first screen.

This is the declaration in the first view controller.h

int total;

This is what I am doing in the 2nd view controller.m

FirstViewController *fvc = [[FirstViewController alloc] init];
[fvc setTotal:1]; //or the following, i can't see what the difference is
fvc.total = 1;

And this seems to do the trick, it sets the variable that I’ve declared in another view.

However, in the 3rd view controller.m, I do this:

FirstViewController *fvc = [[FirstViewController alloc] init];
NSLog(@"Current total: %x", fvc.total);

This works, as in I don’t get errors, but the integer has reset to 0. Ideally, it would say ‘1’, to show it’s retained the value…

My question is; how can I reuse and add to this integer throughout my entire app?

Please also make answers relevant to strings, as I would like to do that also.

Sorry, I’m still learning.

Thanks in advance.

Answer 1

If you define a variable as static, it retains its value across different instances. You could do this in your class like this:

@interface FirstViewController : UIViewController

- (NSInteger)sharedValue;
- (void)setSharedValue:(NSInteger)value;

@end


@implementation FirstViewController

static NSInteger sharedValue = 0;

- (NSInteger)sharedValue
{
    return sharedValue;
}

- (void)setSharedValue:(NSInteger)value
{
    sharedValue = value;
}

@end

Now any instances of FirstViewController that you create will share the same copy of sharedValue, and if you set it on one, and get it from another it will work the way you want. The same process works with strings, etc, but if you aren’t using ARC, be careful about retaining and releasing the sharedValue.

A better approach than creating multiple static values on your class is to create a single shared class on which you can set and get multiple properties. These shared classes are sometimes called “Singletons” because they are classes that have a single instance.

You can see lots of places where Apple uses singletons, for example [UIApplication sharedApplication], [NSNotificationCenter defaultCenter], [NSUserDefaults standardUserDefaults] – these are all examples of classes where there is a single instance re-used throughout the app.

To create a singleton, create a new class of type NSObject, like this. You can then add any properties you want to it and access them anywhere in the app via the sharedInstance:

@interface MyObject : NSObject

@property (nonatomic, assign) NSInteger someInteger;
@property (nonatomic, strong) NSString *someString;

+ (MyObject *)sharedInstance;

@end


@implementation MyObject

@synthesize someInteger, someString;

+ (MyObject *)sharedInstance
{
    static MyObject *sharedInstance = nil;
    if (sharedInstance == nil)
    {
        sharedInstance = [[self alloc] init];
    }
    return sharedInstance;
}

@end

You can now access the shared properties anywhere in your app by saying

[MyObject sharedInstance].someInteger = 5;

...

NSInteger value = [MyObject sharedInstance].someInteger;

This is definitely the best way to store global data in your app.