Calling a method from other class and set the UILabel iOS - ios

Question

I have 2 classes which names are A and B, I have UIScrollView with pagecontroller in class A and I have a UILabel and NSMutableArray in B.
I used this event for get pagecontroller's page and i am sending number of page to classB for use array's element.
//ClassA
-(void)scrollViewDidScroll:(UIScrollView *)scrollView
{
CGFloat pageWidth = self.imageScrollView.frame.size.width;
int page = floor((self.imageScrollView.contentOffset.x - pageWidth / 2) / pageWidth) + 1;
ClassB *obj = [[ClassB alloc]init];
[obj changeDiscount:page];
}
//ClassB
- (void)viewDidLoad
{
numbers = [[NSMutableArray alloc]initWithObjects:#"15",#"25",nil];
}
-(void) changeDiscount:(int)currentPagePresentation{
NSLog(#"currentI = %i",currentPagePresentation);
_discountLabel.text = [NSString stringWithFormat:#"%# Discount",[numbers objectAtIndex:currentPagePresentation]];
}
I can call the changeDiscount method but array is coming null every time and I can't set the string to label.
What am I doing wrong?
Thanks for your answer and advice.

Answer 1

The reason why your _discountLabel.text's string is equal to null is because the numbers array that you are accessing has not even been initialised.
The reason why your numbers array has not been initialised is because the viewDidLoad method only gets called as the method states: WHEN the view has loaded ;)
If you want to access the array after creating an instance of your class, its best to setup the numbers array in an init method or so.
All you've done is:
//This creates a new instance of your second class B.
ClassB *obj = [[ClassB alloc]init];
//Youre trying to access the numbers array when you havent even loaded the view
//All you've done is create an instance of it and then calling a method with an empty numbers array.
[obj changeDiscount:page];
And that doesn't sit well with your existing code. Please continue to read to understand why.
You also definitely don't want to be creating new instances of your class every time your scroll view delegate method is called. I highly suggest you revisit that code and find an appropriate place for that code.
Solution
I suggest you revise the view controllers programmers guide on the apple website before doing anything else.
Follow step 1.
Follow step 1 again.
Then something you can do is:
Method 1: - lazy method In class B you could create an instance method like so:
//.h
-(void)setupArray;
//.m
-(void)setupArray{
numbers = [[NSMutableArray alloc]initWithObjects:#"15",#"25",nil];
}
//Then you can do something like this in class a
ClassB *obj = [[ClassB alloc]init];
[obj setupArray];
[obj changeDiscount:page];
Method 2: more appropriate If you want to do it in one go you can do this, and create an init method.
//.h
//in your Class B .h file you create an instance method like so:
-(void)init;
//.m file
-(id)init{
self=[super init];
if(self)
numbers = [[NSMutableArray alloc]initWithObjects:#"15",#"25",nil];
return self;
}
//Then in your class a method you can do this:
//Like before.
ClassB *obj = [[ClassB alloc]init];
[obj changeDiscount:page];

Answer 2

Ok, there are a few things causing issues here:
You are creating an instance of ClassB within the scope of scrollViewDidScroll of classA.
As soon as that method completes, that new object will be deallocated.
ClassB initialises the numbers array in viewDidLoad. This method will be called only when a UIViewController subclass loads it’s UIView, so ClassB must be a UIViewController subclass and you need to have presented it.

Answer 3

viewDidLoad is called when you first time access view property of that viewController.until then view is nil.
So your numbers array wont be initialised because you are calling changeDiscount method before viewDidLoad is executed.
So, move the initialising from viewDidLoad to init or initWithNib.
-(id)init{
self=[super init];
if(self)
numbers = [[NSMutableArray alloc]initWithObjects:#"15",#"25",nil];
return self;
}