How do I call this methods with a parameter in ObjC

Question

Hi everyone,

I'm quiet new in ObjC and app programming. All I want to do is fade in an image. For that I did this:

-(IBAction)changeImg:(id)sender{
    myImgView.image = [UIImage imageNamed:@"image.png"];
    myImgView.alpha = 0;
    [UIView beginAnimations:nil context:nil];
    [UIView setAnimationDuration:1];
    myImgView.alpha = 1;
    [UIView commitAnimations];
}

That works quiet good. But now I want to put the fade function into an own method and I don't know how to call that method:

-(IBAction)changeImg:(id)sender{
    myImgView.image = [UIImage imageNamed:@"image.png"];
    [myImgView fadeIn:self];
}

-(void) fadeIn:(UIImageView *) imgFade{
    imgFade.alpha = 0;
    [UIView beginAnimations:nil context:nil];
    [UIView setAnimationDuration:1];
    imgFade.alpha = 1;
    [UIView commitAnimations];
}

When I call the method like this I get a warning:"'UIImageView' may not respond to '-fadeIn:'"

Can anyone help me please?

Answer 1

That warning usually comes up because there is no written declaration in the the interface file or .h file.

I assume that fadeIn is in myImgView class, so why are you passing self? Just make it [myImgView fadeIn]; and replace imgFade with self.

Code for source .m

@implementation MyClass
-(IBAction)changeImg:(id)sender{
    myImgView.image = [UIImage imageNamed:@"image.png"];
    [myImgView fadeIn];
}

-(void) fadeIn{
    self.alpha = 0;
    [UIView beginAnimations:nil context:nil];
    [UIView setAnimationDuration:1];
    selfe.alpha = 1;
    [UIView commitAnimations];
}
@end

Add to your header .h

  - (void) fadeIn;

Or as someone said, put at the top of the source file .m before @implementation.

@interface MyClass ()
  - (void) fadeIn;
@end 

To make it private

Answer 2

Put -changeImg: after -fadeIn:. Better yet, declare your private methods in a category at the top of the file:

@interface MyClass (PrivateMethods)
- (void)fadeIn: (UIImageView *)imgFade;
@end

You don't need to pass self around like that--every method you call on the same object will have the same self.

Answer 3

Sorry but I don't get it to work. The warning stays. And when I do it like kuroutadori suggested:

...
self.alpha = 0;
...

I get an error: "request for member 'alpha' in something not a structure or union"

Answer 4

You've got it backwards.

[self fadeIn:myImgView];

Answer 5

You are declaring fadeIn: on your current class (we would need more code to see which one it is) yet calling it on an UIImageView instance (in your case myImgView).

You should either declare it privately in your current class implementation file (as Jonathan Grynspan suggested). Or if you're extensively using this animation, put it into UIImageView itself through a category:

@interface UIImageView (MyFade)

- (void) fadeIn;

@end

and then implement it with

@implementation UIImageView (MyFade)

- (void) fadeIn
{
    self.alpha = 0;
    [UIView beginAnimations:nil context:nil];
    [UIView setAnimationDuration:1];
    selfe.alpha = 1;
    [UIView commitAnimations];
}

@end

Answer 6

Finally it works. I first had to deal with categories. But now I think I got it. Thanks to all of you. Great support!