Question

I am trying to learn about Objective-C for iPhone development, and while going through examples about actions I often saw things like:

- (IBAction)sliderMoved:(id)sender
{
    UISlider *aSlider = (UISlider *)sender;
    //rest of code goes here...
}

What is the purpose of taking in type (id) and then immediately casting it to (UISlider *). Why not just have the following:

- (IBAction)sliderMoved:(UISlider *)aSlider;
{
    //rest of code goes here...
}

Is there a technical reason for this, or is it simply convention, or done for readability?

Thanks in advance,

• Timo

Answer 1

I believe it is a convention that stems from the fact that there is not always a 1:1 mapping of UIControl to IBAction. For example, you could have a UISwitch, a UIButton, and a UITextField all mapped to a - (IBAction)syncUI:(id)sender; method. In that case, it is impossible to specify the type.

I admit that this is just my own mental justification, and there is certainly nothing preventing you from being more explicit in the type of sender if you can be. I usually am.