Error: Incompatible types in initialization? (iPhone,Objective-C)

Question

I'm getting the error on "float pos = [sender value];"...sender should be a UISlider that I set up in Interface Builder.

ButtonViewController.m

- (IBAction)slide: (id)sender {
 float pos = [sender value];
 loadValue.progress = pos;
}

ButtonViewController.h

@interface Button_Fun4ViewController : UIViewController {
 IBOutlet UIProgressView *loadValue;
}

- (IBAction)slide: (id)sender;

THANKS.

Answer 1

Since sender is of type 'id', the compiler doesn't know what it will return from its messages, and assumes they'll return an 'id' typed value. You need to either cast sender to UISlider (so the compiler knows that -value returns a float), or cast the returned value of the message to a float.

Answer 2

While sender "should" be a UISlider, the compiler doesn't know that. All it sees is id, which it binds to the first matching method signature it can find; probably one that returns something other than a float (and as a final guess, the compiler will assume it returns an id).

You'll need to typecast this over to UISlider in this case:

- (IBAction)slide: (id)sender {
    float pos = [(UISlider *)sender value];
    loadValue.progress = pos;
}

Using NSAssert() or an if() to double-check that this is fact a UISlider wouldn't be a bad idea, but isn't strictly necessary.

Answer 3

Since sender is of type id you should indicate what type it is.

Use the following:

float pos = [(UISlider *)sender value];

Alternatively, you can use protocols. That way, you can receive different types of objects which, as long as they adhere to the protocol, can all be used in the same way.

@protocol myProtocol {
-(float)value;
}

-(void)myMethod:(id<myProtocol>)sender {
    float pos = [sender value];
}