How to detect changes on UIPickerView?

I want to detect changes of UIPickerView value.

If UIPickerView respond to addTarget I used a code like this:

-(void) valueChange:(id)sender {
    change = YES;
} 

UIPickerView *questionPicker = [[UIPickerView alloc] init]; 
[questionPicker addTarget:self action:@selector(valueChange:) forControlEvents:UIControlEventValueChanged];

How can I do same things but in a correct way ?