when to use respondsToSelector in objective-c

Question

- (void)someMethod
{
    if ( [delegate respondsToSelector:@selector(operationShouldProceed)] )
    {
        if ( [delegate operationShouldProceed] )
        {
             


        

Answer 1

As kubi mentioned respondsToSelector is normally used when you have a an instance of a method that conforms to a protocol.

// Extend from the NSObject protocol so it is safe to call `respondsToSelector` 
@protocol MyProtocol  

// @required by default
- (void) requiredMethod;

@optional

- (void)optionalMethod;

@end

Given and instance of this protocol we can safely call any required method.

id  myObject = ... 
[myObject requiredMethod];

However, optional methods may or may not be implemented, so you need to check at runtime.

if ([myObject respondsToSelector:@selector(optionalMethod)]) 
{
     [myObject optionalMethod];
}

Doing this will prevent a crash with an unrecognised selector.

---

Also, the reason why you should declare protocols as an extension of NSObjects, i.e.

@protocol MyProtocol  

Is because the NSObject protocol declares the respondsToSelector: selector. Otherwise XCode would think that it is unsafe to call it.