Question

Here’s my scenario. I have a class A. Inside its implementation I create object of type B and set B’s delegate to self (So B.delegate = self somewhere inside class A’s implementation).

And class A has an instance method - (void)printThis;

Now inside B’s implementation, when I try to do [delegate printThis];, it gives me this error:
“No known instance method for selector printThis”

Of course this is when I have enabled ARC. The above delegation pattern used to work fine in iOS 4.x without the ARC. And it still does when I switch OFF ARC. What has ARC got to do with passing messages to delegates?

Skeleton code:

A.h

@class B;

@interface A: blah blah
{
    B objB;
}

-(void) printThis;

A.m

objB = [[B alloc] init];
objB.delegate = self;

- (void)printThis {
    //doSomething
}

B.h

@interface B: blah blah
{
    //id delegate; //used to be there, now I just property & synthesize
}

@property (nonatomic,weak) id delegate;

B.m

@synthesize delegate;

[delegate printThis]; //error with ARC ON, works with OFF

IMPORTANT EDIT:

And mind you this happens for a method here and there. For instance I have a few other methods in A like printThat etc etc which work without errors. I’m clueless as to what is happening!

Answer 1

You need to define -printThis in a protocol and make A implement this protocol. You also need to mark the delegate as conforming to this delegate.

i.e.:

@protocol Printer <NSObject>

- (void)printThis;

@end

@interface A : NSObject <Printer>
//...
@end

@interface B : //...

@property (nonatomic, weak) id<Printer> delegate;

@end

ARC needs to know about the interface for method calls in order to properly manage the memory correctly. If there isn’t a definition then it’ll complain.