Objective-C: Calling selectors with multiple arguments

In MyClass.m, I've defined

- (void) myTest: (NSString *) withAString{
    NSLog(@"hi, %@", withAString);
}

and the appropriate declaration in MyClass.h . Later I want to call

[self performSelector:@selector(mytest:withAString:) withObject: mystring];

in MyClass.m but I get an error similar to * Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '* -[MyClass myTest:withAtring:]: unrecognized selector sent to instance 0xe421f0'

I tried a simpler case with a selector that took no arguments that printed a string to console and that worked just fine. What's wrong with the code and how can I fix it? Thanks.

Solution 1:

In Objective-C, a selector's signature consists of:

1. The name of the method (in this case it would be 'myTest') (required)
2. A ':' (colon) following the method name if the method has an input.
3. A name and ':' for every additional input.

Selectors have no knowledge of:

1. The input types
2. The method's return type.

Here's a class implementation where performMethodsViaSelectors method performs the other class methods by way of selectors:

@implementation ClassForSelectors
- (void) fooNoInputs {
    NSLog(@"Does nothing");
}
- (void) fooOneIput:(NSString*) first {
    NSLog(@"Logs %@", first);
}
- (void) fooFirstInput:(NSString*) first secondInput:(NSString*) second {
    NSLog(@"Logs %@ then %@", first, second);
}
- (void) performMethodsViaSelectors {
    [self performSelector:@selector(fooNoInputs)];
    [self performSelector:@selector(fooOneInput:) withObject:@"first"];
    [self performSelector:@selector(fooFirstInput:secondInput:) withObject:@"first" withObject:@"second"];
}
@end

The method you want to create a selector for has a single input, so you would create a selector for it like so:

SEL myTestSelector = @selector(myTest:);

Solution 2:

Your method signature is:

- (void) myTest:(NSString *)

withAString happens to be the parameter (the name is misleading, it looks like it is part of the selector's signature).

If you call the function in this manner:

[self performSelector:@selector(myTest:) withObject:myString];

It will work.

But, as the other posters have suggested, you may want to rename the method:

- (void)myTestWithAString:(NSString*)aString;

And call:

[self performSelector:@selector(myTestWithAString:) withObject:myString];