foo.send("bar")

if ([target respondsToSelector:theSelector]) {
    NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:
        [target methodSignatureForSelector:theSelector]];
    [invocation setTarget:target];
    [invocation setSelector:theSelector];
    // Note: Indexes 0 and 1 correspond to the implicit arguments self and _cmd,
    // which are set using setTarget and setSelector.
    [invocation setArgument:arg1 atIndex:2];
    [invocation setArgument:arg2 atIndex:3];
    [invocation setArgument:arg3 atIndex:4];
    // ...and so on
    [invocation invoke];
    [invocation getReturnValue:&retVal]; // Create a local variable to contain the return value.
}

if ( [foo respondsToSelector:NSSelectorFromString(@"bar")] ) [foo performSelector:NSSelectorFromString(@"bar")];