Question

I want to invoke a selector of a method that has the usual NSError** argument:

-(int) getItemsSince:(NSDate *)when dataSelector:(SEL)getDataSelector error:(NSError**)outError  {
    NSArray *data = nil;
    if([service respondsToSelector:getDataSelector]) {
        data = [service performSelector:getDataSelector withObject:when withObject:outError];
        // etc.

… which the compiler doesn’t like:

warning: passing argument 3 of 'performSelector:withObject:withObject:' from incompatible pointer type

Is there any way around this short of encapsulating the pointer in an object?

Answer 1

Check out NSInvocation, which lets you “performSelector” in a much more flexible way.

Here’s some code to get you started:

if ([service respondsToSelector:getDataSelector]) {
    NSArray *data;
    NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:
        [service methodSignatureForSelector:getDataSelector]];
    [invocation setTarget:delegate];
    [invocation setSelector:getDataSelector];
    // Note: Indexes 0 and 1 correspond to the implicit arguments self and _cmd, 
    // which are set using setTarget and setSelector.
    [invocation setArgument:when atIndex:2]; 
    [invocation setArgument:outError atIndex:3];
    [invocation invoke];
    [invocation getReturnValue:&data];
}