iOS: performSelectorOnMainThread: with two arguments

iOS's NSObject class provides a mechanism to run any selector on a different thread. The most often used variant runs the selector on the "main" (i.e. UI) thread, the snappily titled: performSelectorOnMainThread:withObject:waitUntilDone:. However, the limitation with this call is that you can only supply one argument.

Sometimes, one is not enough. There is a two argument non-thread version, here. So why not invent our own two-argument threaded version?

We can add this easily enough to the base NSObject class by opening a new category on it just for the purpose.

Here's how...

1. Define our API

We open a category on NSObject (note the category name in brackets)

@interface NSObject (PGPerformSelectorOnMainThreadWithTwoObjects)

- (void) performSelectorOnMainThread:(SEL)selector withObject:(id)arg1 withObject:(id)arg2 waitUntilDone:(BOOL)wait;

@end

Naturally, it follows the existing performSelector API signatures, but just adds our extra withObject: argument.

2. Implement it

Here's the secret sauce. Careful application of NSMethodSignature, and NSInvocation.

@implementation NSObject (PGPerformSelectorOnMainThreadWithTwoObjects)

- (void) performSelectorOnMainThread:(SEL)selector withObject:(id)arg1 withObject:(id)arg2 waitUntilDone:(BOOL)wait

{

NSMethodSignature *sig = [self methodSignatureForSelector:selector];

if (!sig) return;

NSInvocation* invo = [NSInvocation invocationWithMethodSignature:sig];

[invo setTarget:self];

[invo setSelector:selector];

[invo setArgument:&arg1 atIndex:2];

[invo setArgument:&arg2 atIndex:3];

[invo retainArguments];

[invo performSelectorOnMainThread:@selector(invoke) withObject:nil waitUntilDone:wait];

}

@end

That's it.

Now you have your own simple way to invoke a call to selector on the main thread from wherever you are in your code. Handy.