@implementation commonClass
- (void)CommonMethod:(id)sender
{
//So some awesome generic stuff...
    }
@end

@implementation ViewController2

- (void)do_something... {
    commonClass *c=[[commonClass alloc] init];
    [c CommonMethod];
    [c release];
}

@end

http://stackoverflow.com/q/1658471/907953

Accessing Method from other Classes Objective-C

Use function from another class

c, iphone

Objective C