Modelo de código de fundo da IU iOS

/* learned from http://www.objc.io/issue-2/common-background-practices.html */
__weak id weakSelf
= self;
[self.operationQueue addOperationWithBlock:^{
NSNumber* result = findLargestMersennePrime();
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
MyClass* strongSelf = weakSelf;
strongSelf
.textLabel.text = [result stringValue];
}];
}];