Question

Is it possible to, say, use a block as a completion handler in a View Controller’s init method so that the parent view controller is able to fill in the details in a block without having to create a custom initWithNibName:andResourceBundle:andThis:andThat: for each possible properties ?

// ... in the didSelectRowAtIndexPath method of the main view controller :
SubViewController *subviewController = [[SubViewController alloc] initWithNibName:nil bundle:nil completionHandler:^(SubViewController * vc) {
    vc.property1 = NO;
    vc.property2 = [NSArray array];
    vc.property3 = SomeEnumValue;
    vc.delegate = self;
}];
[self.navigationController pushViewController:subviewController animated:YES];
[subviewController release];

in SubViewController.m :

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil completionHandler:(void (^)(id newObj))block {
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        block(self);
    }
    return self;
}

instead of

// ... in the didSelectRowAtIndexPath method of the main view controller :
SubViewController *subviewController = [[SubViewController alloc] initWithNibName:nil bundle:nil andProperty1:NO andProperty2:[NSArray array] andProperty3:SomeEnumValue andDelegate:self];
[self.navigationController pushViewController:subviewController animated:YES];
[subviewController release];

with

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil andProperty1:(BOOL)p1 andProperty2:(NSArray *)p2 andProperty3:(enum SomeEnum)p3 andDelegate:(id<MyDelegateProtocol>)myDelegate {
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
       self.property1 = p1;
       self.property2 = p2;
       self.property3 = p3;
       self.delegate = myDelegate;
    }
    return self;
}

So that I can do whatever I want in the main controller vs calling a predefined init method (and having to write one for each possible initialization).

Is it something bad ? will there be retain cycles ?

Answer 1

Which advantages do you see in using a block? The initializer is commonly used to set up private state of the instance. This private state could not be accessed from the block since the block is implemented somewhere else.

If you only use public properties why not setting them up after initialization?

SubViewController *vc = [[SubViewController alloc] initWithNibName:nil bundle:nil];
vc.property1 = NO;
vc.property2 = [NSArray array];
vc.property3 = SomeEnumValue;
vc.delegate = self;

That’s exactly what the block version does (without the hassle).

Is it something bad ? will there be retain cycles ?

No, but I would dismiss your proposition for architectural reasons: You are either breaking encapsulation of the class or do not gain any advantage over just doing what the block does after initialization.