How do runtime-created sub views communicate with the view controller?

Question

Hi

In my iPhone project I have a UIViewController in which I add an instance of a subclass of UIView. From this instance I need to communicate back an integer to my view controller. My current solution is to send a message from my instance to my App Delegate which sends a message to the view controller. It works, but it feels a bit messy.

Is there anyway I can send a message straight back to the view controller?

I would love to be able to do something like [super doSomething:123];

Any ideas?

Thanks

Answer 1

Um, I'm not sure I understand your problem correctly. You have a class derived from UIView which needs to send a message to another class derived from a UIViewController. It sounds like you are creating the UIView instance programmatically. Is there any reason my you could not have a property on the UIView which refers to the UIVIewController and just use that to send it a message directly.

You cannot use [super ...] because the super of your UIView derived class would be UIView.

Or am I miss-understanding the issue :-)

Answer 2

Simply add an outlet to your UIView subclass, connect it to its view controller in Interface Builder, and call your method on that. Here’s how that might look:

MyUIView.h:

@interface MyUIView : UIView
{
  UIViewController *viewController;
}
@property (assign) IBOutlet UIViewController *viewController;
@end

MyUIView.m:

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
  [[self viewController] setTitle:@"Hello from MyUIView"];
}

Answer 3

If I understand correctly, you want to send a message from your subclass of UIView to the view controller.

That means your subclass of UIView needs to have a property or ivar which is the view controller. The easiest way to do this is to add it as an outlet and connect it to the view controller in the nib file.

Answer 4

Generally you should not go via the app delegate. Having a typed pointer link is also less than ideal.

The optimal way of communicating - Apple does it like this as well - is to create a delegate protocol. When creating the view controller you pass a pointer to the delegate as id . Then when it gets to sending the message you ask the delegate:

if ([delegate respondsToSelector(didFinishSomething:)])
{
  [delegate didFinishSomething:info_to_pass];
}

If you want to be extra-sophisticated then you can also add a pointer to the calling class instance. Like:

[delegate myViewController:self didFinishSomething:info_to_pass];

This way you always know what kind of class the message is coming from.

If there is more than one place that needs to be notified of a change, then instead of delegation you will use notifications.

Answer 5

This is the kind of thing that NSNotificationCenter was provided for. Once you get handy with sending and receiving notifications, your message-passing gets a WHOLE lot simpler.

One of the classic things people confront is how to get a pointer to the object they want, in order to tell it about something. How do I, for instance, tell the ViewController two slots back up the UINavigationController stack that the user just changed this data field? So you dig into the stack, offset back by some magic number of elements in the stack, build public setters on the fields you want talk to... It's super cumbersome.

Compared to registering as a notification receiver in one place, and then firing a notification in some complete other place when the data changes. It's kind of magical, after doing all the "dig through the view hierarchy" work.

Answer 6

In my iPhone project I have a UIViewController in which I add an instance of a subclass of UIView.

This implies that you have both a reference to the instance of the UIView subclass and the UIViewController in the same scope. I.e. something equivalent to:

UIViewControllerSubclass *myViewController;
UIViewSubclass *myView;

(It doesn't matter if they are actually instance variables or, even, globals)

And once those two variables are initialized, somewhere you do something like:

myViewController.view = myView;

In your UIViewSubclass, add a property that points back to your UIViewControllerSubclass:

@property(assign) UIViewControllerSubclass *myController;

Then, when you do the above assignment, add:

myView.myController = myViewController;

From there, messaging your controller from your view is easy:

[self.myController yoManHereIsAnInt: 42];

Note that I used assign instead of retain because the controller already retains the view. If the view were to also retain the controller, you would have a cycle that would eventually lead to a leak.

---

No super about it. super is entirely related to the inheritance hierarchy of your Objective-C classes. What you are asking has nothing to do with inheritance and everything to do with how the various instances of objects in your application are connected together.