iOS: how can I make one view trigger an update in another?

Question

My top level UIView contains a some labels and a drawing canvas. The drawing canvas is itself a UIView which overrides its drawRect() method and, of course, the various touch methods. When the user has finished drawing, I want to update one or more labels that are outside the canvas (i.e. are all components of the top level UIView).

What's the best way to do it?
I currently have the top level UIViewController pass the various labels into my CanvasViews from the top level viewDidLoad() method and I store them locally. Whenever I need to update a label, I do so from the local copies of these sibling views I have - But I'm not happy with this because it feels kludgy.

To explain further, this is what I do in my top level UIViewController:

@IBOutlet weak var myLabel: UILabel!

override func viewDidLoad() {
    super.viewDidLoad()

    for v in self.view.subviews {
        if v .isKindOfClass(CanvasView) {
            let canvasView = v as CanvasView
            canvasView.setMyLabel(myLabel)
        }
    }
}

And then in my CanvasView.swift file, I have:

private var myLabel: UILabel!

...

override func touchesEnded(touches: NSSet, withEvent event: UIEvent) {
    // do stuff ...
    setNeedsDisplay()
    myLabel.text = "etc.etc."
}

Is there a better way?

Thanks.
Ave

create and use delegates. – Fawad Masud Jan 31 '15 at 17:47 — Fawad Masud, Jan 31 '15 at 17:47

score 0 · Answer 1 · edited May 23 '17 at 12:05

I would implement the delegate pattern. When creating the canvas pass the parent viewcontroller where you can implement various callback methods.

Here you can find a more detailed explanation of the various methods of implementing a delegate. Some of them might be overkill for this single and simple usecase.

Example: you have a game insance (called game), which defines a delegate protocol. the protocol contains several methods for different events. (e.g. -(void)gameDidFinish etc.)

Now the game implements a method addDelegate which is called by every instance, which wants to be notified for certain events of the game. All of these instances, who register themselves as delegates implement all or some of the methods defined in the games protocol

If one of the game events is triggered the game checks all of its registered delegates if they have implemented the corresponding method and calls it.

Important: as mentioned the protocol ma be overkill, as you have only one delegate.

Update: Ok, the delegate method works, but what is the standard idiom for doing this - e.g. when a shooter game wants to update scores dynamically outside its main drawing frame? — Anand Venkataraman, Jan 31 '15 at 18:05
the exact same thing. i will update my answer with an example — Maximilian Körner, Jan 31 '15 at 18:12

score 0 · Accepted Answer · answered Jan 31 '15 at 18:13

If your canvas is a separate viewController, you can use delegate protocols to access your base view controller from the canvas view controller. Suppose you have a BaseViewController which calls the canvasViewController.

In your canvasViewController.h add the following :

@protocol CanvasDelegate <NSObject>
@required
//examples . add your methods to upgate labels here
    - (void) updateLabels: (NSString*) labelText1 and:(NSString*)labelText2;
@end

@interface canvasViewController : UIViewController
{
    id<CanvasDelegate> delegate;
}

@property (retain) id delegate;

@end

Now from your canvasViewController.m file execute this code whenever you want to update your labels (or ask the baseViewController to do something) :

[[self delegate] updateLabels:string1 and:string2];

Now for your base view controller to accept messages from the canvas, the BaseVC has to "adopt" the CanvasDelegate Protocol and also implement the required methods. In baseViewController.h, add the following :

@interface baseViewController : UIViewController <CanvasDelegate>

In baseViewController.m when you create an object of canvasViewController for pushing, add the following command :

canvasVC.delegate = self;

And don't forget to implement the required methods in baseViewController.m like so :

- (void) updateLabels: (NSString*) labelText1 and:(NSString*)labelText2
{
    //update your labels here
}

iOS: how can I make one view trigger an update in another?

2 Answers2

Linked