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Editorial Team
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Asked: 2026-05-26T11:18:40+00:00

I’m creating one UIViewController that operates a calculator interface, while another UIViewController deals with

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I’m creating one UIViewController that operates a calculator interface, while another UIViewController deals with drawing a graph.

The calculator prompts the graph to redraw itself when the user pushes the graph button. The graph then performs a call back to a protocol method on the calculator to get the y value for each given x coordinate.

I’m wondering what the standard way of handling this kind of communication is? I feel like I should be able to call something akin to setNeedsDisplay on a UIViewController (I’m aware this is actually only a UIView method), but I have a feeling I’m going to have to write another protocol with an similar method. Is there some kind of updateViewController method I’m missing?

Ideally I’d like my Calculator UIViewController to only know the graph as a UIViewController while the graph knows the Calculator through a protocol I define.

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1 Answer

  1. Editorial Team

    You have several methods to do that.

    1. Target/Action
    2. Delegates
    3. NSNotificationCenter
    4. KVO (Key Value observing)

    In your case, KVO seems to be the best option. Basically, what you graph draws depends on your calculator interface (if I understand correctly) so you are observing some variables (read keys) for change. Look at this document to learn more.

    KVO can let you optimize graph drawings as well because you can get previous value, new value etc. Infact, by definition:

    Key-value observing is a mechanism that allows objects to be notified of changes to specified properties of other objects.

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