How can I update a view controller when something happens in a container view controller?

Question

What I need to do is be able to update my main view controller when something happens in a container view inside of it.

For example, if there was a nav bar at the top of a page with a save button. Below the nav bar is a container view and a form. The save button is greyed out until text is entered into the form and then it becomes intractable with the user. I want to do something similar so that the main view controller responds to something happening on the container view.

I can use a singleton create a boolean variable that changes when I want my code to run, but how can I make it so that once this variable changes to true, code on the other view controller runs and responds to this change?

I saw in this post that I could use delegates but it still leaves me with the problem of how the other view controller detects that the variable has changed - the method in this post was just another way of making the variable accessible from the other view controller I think.

I thought that I could have a while loop running checking the variable but this would freeze the main view controller until the while loop had finished - in this case when the variable was true wouldn't it?

Is there a way for a view controller to check if a variable has changed so that once it has, certain code can run afterwards without if freezing the view controller like I think a while loop would? Or a way for a view controller to respond to the value of a variable?

Thank you.

Answer 1

Have the main view controller listen for a NSNotification that tells it something needs saving. When text is entered, the controller that looks after the text field sends the notification.

You can attach the new text (or other information) to the notification if the target needs to know about its details.