Can't update Label from other class in Swift

Question

I have a little problem with my code so that I can't call a function. I created a function func changeLabelText(text: String) { mylabel.stringValue = text } in the ViewController with that I can update the Text in the Label. When I try to call the function from another class in the Project, I get a runtime error(EXC_BAD_Instr...) and the Debugger holds on the line where I try to change the Label's text, with the error: fatal error: unexpectedly found nil while unwrapping an Optional value. Where is the problem? Can someone help me please!

Answer 1

Regarding the "unexpectedly found nil while unwrapping an Optional value" error, the problem is that you have an optional (judging from the code snippet you've provided, it must be an implicitly unwrapped optional) that is nil. Most likely, mylabel is nil. Either print the value or add a breakpoint and examine the property in the debugger to confirm.

If it is nil, you then have to figure out why. In our discussion, it turns out that you're trying to call the Log event in the view controller:

ViewController().Log("asdf")

The problem is that this doesn't call Log in the existing view controller, but rather the ViewController() expression ends up instantiating a new, completely unrelated view controller with its outlets not hooked up to anything. Thus the attempt to update the implicitly unwrapped outlet will cause the error you shared with us.

If you want this separate class (a database manager object) to inform the view controller of event in order to allow the view controller to update the UI, there are three common approaches:

1. Completion/progress handlers that are closures/blocks.

   This is used for simple interface where the database needs to inform view controller when the request is done.

2. Delegate-protocol pattern.

   The delegation pattern (usually conforming to some well-established protocol) is used for rich interfaces where the database needs to inform the view controller of a variety of different types of events.

3. Notification pattern.

   Notifications are used when you want a loosely coupled interface between the database object and whatever is handling these notifications. The view controller might register itself as an observer of any notifications of a particular name with the defaultCenter() of the NSNotificationCenter. The database object can then post notifications of that name (supplying details via the userInfo dictionary) and the view controller will be informed of these events.

Answer 2

A few things are you calling an instance of the view controller properly

e.g if the class name is ViewController, in the the view controller that you are calling the function make sure you use

let VC = ViewController ()

and then use VC.yourFunctionName

also instead of stringValue use .text