Accessing State's value outside of being installed on a View. This will result in a constant Binding of the initial value and will not update.

I sometimes like to explore the weirder error messages that Xcode throws at us even once I've fixed the initial issue. This is one of those:

Accessing State's value outside of being installed on a View. This will result in a constant Binding of the initial value and will not update.

TL;DR: In case you landed here looking for an actual fix to this error: this happens when trying to access a binding to a property on an ObservableObject that's wrapped with @State from within the observing View. The fix is to change the @State to an @Published and update the reference from model.$property to $model.property. That's the existing well published fix and the rest of this post is about the why.

This is a confusing warning message, the phrasing of outside of being installed on a View could give the impression that this is a lifecycle issue and so maybe the fix is to adjust the timing of when the property is accessed, like moving it within a .onAppear block or similar.

What the warning is in fact saying is that you can't use @State on something that isn't a View such as an ObservedObject and then reference that from your View and expect it to behave the same way.

The reason is more or less obvious once you've realised you meant to use an @Published, but none the less. An @State knows that it belongs to the View it was declared in. The backing store for the @State SwiftUI created for us then notifies the View to update when that value is mutated. If we have an @State in an ObservableObject it has no way of knowing what View it is being used in, therefore nothing can be updated when it's mutated. So instead we just get a constant binding of the initial value, or in other words we get the value we initialised the property to and no updated values thereafter.

So instead we can use an @Published (which is likely what we meant to do when we encountered this warning) instead of @State.

This will bring us an error similar to the following Cannot convert value of type 'Published<Double>.Publisher' to expected argument type 'Binding<Double>'. Which is because our model.$property now accesses the projected value of an @Published which is a Publisher, so we need to change this to $model.property which gives us the a binding to an ObservableObject, a binding being the projected value of an ObservableObject.

All that to say @State properties should be private and I can't think of any good reason to use them in an ObservableObject in any case. I just thought this was an interesting error and wanted to run through the why of it.

For me, this is just a reminder of how much is magicked for us when working with SwiftUI.