A ValueBinder creates a two-way binding object to allow sharing of a value between objects.

This is mildly similar to @Binding in SwiftUI but doesn't rely on SwiftUI - meaning it can be used pretty much anywhere Swift can.

You can define a binder using the standard initializer.

This initializer also allows you to supply a callback block that gets triggered when the wrappedValue changes.

let countBinder = ValueBinder<Int>(0) { newValue in
   Swift.print("countBinder changed: \(newValue)")
}
...
countBinder.wrappedValue = 4  // triggers the update callback

You can hand your ValueBinding object to another class which can supply a block to be called when the ValueBinder wrapped value changes.

class AnotherClass {
   init(_ binder: ValueBinder<Int>) {
      binder.register(self) { newValue in
         Swift.print("Binding detected change: \(newValue)")
      }
   }
}

Additionally, if you hold on to the binder object your class can update the ValueBinder value too!

class AnotherClass {
   let countBinder: ValueBinder<Int> 
   init(_ binder: ValueBinder<Int>) {
      countBinder = binder
      countBinder.register(self) { newValue in
         Swift.print("Binding detected change: \(newValue)")
      }
   }
   
   func userPressed() {
      countBinder.wrappedValue += 1
   }
}

Any object that holds a ValueBinder object can update the wrapped value.

_countBinder.wrappedValue += 1

All objects that have registered for change callbacks will be notified of the change in value.

ValueBinding is a property wrapper implementation for the ValueBinder type. Thanks to Mx-Iris for sharing their implementation.

@ValueBinding var countValue = 0

// Register a block for updates
$countValue.register { newValue in 
   Swift.print("countValue is now \(newValue)")
}

countValue = 4  // triggers the update callback
// prints "countValue is now 4"

// Register for combine updates
$countValue.publisher?.publisher
   .receive(on: DispatchQueue.global(qos: .background))
   .sink { newValue in
      // Do something with 'newValue'
   }
   .store(in: &subscribers)

A KeyPathBinder is a specialization of the ValueBinder that can track a dynamic keypath

// The dynamic property to bind to. This might be (for example) bound to a control from interface builder.
@objc dynamic var state: NSControl.State = .on

// Our binding object
lazy var boundKeyPath: KeyPathBinder<MyViewController, NSControl.StateValue> = {
   return try! .init(self, keyPath: \.buttonState) { newValue in
      Swift.print("boundKeyPath notifies change: \(String(describing: newValue))")
   }
}()

An EnumKeyPathBinder is a keypath binder for observing Swift enum types.

I had a situation where I was tring to use a KeyPathBinder on the size mode for a toolbar which is of type NSToolbar.SizeMode, and it continually failed. However, binding to NSControl.StateValue on a control worked fine.

The result was that NSControl.StateValue, while appearing like an enum is actually a struct, whereas NSToolbar.SizeMode is an enum (specifically, a RawRepresentable). The issue appears when trying to observe enum type changes, so this class is a specialization of KeyPathBinder specifically for observing such enum types.

Both binder types expose a property publisher which you can hook up to your combine workflow.

If the OS doesn't support Combine, the publisher property will be nil.

let binder = ValueBinder(0) 
...
let cancellable = binder.publisher?.sink { newValue in
   // do something with `newValue`
}

MIT License

Copyright (c) 2023 Darren Ford

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