Automatic trigger for objectWillChange publisher

[jlsiewert]

Automatically triggers the objectWillChange publisher in Combines ObservableObject.

Heavily inspired by John Sundell and the @propertyWrapper Swift Evolution Proposal.

fixes #5

[jlsiewert]

By adding the public subscript

public static subscript<T: ObservableObject>(
         _enclosingInstance instance: T,
         wrapped wrappedKeyPath: ReferenceWritableKeyPath<T, Value>,
         storage storageKeyPath: ReferenceWritableKeyPath<T, Self>
     ) -> Value where T.ObjectWillChangePublisher == ObservableObjectPublisher

we are in fact enforcing that the property wrapper can only be used within an ObservableObject with the default ObservableObjectPublisher, what is probably not what we want.

The best solution would probably be to provide two implementation, one more specialised then the other, like so:

public static subscript<T: AnyObject>(
        _enclosingInstance instance: T,
        wrapped wrappedKeyPath: ReferenceWritableKeyPath<T, Value>,
        storage storageKeyPath: ReferenceWritableKeyPath<T, Self>
    ) -> Value {
        get {
            // Return the saved wrapped value
            instance[keyPath: storageKeyPath].wrappedValue
        }
        set {
            instance[keyPath: storageKeyPath].wrappedValue = newValue
        }
    }

    public static subscript<T: ObservableObject>(
        _enclosingInstance instance: T,
        wrapped wrappedKeyPath: ReferenceWritableKeyPath<T, Value>,
        storage storageKeyPath: ReferenceWritableKeyPath<T, Self>
    ) -> Value where T.ObjectWillChangePublisher == ObservableObjectPublisher {
        get {
            // Return the saved wrapped value
            instance[keyPath: storageKeyPath].wrappedValue
        }
        set {
            instance.objectWillChange.send()
            instance[keyPath: storageKeyPath].wrappedValue = newValue
        }
    }

However, that currently produces the error Property wrapper type 'AsyncValue' has multiple enclosing-self subscripts 'subscript(_enclosingInstance:wrapped:storage:)', as described as in apple/swift#54777

For now, we could fall back for a runtime check:

public static subscript<T: AnyObject>(
        _enclosingInstance instance: T,
        wrapped wrappedKeyPath: ReferenceWritableKeyPath<T, Value>,
        storage storageKeyPath: ReferenceWritableKeyPath<T, Self>
    ) -> Value {
        get {
            // Return the saved wrapped value
            instance[keyPath: storageKeyPath].wrappedValue
        }
        set {
            if let instance = instance as? any ObservableObject,
               let publisher = (instance.objectWillChange as any Publisher) as? ObservableObjectPublisher {
                // Trigger the `ObjectWillChangePublisher`
                publisher.send()
            }
            instance[keyPath: storageKeyPath].wrappedValue = newValue
        }
    }

[BrentMifsud]

Hey thanks for contributing! I was looking into adding something like this so users don't have to keep adding a willSet.

I'll take a closer look tomorrow when I have some time.

[BrentMifsud]

Hey looks good.

Going to force merge this as there's a permission issue with the GitHub actions.