November 18, 2022 (Updated: December 10, 2022)

When you want to add a custom @propertyWrapper to an existing @Published variable in your SwiftUI code, e.g. in your ViewModel, you will quickly discover that @Published does not play well with other property wrappers.

The solution I came up with is to wrap the functionality of @Published within the custom property wrapper:

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@propertyWrapper
struct CustomFormatted {

    var wrappedValue: String {
        didSet {
            objectWillChange?()

            // assuming the functionality of `.formatted` exists in an extension
            wrappedValue = wrappedValue.formatted
        }
    }

    var objectWillChange: (() -> Void)?

    init(wrappedValue: String, objectWillChange: (() -> Void)? = nil) {
        self.wrappedValue = wrappedValue.formatted
        self.objectWillChange = objectWillChange
    }
}

Then, when used in a view model it can simply pass a custom operation in:

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final class MyViewModel: ObservableObject {

    @CustomFormatted
    var myVariable: String = ""

    init() {
        _myVariable.objectWillChange = { objectWillChange.send() }
    }

}

It is clunky but I haven’t seen a better solution yet.