StateObject that works in iOS 13

StateObject.swift

import Combine
import PublishedObject // https://github.com/Amzd/PublishedObject
import SwiftUI

/// A property wrapper type that instantiates an observable object.
@propertyWrapper
public struct StateObject<ObjectType: ObservableObject>: DynamicProperty
where ObjectType.ObjectWillChangePublisher == ObservableObjectPublisher {
    
    /// Wrapper that helps with initialising without actually having an ObservableObject yet
    private class ObservedObjectWrapper: ObservableObject {
        @PublishedObject var wrappedObject: ObjectType? = nil
        init() {}
    }
    
    private var thunk: () -> ObjectType
    @ObservedObject private var observedObject = ObservedObjectWrapper()
    @State private var state = ObservedObjectWrapper()
    
    public var wrappedValue: ObjectType {
        if state.wrappedObject == nil {
            // There is no State yet so we need to initialise the object
            state.wrappedObject = thunk()
            // and start observing it
            observedObject.wrappedObject = state.wrappedObject
        } else if observedObject.wrappedObject == nil {
            // Retrieve the object from State and observe it in ObservedObject
            observedObject.wrappedObject = state.wrappedObject
        }
        return state.wrappedObject!
    }
    
    public var projectedValue: ObservedObject<ObjectType>.Wrapper {
        ObservedObject(wrappedValue: wrappedValue).projectedValue
    }
    
    public init(wrappedValue thunk: @autoclosure @escaping () -> ObjectType) {
        self.thunk = thunk
    }
    
    public mutating func update() {
        // Not sure what this does but we'll just forward it
        _state.update()
        _observedObject.update()
    }
}

Currently don’t have access to a Mac but you can probably do a couple if #available checks inside a separate property wrapper to switch between wrappers (you can do SwiftUI.StateObject to get the SwiftUI type)

Update func is supposed to init the object

Yes, update function should reinitialise the observed object. (as it gets reset whenever StateObject.init is called)

I have found a "hacky solution" to detect this case: ObservedObject has an internal property _seed which is some internal state.
Whenever the value is 1 our StateObject should recreate a new object.

    public mutating func update() {
        // Not sure what this does but we'll just forward it
        _state.update()
        _observedObject.update()

        // HACK! We rely on the internal _seed variable of `ObservedObject` to learn when we should initialize
        let mirror = Mirror(reflecting: _observedObject)
        guard let seed = mirror.descendant("_seed") as? Int else {
            return
        }
        if seed == 1 {
            state.wrappedObject = thunk()
            observedObject.wrappedObject = state.wrappedObject
        }
    }

Correct me if I'm wrong, but since we call the _state.update() that should reinit its ObservedObjectWrapper causing state.wrappedObject and observedObject.wrappedObject to not be the same object? @pd95 @malhal

If that's the case I think the best would be to just change the wrappedValue getter to always update the observedObject.wrappedObject?

    public var wrappedValue: ObjectType {
        if state.wrappedObject == nil {
            // There is no State yet so we need to initialise the object
            state.wrappedObject = thunk()
-        }
-       if observedObject.wrappedObject == nil {
            // Retrieve the object from State and observe it in ObservedObject
            observedObject.wrappedObject = state.wrappedObject
        }
        return state.wrappedObject!
    }

I know the init happens when you get wrappedValue vs when update is called but I don't like relying on undocumented behaviour.

you are a legend!

[Is there a way to conditionally use @StateObject while targeting iOS 13 but use apple's @StateObject in iOS 14

If you are using Combine's ObservableObject as the @StateObject you could use Async/await to replace the functionality and that is backwards compatible with iOS 13. Could be tricky without the task(priority:_:) modifier though.

[Is there a way to conditionally use @StateObject while targeting iOS 13 but use apple's @StateObject in iOS 14

try this?

@available(iOS 13, obsoleted: 14)

@available(iOS 13, obsoleted: 14)

@calvingit this not work
I insert a break point, it also called iOS 13 method...