Add StateMutationSink

[n8chur]

Background

From @dhavalshreyas's proposal in Notion (with slightly updated examples):

Sinks are used to process Actions, either from SideEffects or from user-interaction events.

We currently have makeSink to process any Action and makeOutputSink, which provides a Sink that sends the Output.

The proposal here is to introduce makeStateMutationSink, which would provide a Sink that automatically mutates State without an intermediate Action.

Before:

struct MyWorkflow: Workflow {
    enum Action: WorkflowAction {
        typealias WorkflowType = MyWorkflow

        case updateTapCount(Int)

        func apply(toState state: inout Int) -> MyWorkflow.Output? {
            switch self {
            case .updateTapCount(let count):
                state.tapCount = count
                return nil
            }
            return nil
        }
    }

    func render(state: State, context: RenderContext<Self>) -> Rendering {
        let sink = context.makeSink(of: Action.self)
        return Screen(onTap: { _ in
            sink.send(.updateTapCount(state.tapCount + 1))
        })
    }
}

After:

struct MyWorkflow: Workflow {
    func render(state: State, context: RenderContext<Self>) -> Rendering {
        let stateMutationSink = context.makeStateMutationSink()
        return Screen(onTap: { _ in
            stateMutationSink.send(\.tapCount, value: state.tapCount + 1)
        })
    }
}

---

This convenience has been baking in ios-register's WorkflowExperimental framework for quite some time now and has proven useful to many.

Additions to existing convenience

These changes area a direct port of that code (including the associated tests) with one additional convenience which would allow for mutating state with a closure rather than just the KeyPath/Value syntax:

struct MyWorkflow: Workflow {
    func render(state: State, context: RenderContext<Self>) -> Rendering {
        let stateMutationSink = context.makeStateMutationSink()
        return Screen(onTap: { _ in
            stateMutationSink.send { state in
               state.tapCount += 1
               state.tapped = true
            }
        })
    }
}

Checklist

• Unit Tests
• UI Tests
• Snapshot Tests (iOS only)
• I have made corresponding changes to the documentation

[n8chur]

Let me know if you feel this deserves documentation outside of the code doc comment, and if so, where it should go!

[jamieQ]

this doesn't really affect the semantics of the tests at all, but i think the expectation machinery isn't strictly needed in this instance. the processing should happen synchronously as soon as the values are received.

[n8chur]

Would you prefer that I try removing it?

I just directly ported the test from ios-register and didn't dig into how I might be able to improve these tests much...

[jamieQ]

ah, no it's fine as-is, just noticed this. i didn't realize initially that this code was just being moved.