Reimagining SwiftCurrent on SwiftUI

[Tyler-Keith-Thompson]

I'm quite convinced we have yet to nail down an appropriate SwiftUI API. We have gotten closer, but we lack some features and we lack some focus. I am quite confident we can do better.

Motivation

As it stands, our SwiftUI implementation is complicated, not only because of recursive views but because we are dealing with this underlying Workflow reference type. It's also not extensible. Apple broke the NavigationLink contract we used in SwiftUI 4...the code compiles but navigation doesn't work. We're hamstrung releasing something new because GitHub runner aren't on the latest OS and our consumers can't make it work because there's not a great place for them to extend functionality.

We also had this notion that "we want views to work whether they're in a Workflow or not." which is counter to how SwiftUI works in general. A ScrollViewReader only works in a ScrollView. We need to shed some expectations that only exist because "that's how UIKit did it." So I'm taking a stronger stance with this pitch.

Goals

• Get rid of _workflowPointer
• Loosen the requirements on FlowRepresentable so that it's either unnecessary, or only necessary for certain circumstances
• Tie into the SwiftUI ecosystem better
• Grant flexibility with shouldLoad so that it can be placed inside a view, but can also be placed outside that view in the WorkflowView
• Tweak the result builder so that it's trivial to store a WorkflowGroup outside a WorkflowView definition, thus allowing workflows to be extracted, named, and reused.
• Don't tie ourselves to "how UIKit did it" nearly as much.
• Allow for custom presenters to be added, making it easy to have a WorkflowItem display in a modal, nav link, or any custom context (like a custom built bottom drawer)
• Force presenters to be very simple about defining their navigation logic, thus taking an area where we've had lots of issues and simplifying it.
• Rethink abandon so that it becomes more efficient and simpler.
• Do not add complexity to the public interface, either remove complexity or keep it the same.
• Add a WorkflowReader type to replace the need for _workflowPointer there's enough convention on this that a discussion regarding that type shouldn't be all the necessary. The last discussion Proposal: FlowRepresentableView #123 did not result in a conversation that helped move this forward, but may provide insight into there being better naming conventions or documentation.

I'll be working on this as I have time, after I know whether it even works I'll submit a PR and we can tailor this discussion to reality of the thing, rather than a hypothetical, which I think hindered us last time.

[Tyler-Keith-Thompson]

I have a pretty reasonable prototype over on the reimagine-swiftui branch. These changes are feeling pretty great, but as of the time of writing there are unanswered questions and still missing functionality.

Overview of the changes:

Workflow Changes:

• The type Workflow is now a protocol, allowing for workflows to be extracted from view contexts, example:
• WorkflowItem has a new initializer!

// old
WorkflowView {
    WorkflowItem(Screen1.self) // not possible to extract :(
}

// new -- without extracting
WorkflowView {
    WorkflowItem { Screen1() }
}

// new
WorkflowView {
    workflow
}

@WorkflowBuilder var workflow: some Workflow { // extracted!
    WorkflowItem { Screen1() }
}

• FlowRepresentable is gone! It's been replaced by a series of EnvironmentValues and its functionality has been replaced by view operators/modifiers.
• The old imperative methods have been replaced with view modifiers:

// old
import SwiftCurrent
struct Screen1: View, FlowRepresentable {
    weak var _workflowPointer: AnyFlowRepresentable?
    
    var body: some View {
        VStack {
            Text("Hello, World!")
        }
    }
    
    func shouldLoad() -> Bool {
        true
    }
}

// new
import SwiftCurrent_SwiftUI
struct Screen1: View {
    @State var shouldProceed = false
    @State var shouldAbandon = false
    @State var shouldBackUp = false
    var body: some View {
        VStack {
            Text("This is \(String(describing: Self.self))")
            Button("Navigate forward") { shouldProceed = true }
            Button("Navigate backward") { shouldBackUp = true }
            Button("Abandon") { shouldAbandon = true }
        }
        .workflowLink(isPresented: $shouldProceed, value: "my string")
        .backUpInWorkflow($shouldBackUp)
        .abandonWorkflow($shouldAbandon)
        .shouldLoad { true }
    }
}