PropertyWrapper: @transaction binding for SwiftUI to make changes to data supporting commit/rollback

Example_Complex.swift

struct User: Equatable {
    var firstName: String
    var lastName: String
}

@main
struct MyApp: App {
    @State var value = User(firstName: "", lastName: "")
    @State var showEdit = false

    var body: some Scene {
        WindowGroup {
            VStack {
                Text("First Name: \(value.firstName)")
                Text("Last Name: \(value.lastName)")
                Button("Edit") { showEdit = true }
            }
            .sheet(isPresented: $showEdit) {
                UserEditView(value: $value.transaction())
            }
        }
    }
}

struct UserEditView: View {
    @Transaction var value: User

    var body: some View {
        VStack {
            TextField("First Name", text: $value.firstName)
            TextField("Last Name", text: $value.lastName)
            Divider()
            Button("Commit", action: $value.commit).disabled(!$value.hasChanges)
            Button("Rollback", action: $value.rollback).disabled(!$value.hasChanges)
        }
    }
}

Example_Simple.swift

@main
struct MyApp: App {
    @State var value = 0

    var body: some Scene {
        WindowGroup {
            MyView(value: $value.transaction())
        }
    }
}
struct MyView: View {
    @Transaction var value: Int

    var body: some View {
        VStack {
            Text("Value: \(value)")
            Text("HasChanges: \($value.hasChanges ? "yes" : "no")")
            Divider()
            Button("Increase") { value += 1 }
            Button("Commit", action: $value.commit)
            Button("Rollback", action: $value.rollback)
        }
    }
}

Transaction.swift

import SwiftUI

@propertyWrapper
@dynamicMemberLookup
public struct Transaction<Value>: DynamicProperty {
    @State private var derived: Value
    @Binding private var source: Value

    fileprivate init(source: Binding<Value>) {
        self._source = source
        self._derived = State(wrappedValue: source.wrappedValue)
    }

    public var wrappedValue: Value {
        get { derived }
        nonmutating set { derived = newValue }
    }

    public var projectedValue: Transaction<Value> { self }

    public subscript<T>(dynamicMember keyPath: WritableKeyPath<Value, T>) -> Binding<T> {
        return $derived[dynamicMember: keyPath]
    }

    public var binding: Binding<Value> { $derived }

    public func commit() {
        source = derived
    }
    public func rollback() {
        derived = source
    }
}

extension Transaction where Value: Equatable {
    public var hasChanges: Bool { return source != derived }
}
extension Binding {
    public func transaction() -> Transaction<Value> { .init(source: self) }
}

Hello @IanKeen, how that would work in a preview?

struct ChangeMindfulMinutesGoal_Previews: PreviewProvider {
    static var previews: some View {
        ChangeMindfulMinutesGoal(goal: 20)
    }
}

And I get this error:

Cannot convert value of type 'Int' to expected argument type 'Transaction<Int>'

hey @agisilaos , currently this requires you base the transaction off a Binding so you should be able to do something like:

ChangeMindfulMinutesGoal(goal: Binding.constant(20).transaction())

if you want to provide just a value you could do something like

extension Transaction {
    public init(source: Value) {
        var source = source
        let binding = Binding(get: { source }, set: { source = $0 })
        self.init(source: binding)
    }
}

then you can do

ChangeMindfulMinutesGoal(goal: .init(source: 20))

but this would be a detached object, commits don't propagate anywhere (which is fine for previews)