Attach async tasks to SwiftUI views using a trigger mechanism.

TaskTrigger.swift

import SwiftUI

struct TaskTrigger<T: Equatable>: Equatable {

    fileprivate enum TaskState<S: Equatable>: Equatable {
        case inactive
        case active(value: S, uniqueId: UUID? = nil)
    }

    fileprivate var state: TaskState<T> = .inactive

    mutating func trigger(value: T, id: UUID? = UUID()) {
        self.state = .active(value: value, uniqueId: id)
    }

    mutating func cancel() {
        self.state = .inactive
    }
}

extension TaskTrigger where T == Bool {
    mutating func trigger() {
        self.state = .active(value: true)
    }
}

struct TaskViewModifier<T: Equatable>: ViewModifier {

    @Binding var trigger: TaskTrigger<T>
    let action: @Sendable (_ arg: T) async -> Void

    func body(content: Content) -> some View {
        content
            .task(id: trigger.state) {
                guard case .active(let arg, _) = trigger.state else {
                    return
                }

                await action(arg)
                
                if !Task.isCancelled {
                    self.trigger.cancel()
                }
            }
    }
}

extension View {
    func task<T: Equatable>(
        _ trigger: Binding<TaskTrigger<T>>, 
        _ action: @escaping @Sendable @MainActor (_ arg: T) async -> Void
    ) -> some View {
        modifier(TaskViewModifier(trigger: trigger, action: action))
    }

    func task(
        _ trigger: Binding<TaskTrigger<Bool>>, 
        _ action: @escaping @Sendable () async -> Void
    ) -> some View {
        modifier(TaskViewModifier(trigger: trigger, action: { _ in await action() }))
    }
}

/************************
******* Examples *******
************************/

/*
This first example simply acts as a boolean trigger without any 
attached value. Once you call `trigger()`, the async task will execute.
By being attached to the view it will also get cancelled once the
view gets dismissed.
*/

struct ContentView: View {

    @State private var asyncTrigger: TaskTrigger<Bool> = TaskTrigger()
    
    var body: some View {
        Button("Do async stuff") {
            asyncTrigger.trigger()
        }
        .task($asyncTrigger) {
            await someAsyncFunction()
        }
    }    
}

/*
This example attaches any equatable value (e.g. an integer) to the trigger. 
Once you call `trigger(value:)`, the async task will execute and the attached 
value is passed to the task. This might be useful when fetching some API based 
on variable parameters.
*/

struct ContentView: View {

    @State private var asyncTrigger: TaskTrigger<Int> = TaskTrigger()
    
    var body: some View {
        Button("Do async stuff") {
            asyncTrigger.trigger(value: 42)
        }
        .task($asyncTrigger) { value in
            await someAsyncFunctionWithInteger(value)
        }
    }    
}

This is now available as a Swift Package as well: https://github.com/lukepistrol/TaskTrigger