Unexpected events emitted by Swift Combine PassThroughSubject - swift

Question

I'm currently playing with Combine and SwiftUI and have built a prototype app using the MVVM pattern. The app utilises a timer and the state of the button controlling this is bound (inelegantly) to the view model utilising a PassThroughSubject.
When the button is pressed, this should toggle the value of a state variable; the value of this is passed to the view model's subject (using .send) which should send a single event per button press. However, there appears to be recursion or something equally weird going on as multiple events are sent to the subject and a runtime crash results without the UI ever being updated.
It's all a bit puzzling and I'm not sure if this is a bug in Combine or I've missed something. Any pointers would be much appreciated. Code below - I know it's messy ;-) I've trimmed it down to what appears to be relevant but let me know if you need more.
View:
struct ControlPanelView : View {
#State private var isTimerRunning = false
#ObjectBinding var viewModel: ControlPanelViewModel
var body: some View {
HStack {
Text("Case ID") // replace with binding to viewmode
Spacer()
Text("00:00:00") // repalce with binding to viewmodel
Button(action: {
self.isTimerRunning.toggle()
self.viewModel.apply(.isTimerRunning(self.isTimerRunning))
print("Button press")
}) {
isTimerRunning ? Image(systemName: "stop") : Image(systemName: "play")
}
}
// .onAppear(perform: { self.viewModel.apply(.isTimerRunning(self.isTimerRunning)) })
.font(.title)
.padding(EdgeInsets(top: 0, leading: 32, bottom: 0, trailing: 32))
}
}
Viewmodel:
final class ControlPanelViewModel: BindableObject, UnidirectionalDataType {
typealias InputType = Input
typealias OutputType = Output
private let didChangeSubject = PassthroughSubject<Void, Never>()
private var cancellables: [AnyCancellable] = []
let didChange: AnyPublisher<Void, Never>
// MARK:- Input
...
private let isTimerRunningSubject = PassthroughSubject<Bool, Never>()
....
enum Input {
...
case isTimerRunning(Bool)
...
}
func apply(_ input: Input) {
switch input {
...
case .isTimerRunning(let state): isTimerRunningSubject.send(state)
...
}
}
// MARK:- Output
struct Output {
var isTimerRunning = false
var elapsedTime = TimeInterval(0)
var concernId = ""
}
private(set) var output = Output() {
didSet { didChangeSubject.send() }
}
// MARK:- Lifecycle
init(timerService: TimerService = TimerService()) {
self.timerService = timerService
didChange = didChangeSubject.eraseToAnyPublisher()
bindInput()
bindOutput()
}
private func bindInput() {
utilities.debugSubject(subject: isTimerRunningSubject)
let timerToggleStream = isTimerRunningSubject
.subscribe(isTimerRunningSubject)
...
cancellables += [
timerToggleStream,
elapsedTimeStream,
concernIdStream
]
}
private func bindOutput() {
let timerToggleStream = isTimerRunningSubject
.assign(to: \.output.isTimerRunning, on: self)
...
cancellables += [
timerToggleStream,
elapsedTimeStream,
idStream
]
}
}

Answer 1

In your bindInput method isTimerRunningSubject subscribes to itself. I suspect this is not what you intended and probably explains the weird recursion you are describing. Maybe you're missing a self. somewhere?
Also odd is that both bindInput and bindOutput add all streams to the cancellables array, so they'll be in there twice.
Hope this helps.

Answer 2

This example works as expected but I discovered during the process that you cannot use the pattern in the original code (internal Structs to define Input and Output) with #Published. This causes some fairly odd errors (and BAD_ACCESS in a Playground) and is a bug that was reported in Combine beta 3.
final class ViewModel: BindableObject {
var didChange = PassthroughSubject<Void, Never>()
#Published var isEnabled = false
private var cancelled = [AnyCancellable]()
init() {
bind()
}
private func bind() {
let t = $isEnabled
.map { _ in }
.eraseToAnyPublisher()
.subscribe(didChange)
cancelled += [t]
}
}
struct ContentView : View {
#ObjectBinding var viewModel = ViewModel()
var body: some View {
HStack {
viewModel.isEnabled ? Text("Button ENABLED") : Text("Button disabled")
Spacer()
Toggle(isOn: $viewModel.isEnabled, label: { Text("Enable") })
}
.padding()
}
}