getting unresolved identifier 'self' in SwiftUI code trying to use Timer.scheduledTimer - swift

Question

In SwiftUI I'm noting to use a Timer that:
Try 1 - This doesn't work as get "Use of unresolved identifier 'self'"
var timer2: Timer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true) {
self.angle = self.angle + .degrees(1)
}
Try 2 - Works, but have to put in an "_ = self.timer" to start it later
var timer: Timer {
Timer.scheduledTimer(withTimeInterval: 0.1, repeats: true) {_ in
self.angle = self.angle + .degrees(1)
}
}
// then after need to use " .onAppear(perform: {_ = self.timer}) "
Is there a way to get my Try1 working? That is where in a SwiftUI file I can create the timer up front? Or actually where in SwiftUI would one normally start and stop the timer? i.e. where are lifecycle methods
Whole file:
import SwiftUI
struct ContentView : View {
#State var angle: Angle = .degrees(55)
// var timer2: Timer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true) {
// self.angle = self.angle + .degrees(1)
// }
var timer: Timer {
Timer.scheduledTimer(withTimeInterval: 0.1, repeats: true) {_ in
self.angle = self.angle + .degrees(1)
}
}
private func buttonAction() {
print("test")
self.angle = self.angle + .degrees(5)
}
var body: some View {
VStack{
Text("Start")
ZStack {
Circle()
.fill(Color.blue)
.frame(
width: .init(integerLiteral: 100),
height: .init(integerLiteral: 100)
)
Rectangle()
.fill(Color.green)
.frame(width: 20, height: 100)
// .rotationEffect(Angle(degrees: 25.0))
.rotationEffect(self.angle)
}
Button(action: self.buttonAction) {
Text("CLICK HERE")
}
Text("End")
}
.onAppear(perform: {_ = self.timer})
}
}

Answer 1

It isn't clear to me that you need a timer for your example, but since there is a great deal of misinformation out there about how to include a Timer in a SwiftUI app, I'll demonstrate.
The key is to put the timer elsewhere and publish each time it fires. We can easily do this by adding a class that holds the timer as a bindable object to our environment (note that you will need to import Combine):
class TimerHolder : BindableObject {
var timer : Timer!
let didChange = PassthroughSubject<TimerHolder,Never>()
var count = 0 {
didSet {
self.didChange.send(self)
}
}
func start() {
self.timer?.invalidate()
self.count = 0
self.timer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true) {
[weak self] _ in
guard let self = self else { return }
self.count += 1
}
}
}
We will need to pass an instance of this class down through the environment, so we modify the scene delegate:
window.rootViewController =
UIHostingController(rootView: ContentView())
becomes
window.rootViewController =
UIHostingController(rootView: ContentView()
.environmentObject(TimerHolder()))
Finally, lets put up some UI that starts the timer and displays the count, to prove that it is working:
struct ContentView : View {
#EnvironmentObject var timerHolder : TimerHolder
var body: some View {
VStack {
Button("Start Timer") { self.timerHolder.start() }
Text(String(self.timerHolder.count))
}
}
}
EDIT Update for those who have not been following the plot: BindableObject has migrated into ObservableObject, and there is no need any more for manual signalling. So:
class TimerHolder : ObservableObject {
var timer : Timer!
#Published var count = 0
// ... and the rest is as before ...

Answer 2

Based on matt's answer above, I think there is a less complicated version possible.
class TimerHolder : ObservableObject {
var timer : Timer!
#Published var count = 0
func start() {
self.timer?.invalidate()
self.count = 0
self.timer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true) {
_ in
self.count += 1
print(self.count)
}
}
}
With : ObservableObjectand #Publish var count
and in the View
#ObservedObject var durationTimer = TimerHolder()
...
Text("\(durationTimer.count) Seconds").onAppear {
self.durationTimer.start()
}
you can observe any change to the count variable and update the Text in the view accordingly.
As mentioned in other comments, make sure though, that the timer is not restarted unexpectedly due to an action recreating the view.