Swift DispatchSourceTimer makes unrelated views reload every second - ios

Question

[28/10/2019: I have recreated the same test project with UIKit... unsurprisingly, the timer does not make any of the view controller views reload!!! So this looks like a SwiftUI bug. UIKIt version: https://github.com/DominikButz/DispatchSourceTimerUIKit.git]
[19/10/2019: Please check out https://github.com/DominikButz/DispatchTimerSwiftUIReloadBug.git for better understanding of the issue. When you launch this in the simulator, just tap on "Launch timer". Check the debug console, you will see that all views are reloaded every second.]
Original post:
I'm creating an iOS app in SwiftUI for which I need to use a timer. I'm posting the class below.
The timer works fine (except for not continuing when the app is in the background, but that is another story...). As you can see in the resumeTimer function, the timer is set to repeat every second.
There is a view in which a Text label gets updated every time the timer fires. This works fine. However, the problem is that other views that are loaded after the timer starts all get reloaded every time the timer fires, that means every second. e.g. an unrelated list (where the timer text label is not placed) gets reloaded every second, which makes it unusable.
I know this because when I pause or cancel the timer through the user interface, the debugger shows that the views don't get reloaded every second anymore. I placed a breakpoint (with the option "continue after evaluation") in the body of each of those views and the debugger shows that the views gets reloaded every time the timer fires.
The timer class below is created in the SceneDelegate and then passed to the Content view as environment object. I had it as ObservedObject before only in the view where the timer Text label is placed, but the result is the same. See below a view that gets reloaded when the timer fires... (gif screenshot).
Could this be a bug in SwiftUI? or might there be an error in my timer class? thanks for any hints in the right direction
Here is the content view
struct ContentView: View {
#EnvironmentObject var workoutModel: WorkoutModel
#EnvironmentObject var workoutWatch: WorkoutStopWatchTimer
#State var selectedMenuIndex = UserDefaults.standard.integer(forKey: UserDefaultKeys.selectedMenuIndex)
#State private var workoutViewPresenting = false
func setMenuIndex() {
UserDefaults.standard.set(selectedMenuIndex, forKey: UserDefaultKeys.selectedMenuIndex)
}
var body: some View {
TabView(selection:$selectedMenuIndex) {
WorkoutSelectionView(workoutViewPresenting: self.$workoutViewPresenting).onAppear(perform: self.setMenuIndex)
.tabItem {
Image(systemName: "1.square.fill")
Text("Start Workout")
}.tag(0)
Text("History").onAppear(perform: self.setMenuIndex) // I set a breakpoint here (with continue after evaluation checked)
.tabItem {
Image(systemName: "2.square.fill")
Text("History")
}.tag(1)
ExerciseListView().onAppear(perform: self.setMenuIndex) // I set a breakpoint here (with continue after evaluation checked)
.tabItem {
Image(systemName: "3.square.fill")
Text("Exercises")
}.tag(2)
Text("Profile").onAppear(perform: self.setMenuIndex)
.tabItem {
Image(systemName: "person.fill")
Text("Profile")
}.tag(3)
}.sheet(isPresented: self.$workoutViewPresenting) {
// the timer is passed to this view in which a Text label is updated every second to show the time
WorkoutView().environmentObject(self.workoutModel).environmentObject(self.workoutWatch)
}
}
}
debugger console: see the comments I put in the content view above to see where I placed the breakpoints. I did not open and close those views that many times, they are reloaded automatically every second when the timer is running.
...
3 loading History
3 loading Exercise List View
4 loading History
4 loading Exercise List View
5 loading History
5 loading Exercise List View
6 loading History
6 loading Exercise List View
7 loading History
7 loading Exercise List View
...
class WorkoutStopWatchTimer: ObservableObject {
private var sourceTimer: DispatchSourceTimer?
private let queue = DispatchQueue.init(label: "stopwatch.timer", qos: .background, attributes: [], autoreleaseFrequency: .never, target: nil)
private var counter: Int = 0 // seconds
var endDate: Date?
var duration: TimeInterval?
#Published var timeDisplayString = "0:00"
var paused = false
var isActive: Bool {
return self.sourceTimer != nil
}
func start() {
self.paused = false
guard let _ = self.sourceTimer else {
self.startTimer()
return
}
self.resumeTimer()
}
func finish() {
guard self.sourceTimer != nil else {return}
self.endDate = Date()
self.duration = TimeInterval(exactly: Double(self.counter))
self.sourceTimer?.setEventHandler {}
self.sourceTimer?.cancel()
if self.paused == true {
self.sourceTimer?.resume()
}
self.sourceTimer = nil
self.reset()
}
func pause() {
self.paused = true
self.sourceTimer?.suspend()
}
private func reset() {
self.timeDisplayString = "0:00"
self.counter = 0
}
private func startTimer() {
self.sourceTimer = DispatchSource.makeTimerSource(flags: DispatchSource.TimerFlags.strict,
queue: self.queue)
self.resumeTimer()
}
private func resumeTimer() {
self.sourceTimer?.setEventHandler { [weak self] in
// self.eventHandler = {
self?.updateTimer()
// }
}
self.sourceTimer?.schedule(deadline: .now(),
repeating: 1)
self.sourceTimer?.resume()
}
private func updateTimer() {
self.counter += 1
DispatchQueue.main.async {
self.timeDisplayString = WorkoutStopWatchTimer.convertCountToTimeString(counter: self.counter)
}
}
}