How to allow timer to count down in background on Watch OS - swift

Question

Made a pretty simple timer app, but I've found that Apple Watch documentation is no where near as good as the phone. It's been difficult to find any answers on this.
Here is my content view. The seconds for the timer is kept in '''timeVal'''
struct ContentView: View {
#State var timeVal = 0.0
#State var timerScreenShow:Bool = false
var body: some View {
VStack {
HStack{
Spacer()
Text("\(self.timeVal.hourMinute)")
Spacer()
}
//various buttons to set timer
NavigationLink(
destination: TimerView(timerScreenShow: self.$timerScreenShow, timeVal: Double(Int(self.timeVal)), initialTime: Int(self.timeVal)),
isActive: $timerScreenShow,
label: {
Text("Start")
}).background(Color.green).cornerRadius(22)
}
}
}
Then, here is the bit that keeps time:
struct TimerView: View {
#Binding var timerScreenShow:Bool
#State var timeVal:Double
let initialTime:Int
var body: some View {
if timeVal > -1 {
VStack {
ZStack {
Text("\(self.timeVal.hourMinuteSecond)").font(.system(size: 20))
.onAppear() {
Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { _ in
if self.timeVal > -1 {
self.timeVal -= 1
}
}
}
//Text("\(self.timeVal.hourMinuteSecond)")
ProgressBar(progress: Int(self.timeVal), initial: self.initialTime).frame(width: 90.0, height: 90.0)
}
Button(action: {
self.timerScreenShow = false
}, label: {
Text("Cancel")
.foregroundColor(Color.red)
})
.padding(.top)
}
} else {
Button(action: {
self.timerScreenShow = false
}, label: {
Text("Done!")
.foregroundColor(Color.green)
}).onAppear() {
WKInterfaceDevice.current().play(.notification)
}
}
}
}
What do I need to do to keep the timer running when the watch app is closed?

Answer 1

Don't keep track of how much time is left on the counter. Instead, store the startTime = Date(). Use the timer only as a signal to update the UI. When the timer ticks, compute the time left as startAmount - (Date().timeInterval(since: startTime)). Store startAmount and startTime and Bool isTimerRunning in persistent storage such as #AppStorage. When the app restarts, pick up where you left off.