Observe singleton timer value changes with Publisher in Combine - swift

Question

One of the requirements of my application is the ability to start multiple timers, for reporting purposes.
I've tried to store the timers and seconds passed in an #EnvironmentObject with #Published variables, but every time the object refreshes, any view that observes the #EnvironmentObject refreshes too.
Example
class TimerManager: ObservableObject {
#Published var secondsPassed: [String: Int]
var timers: [String:AnyCancellable]
func startTimer(itemId: String) {
self.secondsPassed[itemId] = 0
self.timers[itemId] = Timer
.publish(every: 1, on: .main, in: .default)
.autoconnect()
.sink(receiveValue: { _ in
self.secondsPassed[itemId]! += 1
})
}
func isTimerValid(itemId: String) -> Bool {
return self.timers[itemId].isTimerValid
}
// other code...
}
So for example, if in any other view I need to know if a particular timer is active by calling a function isTimerValid, I need to include this #EnvironmentObject in that view, and it won't stop refreshing it because the timer changes secondsPassed which is Published, causing lags and useless redrawings.
So one thing I did was to cache the itemId of the active timers somewhere else, in a static struct that I update every time I start or stop a timer.
It seemed a bit hacky, so lately I've been thinking to move all this into a Singleton, like this for example
class SingletonTimerManager {
static let singletonTimerManager = SingletonTimerManager()
var secondsPassed: [String: Int]
var timers: [String:AnyCancellable]
func startTimer(itemId: String) {
self.secondsPassed[itemId] = 0
self.timers[itemId] = Timer
.publish(every: 1, on: .main, in: .default)
.autoconnect()
.sink(receiveValue: { _ in
self.secondsPassed[itemId]! += 1
})
}
// other code...
}
and only let some Views observe the changes to secondsPassed. On the plus side, I can maybe move the timer on the background thread.
I've been struggling how to do this properly.
These are my Views (albeit a very simple extract)
struct ContentView: View {
// set outside the ContentView
var selectedItemId: String
// timerValue: set by a publisher?
var body: some View {
VStack {
ItemView(seconds: Binding.constant(timerValue))
}
}
}
struct ItemView: View {
#Binding var seconds: Int
var body: some View {
Text("\(self.seconds)")
}
}
I need to somehow observe the SingletonChronoManager.secondsPassed[selectedItemId] so the ItemView updates in real time.

Answer 1

By putting the timer publisher results into Environment, you are propagating change notifications to all views within the tree that define that environment object, which I'm sure will cause un-needed redraws and performance issues (and as you've seen).
A better mechanism is strongly limiting the views (or subviews) that need to display the constantly updating time, and pass in a reference to a timer publisher directly to them rather than layering it into the environment. Putting the timer itself into a singleton is one option but not critical to this, and won't effect the cascading redraws you're seeing.
How to use a timer with SwiftUI has a "shoving a timer into the view itself", which may work for what you're trying to do, but slightly better is the video here: https://www.hackingwithswift.com/books/ios-swiftui/triggering-events-repeatedly-using-a-timer
In Paul's example, he's stuffing the timer into the view itself - wouldn't be my choice, but for a simple real-time clock view it's not bad. You could just as easily pass in the timer publisher from an external object - like your singleton for example.

Answer 2

I've ended up using the following solution, combining #heckj suggestion and this one from #Mykel.
What I did was separating the AnyCancellable from the TimerPublishers by saving them in specific dictionaries of SingletonTimerManager.
Then, every time an ItemView is declared, I instantiate an autoconnected #State TimerPublisher. Every Timer instance now runs in the .common RunLoop, with a 0.5 tolerance to better help the perfomance as suggested by Paul here: Triggering events repeatedly using a timer
During the .onAppear() call of the ItemView, if a publisher with the same itemId already exists in SingletonTimerManager, I just assign that publisher to the one of my view.
Then I handle it like in #Mykel solution, with start and stopping both ItemView's publisher and SingletonTimerManager publisher.
The secondsPassed are shown in a text stored inside #State var seconds, which gets updated with a onReceive() attached to the ItemView's publisher.
I know that I'm probably creating too many publishers with this solution and I can't pinpoint exactly what happens when copying a publisher variable into another, but overall perfomance is much better now.
Sample Code:
SingletonTimerManager
class SingletonTimerManager {
static let singletonTimerManager = SingletonTimerManager()
var secondsPassed: [String: Int]
var cancellables: [String:AnyCancellable]
var publishers: [String: TimerPublisher]
func startTimer(itemId: String) {
self.secondsPassed[itemId] = 0
self.publisher[itemId] = Timer
.publish(every: 1, tolerance: 0.5, on: .main, in: .common)
self.cancellables[itemId] = self.publisher[itemId]!.autoconnect().sink(receiveValue: {_ in self.secondsPassed[itemId] += 1})
}
func isTimerValid(_ itemId: String) -> Bool {
if(self.cancellables[itemId] != nil && self.publishers[itemId] != nil) {
return true
}
return false
}
}
ContentView
struct ContentView: View {
var itemIds: [String]
var body: some View {
VStack {
ForEach(self.itemIds, id: \.self) { itemId in
ItemView(itemId: itemId)
}
}
}
}
struct ItemView: View {
var itemId: String
#State var seconds: Int
#State var timerPublisher = Timer.publish(every: 1, tolerance: 0.5, on: .main, in: .common).autoconnect()
var body: some View {
VStack {
Button("StartTimer") {
// Call startTimer in SingletonTimerManager....
self.timerPublisher = SingletonTimerManager.publishers[itemId]!
self.timerPublisher.connect()
}
Button("StopTimer") {
self.timerPublisher.connect().cancel()
// Call stopTimer in SingletonTimerManager....
}
Text("\(self.seconds)")
.onAppear {
// function that checks if the timer with this itemId is running
if(SingletonTimerManager.isTimerValid(itemId)) {
self.timerPublisher = SingletonTimerManager.publishers[itemId]!
self.timerPublisher.connect()
}
}.onReceive($timerPublisher) { _ in
self.seconds = SingletonTimerManager.secondsPassed[itemId] ?? 0
}
}
}
}