Pausing a Notification publisher in SwiftUI - ios

Question

When I call a backend service (login, value check…) I use a Notification publisher on the concerned Views to manage the update asynchronously.
I want to unsubscribe to the notifications when the view disappear, or « pause » the publisher.
I went first with the simple « assign » option from the WWDC19 Combine and related SwiftUI talks, then I looked at this great post and the onReceive modifier. However the view keeps updating with the published value even when the view is not visible.
My questions are:
Can I « pause » this publisher when the view is not visible ?
Should I really be concerned by this, does it affect resources (the backend update could trigger a big refresh on list and images display) or should I just let SwiftUI manage under the hood ?
The sample code:
Option 1: onReceive
struct ContentView: View {
#State var info:String = "???"
let provider = DataProvider() // Local for demo purpose, use another pattern
let publisher = NotificationCenter.default.publisher(for: DataProvider.updated)
.map { notification in
return notification.userInfo?["data"] as! String
}
.receive(on: RunLoop.main)
var body: some View {
TabView {
VStack {
Text("Info: \(info)")
Button(action: {
self.provider.startNotifications()
}) {
Text("Start notifications")
}
}
.onReceive(publisher) { (payload) in
self.info = payload
}
.tabItem {
Image(systemName: "1.circle")
Text("Notifications")
}
VStack {
Text("AnotherView")
}
.tabItem {
Image(systemName: "2.circle")
Text("Nothing")
}
}
}
}
Option 2: onAppear / onDisappear
struct ContentView: View {
#State var info:String = "???"
let provider = DataProvider() // Local for demo purpose, use another pattern
#State var cancel: AnyCancellable? = nil
var body: some View {
TabView {
VStack {
Text("Info: \(info)")
Button(action: {
self.provider.startNotifications()
}) {
Text("Start notifications")
}
}
.onAppear(perform: subscribeToNotifications)
.onDisappear(perform: unsubscribeToNotifications)
.tabItem {
Image(systemName: "1.circle")
Text("Notifications")
}
VStack {
Text("AnotherView")
}
.tabItem {
Image(systemName: "2.circle")
Text("Nothing")
}
}
}
private func subscribeToNotifications() {
// publisher to emit events when the default NotificationCenter broadcasts the notification
let publisher = NotificationCenter.default.publisher(for: DataProvider.updated)
.map { notification in
return notification.userInfo?["data"] as! String
}
.receive(on: RunLoop.main)
// keep reference to Cancellable, and assign String value to property
cancel = publisher.assign(to: \.info, on: self)
}
private func unsubscribeToNotifications() {
guard cancel != nil else {
return
}
cancel?.cancel()
}
}
For this test, I use a dummy service:
class DataProvider {
static let updated = Notification.Name("Updated")
var payload = "nothing"
private var running = true
func fetchSomeData() {
payload = Date().description
print("DEBUG new payload : \(payload)")
let dictionary = ["data":payload] // key 'data' provides payload
NotificationCenter.default.post(name: DataProvider.updated, object: self, userInfo: dictionary)
}
func startNotifications() {
running = true
runNotification()
}
private func runNotification() {
if self.running {
self.fetchSomeData()
let soon = DispatchTime.now().advanced(by: DispatchTimeInterval.seconds(3))
DispatchQueue.main.asyncAfter(deadline: soon) {
self.runNotification()
}
} else {
print("DEBUG runNotification will no longer run")
}
}
func stopNotifications() {
running = false
}
}

Answer 1

It seems that there are two publishers name let publisher in your program. Please remove one set of them. Also self.info = payload and publisher.assign(to: \.info, on: self)} are duplicating.
}
.onAppear(perform: subscribeToNotifications)
.onDisappear(perform: unsubscribeToNotifications)
.onReceive(publisher) { (payload) in
// self.info = payload
print(payload)
}
.tabItem {
In the following:
#State var cancel: AnyCancellable? = nil
private func subscribeToNotifications() {
// publisher to emit events when the default NotificationCenter broadcasts the notification
// let publisher = NotificationCenter.default.publisher(for: DataProvider.updated)
// .map { notification in
// return notification.userInfo?["data"] as! String
// }
// .receive(on: RunLoop.main)
// keep reference to Cancellable, and assign String value to property
if cancel == nil{
cancel = publisher.assign(to: \.info, on: self)}
}
private func unsubscribeToNotifications() {
guard cancel != nil else {
return
}
cancel?.cancel()
}
Now you can see, cancel?.cancel() does work and the info label no longer update after you come back from tab2. ~~~Publisher pause Here because subscription has been cancelled.~~~
Publisher is not paused as there is another subscriber in the view , so the print(payload) still works.