Swift combine not running properly in iOS app

Question

So I'm very new to swift so I might be getting this all wrong.

I'm learning to work with combine so I used the playground to log at an interval like so

var cancellable:AnyCancellable? = Timer.publish(every: 1, on: .main, in: .default) .autoconnect() .sink { print($0) } PlaygroundPage.current.needsIndefiniteExecution = true

It's very simple and it works like I expect it to, printing a log every second.

Now when I create a new iOS project in Xcode it doesn't seem to work and I can't figure out why. From a blank project, I simply added this in the ContentView

struct ContentView: View { func test() { var cancellable: AnyCancellable? cancellable = Timer.publish(every: 1, on: .main, in: .default) .autoconnect() .sink { print($0) } } var body: some View { VStack { Text("Hello, world!") .padding() } } init() { test() } }

All of a sudden, it doesn't work and the log won't print. I'm sure it's something dumb that I'm not understanding but I've been looking at this forever now. My real case is with a network call that is not running so I tried simplifying as much as I can and this is as simple as I could get.

Any help appreciated!

Thanks!

jnpdx · Accepted Answer · 2021-02-10 01:02:05Z

There's a couple of things going on here:

Your cancellable is immediately going out of scope, so it's not being stored. Change it to an @State property on your View instead.
SwiftUI is really picky about what can/can't happen inside init() of Views. Two options to deal with this: a) Move it to onAppear b) Move it to the init() of an @ObservableObject

Working example:

struct ContentView: View { @State var cancellable : AnyCancellable? func test() { cancellable = Timer.publish(every: 1, on: .main, in: .default) .autoconnect() .sink { print($0) } } var body: some View { VStack { Text("Hello, world!") .padding() }.onAppear { test() } } }

ObservableObject version:

class ViewModel : ObservableObject { private var cancellable : AnyCancellable? init() { cancellable = Timer.publish(every: 1, on: .main, in: .default) .autoconnect() .sink { print($0) } } } struct ContentView: View { var vm = ViewModel() var body: some View { VStack { Text("Hello, world!") .padding() } } }

Steven0351 · Accepted Answer · 2021-02-10 00:47:56Z

0

You’re not calling the function. You need to set the cancellable as a property on your view in this scenario.

answered Feb 10, 2021 at 0:47

Steven0351

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2 Comments

almostalx Over a year ago

I'm calling the function in init() {test()}

Steven0351 Over a year ago

I missed that part. You still need to retain the cancellable. It gets deinitialized immediately because you’re not assigning it to anything outside the scope of the function (e.g., setting it as a property on your view)

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