Image fetched asynchronously sometimes won't show in snapshots

[spelaez]

Describe the bug
Using an image that is fetched asynchronously might won't show sometimes on the light mode snapshot variant

To Reproduce
This should repro mostly on a first run of the tests
There's an ImageProvider dependency that fetches an image in an async fashion
The test is provided with a version of this that returns an image immediately and the snapshots are captured with a wait of 0.1
SnapshotsBug.zip

Expected behavior
Image should show always in the snapshots

Screenshots

Environment

• swift-snapshot-testing version 1.17.2
• Xcode 15.2
• Swift 5.9
• OS: iOS 17.2

[mbrandonw]

Hi @spelaez, your code is producing purple runtime warnings letting you know that something isn't quite right:

Publishing changes from background threads is not allowed; make sure to publish values from the main thread (via operators like receive(on:)) on model updates.

You can fix this by making the model @MainActor, or at least the task method.

Overall I am seeing pretty deterministic test passes, but I understand it may be different for you. This may just be the reality of snapshot testing, which is not an exact science. We do have plans in the future to make assertSnapshot an async function, which could help with situations like this, but it would be a big overhaul to the library and a 2.0 breaking change.

You could emulate the async style of snapshot testing doing something like this:

let view = ContentView(model: viewModel)
let controller = UIHostingController(rootView: view)
_ = controller.view
try await Task.sleep(for: .seconds(0.03))
assertSnapshot(
  of: controller,
  named: "\(density)"
)

Since I don't feel this is directly an issue with the library I am going to convert it to a discussion. Please feel free to continue the conversation over there, or maybe others have tips they can share to help you out.

[spelaez]

I missed that @MainActor annotation but it is definitely there in my actual production code

Thanks for your input, I will try the Task.sleep approach
can I ask, why did you have to use a controller instead of the view directly? and why the wait strategy is not working for this cases?