Using Combine to trigger download on app activation not compiling - swift

Question

I am trying to trigger a download when my app becomes active, but I can't figure out how to do it.
This is my code
NotificationCenter.default.publisher(for: UIApplication.didBecomeActiveNotification).map{ _ in
return self.urlSession.dataTaskPublisher(for: url)
}
.tryMap{ element in
guard let httpResponse = element.response as? HTTPURLResponse,
httpResponse.statusCode == 200 else {
throw URLError(.badServerResponse)
}
.....
I have no idea how to get this to work. I constantly get a compile error stating:
Value of type 'URLSession.DataTaskPublisher' has no member 'response'
Why does my return statement return the complete publisher and not its result?
If I remove the NotificationCenter part and call tryMap directly on dataTaskPublisher like this:
self.urlSession.dataTaskPublisher(for: url)
.tryMap{ element in
the compiler does not complain.
Pretty confused here :)

Answer 1

Why does my return statement return the complete publisher and not its result?
It returns the complete publisher because, well, you told it to return the complete publisher. A map's Output is whatever the map transform returns. Your map transform returns a DataTaskPublisher, so the Output of the map is DataTaskPublisher. Therefore the Input to tryMap—the type of the element variable—is a DataTaskPublisher. Roughly speaking, the type after map (before tryMap) is
Publisher<Publisher<(data: Data, response: URLResponse), URLError>, Never>
(This is not a real type because Publisher is not a real type constructor.)
What you want is to flatten that out into
Publisher<(data: Data, response: URLResponse), Error>
You'll need to use setFailureType(to:) to convert the failure type of the notification publisher from Never to Error. And you'll need to use mapError to convert the failure type of the DataTaskPublisher from URLError to Error.
Once you've got the failure types matched up, there are several operators that can flatten nested publishers. The one you're looking for is probably switchToLatest:
func data(for url: URL) -> AnyPublisher<Data, Error> {
NotificationCenter.default.publisher(for: UIApplication.didBecomeActiveNotification)
.setFailureType(to: Error.self)
.map({ _ in
self.urlSession.dataTaskPublisher(for: url)
.mapError { $0 as Error }
})
.switchToLatest()
.tryMap({ element in
guard let httpResponse = element.response as? HTTPURLResponse,
httpResponse.statusCode == 200 else {
throw URLError(.badServerResponse)
}
return element.data
})
.eraseToAnyPublisher()
}