Combine using optional struct variable - swift

Question

I'm trying to load an array of SCEvents into an array of EventModels using Combine. The variable imagePath is optional, which I'd like to translate it to an empty Data() in its corresponding EventModel.imageData variable.
struct SCEvent {
let name: String
let imagePath: String?
}
struct EventModel {
let name: String
let imageData: Data
}
The following code seems to work, but I can't help but wonder if it is the most optimal way of doing it:
func loadEvents(_ events: [SCEvent]) -> AnyPublisher<[EventModel], Error> {
events.publisher
.flatMap(loadEvent(_:))
.collect()
.eraseToAnyPublisher()
}
func loadEvent(_ event: SCEvent) -> AnyPublisher<EventModel, Error> {
if let imagePath = event.imagePath {
return DataDownloader.downloadData(fromPath: imagePath)
.map { EventModel(name: event.name, imageData: $0) }
.eraseToAnyPublisher()
}
return Just(EventModel(name: event.name, imageData: Data())
.eraseToAnyPublisher()
}
Ideally, I'd like to use a single publisher in the loadEvent function. Maybe something like this (doesn't work, but serves as an example of what I expect):
func loadEvent(_ event: SCEvent) -> AnyPublisher<[EventModel], Error> {
event.imagePath
.flatMap(DataDownloader.downloadData(_:))
.replaceNil(with: Data())
.map {
EventModel(name: event.name, imageData: $0)
}
.eraseToAnyPublisher()
}
Which doesn't work because .replaceNil should be used after event.imagePath to replace a nil string. Another possible approach would be:
func loadEvent(_ event: SCEvent) -> AnyPublisher<[EventModel], Error> {
event.imagePath
.replaceNil(with: "")
.flatMap(
DataDownloader.downloadData(_:)
.replaceError(with: Data())
)
.map {
EventModel(name: event.name, imageData: $0)
}
.eraseToAnyPublisher()
}
But it seems forced. Is it even possible with Combine? Is my initial approach the only valid solution?

Answer 1

You can use the publisher of Optional, which gives you a publisher that publishes one element if the optional is not nil, and an empty publisher otherwise.
You can then replaceEmpty(with: Data()), and map to an EventModel.
func loadEvent(_ event: SCEvent) -> AnyPublisher<EventModel, Error> {
event.imagePath.publisher
.flatMap(DataDownloader.downloadData(fromPath:))
.replaceEmpty(with: Data())
.map {
EventModel(name: event.name, imageData: $0)
}
.eraseToAnyPublisher()
}
However, I don't think replacing with a Data() is a good idea. A better design would be to replace with nil, in which case you'll have to map to an optional first:
struct EventModel {
let name: String
let imageData: Data?
}
...
.flatMap(DataDownloader.downloadData(fromPath:))
.map { $0 as Data? } // here
.replaceEmpty(with: nil)