Avoiding Nested Try/Catch Blocks

A common pattern in Swift that’s easy to avoid.

This is a quick one, and a departure from the Navigator series, but I’ve seen this anti-pattern a lot and just thought I’d mention how easy it is to avoid.

When making API calls, it’s a fairly common practice to want to map any errors that might occur into a more finite set of “known” errors that are easier to handle.

An example might be:

public enum NetworkError: Error {
    case data
    case server
    case status(Int)
    case validation(String)
}

Note the “data” error we want to display in case our data is corrupted or in an unknown format.

Nested Try/Catch

With that in mind, take a peek at the following code.

public func load1(url: URL) async throws -> [FeedItem] {
    do {
        let (data, response) = try await URLSession.shared.data(from: url)

        guard let httpResponse = response as? HTTPURLResponse, httpResponse.statusCode == 200 else {
            throw NetworkError.status((response as? HTTPURLResponse)?.statusCode ?? 500)
        }

        do {
            let decoder = JSONDecoder()
            let items = try decoder.decode([FeedItem].self, from: data)
            return items
        } catch {…