Promise chaining base on condition

[zhouhao27]

In most case, there is only one error catch and chain for the successful operation. But for my case, what I want to do is if the operation success, then I'll return. If it's failed, I'll do another operation. Is it possible and how to express it in PromiseKit?

For example, how to convert this pseudo code to PromiseKit way?

    fetchData1(fromURL: "someURL") { (response, results, error) in
            switch response {
            case .success:
                if !data.isEmpty() {
                   // complete
                } else {
                    fetchData2(for: results) { (response, results, error) in
                        switch response {
                        case .success:
                            // complete
                        }
                    case .fail:
                            // complete with error
                    }
             }
            case .fail:
                    fetchData2(for: results) { (response, results, error) in
                        switch response {
                        case .success:
                            // complete
                        }
                    case .fail:
                            // complete with error
                    }        
            }
        }

[GarthSnyder]

This is a common pattern.

fetchData1 and fetchData2 should follow the general guidelines for creating promise-based APIs: they should be methods that return a promise that either yields a value or errors out. Let the caller handle dealing with the errors.

Something roughly like:

func fetchData1(fromURL: "someURL") -> Promise<MyData> { 
    return initialDataFetch(for: results)
        .get { data in
            if data.isEmpty() {
                throw MyFetch.badInitialData
            }
        }.recover { fetchData2(for: results) }
}

Here, initialDataFetch can be a separate method or just a firstly block.