How to chain functions with a completion handler in Swift?

Question

I am chaining some functions together and I can't figure out how to call a completion handler with a return value once all the functions are done running.

func getAirQuality(completion: (aqi: Int?) -> Void) {
    callAPI()
}

private func callAPI() {
    // ... get data
    self.parseDataForAQI(data: data)
}

private func parseDataForAQI(data: Data) {
    let aqi = aqi
    // Send aqi up to completion handler in getAirQuality
}

So that when everything is said and done I can just do something like this:

getAirQuality(completion: { aqi -> Void in {
    // Do something with aqi
})

Answer 1

My first assumption is that your first 3 functions are part of a class. If so, one approach is to save the completion handler as an instance variable.

class AirQualityProvider {
    var aBlock: ((Int?) -> Void)?

    func getAirQuality(completion: @escaping (Int?) -> Void) {
        aBlock = completion
        callAPI()
    }

    private func callAPI() {
        let data = Data()
        parseDataForAQI(data: data)
    }

    private func parseDataForAQI(data: Data) {
        let aqi = 1
        if let completion = aBlock {
            completion(aqi)
        }
    }
}

Here's an example of a caller as written in a playground.

let aqp = AirQualityProvider()
aqp.getAirQuality { (value) in
    if let value = value {
        print("Value = \(value)")
    }
}