Swift: Escaping closure captures non-escaping parameter 'onCompletion'

You have to mark both completion handlers with @escaping. Usually the compiler offers a fix

class RestApiManager: NSObject {    static let sharedInstance = RestApiManager()    let baseURL = "http://api.randomuser.me/"    func getRandomUser(onCompletion : @escaping (JSON) -> Void) {        makeHTTPGetRequest(path: baseURL, onCompletion: { json, err -> Void in            onCompletion(json)        })    }    func makeHTTPGetRequest(path: String, onCompletion: @escaping ServiceResponse) {        let request = NSMutableURLRequest(url : URL(string: path)! as URL)        let session = URLSession.shared        let task = session.dataTask(with: request as URLRequest, completionHandler: { data, response, error in            let json:JSON = JSON(data as Any)            onCompletion(json, error as NSError?)        })        task.resume()    }}

Answers here were right on adding @escaping before the completion handler parameter declaration, albeit shortly explained.

Here's what I was missing for the full picture, taken from Swift's documentation:

Escaping Closures

A closure is said to escape a function when theclosure is passed as an argument to the function, but is called afterthe function returns. When you declare a function that takes a closureas one of its parameters, you can write @escaping before theparameter’s type to indicate that the closure is allowed to escape.

So basically, if you want a completion handler to be called AFTER the method returns, it is defined as escaping in swift, and should be declared as such:

func makeHTTPGetRequest(path: String, onCompletion: @escaping ServiceResponse)

Use this:

class RestApiManager: NSObject {static let sharedInstance = RestApiManager()let baseURL = "http://api.randomuser.me/"func getRandomUser(onCompletion : @escaping (JSON) -> Void) {    makeHTTPGetRequest(path: baseURL, onCompletion: { json, err -> Void in        onCompletion(json)    })}func makeHTTPGetRequest(path: String, onCompletion: @escaping ServiceResponse) {    let request = NSMutableURLRequest(url : URL(string: path)! as URL)    let session = URLSession.shared    let task = session.dataTask(with: request as URLRequest, completionHandler: { data, response, error in        let json:JSON = JSON(data as Any)        onCompletion(json, error as NSError?)    })    task.resume()}}