如何获取在闭包中分配的变量的值（Swift）

Question

I'm using the Twitter REST API in Swift, and I am trying to get the value of a variable that is assigned inside of a Twitter Request closure, so that I can use that value outside of the closure.

I acquired this code from the Twitter REST API tutorial for Swift, located at: https://dev.twitter.com/twitterkit/ios/access-rest-api

func jsonAvailable() -> Bool {
    // Swift
    let client = TWTRAPIClient()
    let statusesShowEndpoint = "https://api.twitter.com/1.1/statuses/show.json"
    let params = ["id": "20"]
    var clientError : NSError?

    var jsonAvailable: Bool = false
    let request = client.urlRequest(withMethod: "GET", url: 
    statusesShowEndpoint, parameters: params, error: &clientError)

    client.sendTwitterRequest(request) { (response, data, connectionError)-> Void in
    if connectionError != nil {
        print("Error: \(connectionError)")
    }

    do {
        let json = try JSONSerialization.jsonObject(with: data!, options: [])
        print("json: \(json)")
        jsonAvailable = true
    } catch let jsonError as NSError {
        print("json error: \(jsonError.localizedDescription)")
    }

    print("Value of jsonAvailable: \(jsonAvailable)")
    return jsonAvailable
    //always returns false, even if it is changed to true inside of the closure
}

In the last line, jsonAvailable is always false, even when it is changed to true inside of the closure. How can I get the value of jsonAvailable at the end of the function, even as it is modified inside of the sendTwitterRequest closure?

I have tried writing this closure in a separate function and then calling the function to get the value, but because it is a custom closure that requires the client to be called by "sendTwitterRequest" I have found it difficult to pass all these required parameters to fit the API.

Thanks for the help!

Answer 1

Your closure is async. What happens is that you go through all the function body before sendTwitterRequest assigns true to jsonAvailable , resulting in jsonAvailable being false. What you need to do is having a callback instead, providing the json status if you'd like (or the json itself as a nillable object).

EDIT : You could have something like this

func jsonAvailable(callback: ((_ isJsonAvailable: Bool) -> Void)) {
    client.sendTwitterRequest(request) { (response, data, connectionError)-> Void in {
        do {
            let json = try JSONSerialization.jsonObject(with: data!, options: [])
            print("json: \(json)")
            callback(true)
        } catch let jsonError as NSError {
            print("json error: \(jsonError.localizedDescription)")
            callback(false)
        }
    }
}

jsonAvailable(callback: { (_ isJsonAvailable: Bool) in
    print(isJsonAvailable)
})