How to handle waiting for a nested URLSession to complete - swift

Question

I have a function that provides a layer over URLSession. When this function is called I would like to check if the current access token as expired, if it has, I would to pause the current call, make a call to request a new token, replace the existing entry in the Keychain, then continue with the call.
func profile(with endpoint: ProfilesEndpoint, method: HTTPMethod, body: String?, headers: [String: String]?, useAuthToken: Bool = true, completion: #escaping (Either<ProfileResponse>) -> Void) {
var request = endpoint.request
request.httpMethod = method.rawValue
if let body = body {
request.httpBody = body.data(using: .utf8)
}
if useAuthToken {
if !AuthService.shared.isTokenValid {
let group = DispatchGroup()
group.enter()
OAuthService.shared.requestRefreshToken()
group.leave()
}
let (header, token) = AuthService.shared.createAuthHeaderForNetworkRequest()
request.addValue(token, forHTTPHeaderField: header)
}
if let headers = headers {
for (key, value) in headers {
request.addValue(value, forHTTPHeaderField: key)
}
}
execute(with: request, completion: completion)
}
A mechanism existing for handling the Keychain so please assume this is in place.
The function to request a new token looks like
func requestRefreshToken() -> Void {
if let refreshToken = KeychainWrapper.standard.string(forKey: "RefreshToken") {
var postBody = "grant_type=\(refreshTokenGrantType)&"
postBody += "client_id=\(clientId)&"
postBody += "refresh_token=\(refreshToken)&"
let additionalHeaders = [
"Content-Type": "application/x-www-form-urlencoded;"
]
APIClient.shared.identity(with: .token, method: .post, body: postBody, headers: additionalHeaders, useAuthToken: false) { either in
switch either {
case .success(let results):
guard let accessToken = results.accessToken, let refreshToken = results.refreshToken else { return }
AuthService.shared.addTokensToKeyChain(tokens: ["AccessToken": accessToken, "RefreshToken": refreshToken])
case .error(let error):
print("Error:", error)
}
}
}
}
I was expecting the executing to pause here
group.enter()
OAuthService.shared.requestRefreshToken()
group.leave()
However it does not.
How I can await this call to complete before completing the rest of the function?

Answer 1

Add to your requestRefreshToken method completion handler which will get executed when your request for token is completed
func requestRefreshToken(_ completion: #escaping () -> Void) {
if let refreshToken = KeychainWrapper.standard.string(forKey: "RefreshToken") {
var postBody = "grant_type=\(refreshTokenGrantType)&"
postBody += "client_id=\(clientId)&"
postBody += "refresh_token=\(refreshToken)&"
let additionalHeaders = [
"Content-Type": "application/x-www-form-urlencoded;"
]
APIClient.shared.identity(with: .token, method: .post, body: postBody, headers: additionalHeaders, useAuthToken: false) { either in
switch either {
case .success(let results):
guard let accessToken = results.accessToken, let refreshToken = results.refreshToken else {
completion()
return
}
AuthService.shared.addTokensToKeyChain(tokens: ["AccessToken": accessToken, "RefreshToken": refreshToken])
case .error(let error):
print("Error:", error)
}
completion()
}
}
}
then leave dispatchGroup in closure and also add group.wait() (after calling request method) for pausing current thread until group's task has completed
group.enter()
OAuthService.shared.requestRefreshToken {
group.leave()
}
group.wait()
Note: you can add boolean parameter to completion to check if request for token was successful or not

Answer 2

Why not do this underneath instead of on top? This seems like an ideal use for NSURLProtocol. Basically:
The URL protocol snags creation of the request in its init method and saves off all the provided parameters.
The URL protocol allocates a private NSURLSession instance and stores it in a property. That session should not be configured to use the protocol, or else you'll get an infinite loop.
When the protocol gets a startLoading() call, it checks the validity of the token, then:
Fires off a request for a new token if needed, in that private session.
Upon response, or if the token is still valid, fires off the real request — again, in that private session.
With that approach, the entire authentication process becomes basically transparent to the app except for the need to add the protocol into protocolClasses on the session configuration when creating a new session.
(There are a number of websites, including developer.apple.com, that provide examples of custom NSURLProtocol subclasses; if you decide to go with this approach, you should probably use one of those sample code projects as a starting point.)
Alternatively, if you want to stick with the layer-on-top approach, you need to stop thinking about "stopping" the method execution and start thinking about it as "doing the last part of the method later". It's all about asynchronous thinking.
Basically:
Factor out the last part of the method (the code that performs the actual request) into a new method (or a block).
If the token is valid, call that method immediately.
If the token is invalid, asynchronously fetch the new token.
In the token fetch call's completion callback, call the method to perform the actual request.