How do I return data from URLSession dataTask ?
Why do I get nil when I return the data from URLSession dataTask? I confirm that the json is retrieved!
Why do I get error unexpectedly found nil while unwrapping an Optional value when I return the data?
Wtf is It is expecting non-void return statement in void function ?!
Previously I have written a tutorial for using URLSession , then I surveyed around in Reddit / StackOverflow and found quite some people stuck on how to return the data / value retrived from the web API using URLSession.
This post will focus on the context of using URLSession dataTask, but it can also be applied to other function that has completionHandler. Click here to jump straight to the answer if you want to skip explanation of questions listed above.
Lets say you have a function to get user data like this :
override func viewDidLoad() {
super.viewDidLoad()
// Get user with ID 2
let user = fetchUser(userID: 2)
print("user first name is : \(user?.firstName)")
nameLabel.text = user?.firstName
}
func fetchUser(userID: Int) -> User? {
let url = URL(string: "https://reqres.in/api/users/\(userID)")!
let task = URLSession.shared.dataTask(with: url, completionHandler: { data, response, error in
guard let data = data else { return }
do {
// parse json data and return it
let decoder = JSONDecoder()
let jsonDict = try decoder.decode([String: User].self, from: data)
if let userData = jsonDict["data"] {
return userData
}
} catch let parseErr {
print("JSON Parsing Error", parseErr)
}
})
task.resume()
}
The code above won't compile as you will get error mesage like this :
"Wait.. I already mentioned to return User type in the fetchUser function, why does it tell me that Unexpected non-void return? I should return void?"
The problem is that the return userData is inside the completionHandler closure. Remember completionHandler parameter accept a closure (function as variable)? If the return is inside the completionHandler, it will return value for the function of completionHandler, not the outer fetchUser function.
At below, we can see that completionHandler expects a function that accepts Data?, URLResponse?, Error? type as parameters and return void.
If you return a value other than void in the function you passed to completionHandler, it doesn't match the type of completionHandler, hence compiler complains.
"Ahh! I should move the return userData to outside of the completionHandler?"
Lets move the return statement to outside of the completionHandler like this :
func fetchUser(userID: Int) -> User? {
var user : User?
let url = URL(string: "https://reqres.in/api/users/\(userID)")!
let task = URLSession.shared.dataTask(with: url, completionHandler: { data, response, error in
guard let data = data else { return }
do {
// parse json data and return it
let decoder = JSONDecoder()
let jsonDict = try decoder.decode([String: User].self, from: data)
if let userData = jsonDict["data"] {
// save the userData to the outer variable user
user = userData
}
} catch let parseErr {
print("JSON Parsing Error", parseErr)
}
})
task.resume()
// return user outside the completion handler
return user
}
The code compiles but the user firstName is nil and further investigation shows that the user returned is nil, wait what?
This is because task.resume() is asynchronous , the line below task.resume() will be immediately executed without having to wait for the HTTP response to arrive (ie. before executing the code inside completionHandler). The code execution flow will look like this :
Notice that return user is executed before user = userData is executed, hence you will get nil from this function.
If you have used something like user! somewhere in the code , you will get the error unexpectedly found nil while unwrapping an Optional value because user is nil and you force unwrap it with !.
"Huh.. since I can't return the user data inside nor outside of the completionHandler, how should I access and use the user data after URLSession.shared.dataTask has retrieved data?"
One of the solutions is to add a closure parameter to the outer fetchUser() function, which we will explain further below.
Use another closure to use the data received in the completionHandler
Since task.resume() is asynchronous, we can't write code in a sequential way to return the user data. We will add a closure parameter userCompletionHandler to the fetchUser function and also remove the return User -> User? to make it a void function :
// add userCompletionHandler and remove ' -> User?' to make it a void function
func fetchUser(userID: Int, userCompletionHandler: @escaping (User?, Error?) -> Void) {
let url = URL(string: "https://reqres.in/api/users/\(userID)")!
let task = URLSession.shared.dataTask(with: url, completionHandler: { data, response, error in
guard let data = data else { return }
do {
// parse json data and return it
let decoder = JSONDecoder()
let jsonDict = try decoder.decode([String: User].self, from: data)
if let userData = jsonDict["data"] {
userCompletionHandler(userData, nil)
}
} catch let parseErr {
print("JSON Parsing Error", parseErr)
userCompletionHandler(nil, parseErr)
}
})
task.resume()
// function will end here and return
// then after receiving HTTP response, the completionHandler will be called
}
Notice that we have added @escaping for the userCompletionHandler parameter, this is because the fetchUser function will finish execute and return before the userCompletionHandler() function is being called. (ie. execute the last line task.resume() and reaches the end of the function before the userCompletionHandler() is being executed). Still confused? Try looking at the code execution flow image above again, imagine there is an invisible return after the last line task.resume(). Read more about escaping closure on Apple documentation here.
We will also update viewDidLoad() to use the new fetchUser() method :
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
fetchUser(userID: 2, userCompletionHandler: { user, error in
if let user = user {
print("user first name is : \(user.firstName!)")
self.nameLabel.text = user.firstName
}
})
}
Then the flow of code execution will be like this :
(I thought of explaning it using words but figured out using picture with arrow diagram will be much faster and clearer, phew)
userCompletionHandler function will be called inside the completionHandler of URLSession.shared.dataTask(). You can pass the user data or relevant error to userCompletionHandler(). Then you can access these data in viewDidLoad() by calling fetchUser()!
Whenever you want to get and use user data from Web API, just call fetchUser() and use its userCompletionHandler : { user, error in }.
Hope the picture above also gave you an idea on how completion handler works đ.
