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How to Use withCheckedContinuation Function in Swift

Swift, Concurrency, Asynchronous Code1 min read

Swift’s withCheckedContinuation function is a powerful tool that allows developers to manage asynchronous code more efficiently. This function provides a way to create a continuation, which is an object that represents the state of an asynchronous operation. By using a continuation, you can suspend the current execution of your code and resume it later when the operation is complete.

To use withCheckedContinuation, you must first import the Swift Concurrency module into your project. Once you have done that, you can use the function to create a continuation object. Here is the basic syntax:

1func myFunction() async -> String {
2 return await withCheckedContinuation { continuation in
3 // Perform asynchronous operation
4 // When complete, call continuation.resume(returning: result)
5 }

In this example, the function myFunction is declared as asynchronous, which means that it can be suspended and resumed as needed. Within the function, we use withCheckedContinuation to create a new continuation object. The closure passed to withCheckedContinuation is where we perform our asynchronous operation.

Once the operation is complete, we call continuation.resume(returning: result) to resume the function and return the result of the operation. The result can be of any type, and it is specified in the function signature (in this case, we are returning a string).

It’s important to note that when you call continuation.resume(returning: result), you must provide the result that you want to return. If you forget to do this, your function will not return anything and your code will not work as expected.

Here is an example that demonstrates how to use withCheckedContinuation to perform a network request:

1func fetch(url: URL) async -> (Data?, Error?) {
2 return await withCheckedContinuation { continuation in
3 let task = URLSession.shared.dataTask(with: url) { data, response, error in
4 if let error = error {
5 continuation.resume(throwing: error)
6 } else {
7 continuation.resume(returning: (data, nil))
8 }
9 }
10 task.resume()
11 }

In this example, we use withCheckedContinuation to perform a network request using URLSession. We create a new data task and provide a closure that will be executed when the task is complete. If there is an error, we call continuation.resume(throwing: error) to resume the function and throw an error. If the request is successful, we call continuation.resume(returning: (data, nil)) to resume the function and return the data that we received.