A Junior Developer's Story on Swift Concurrency: The async/await Edition

Before Swift 5.5, asynchronous programming was done using GCD and completion handlers. However, this approach had limitations such as reduced readability due to nested code, complex error handling, and the possibility of retain cycles, making it difficult for the compiler to detect errors caused by developer mistakes in advance.

The new async/await syntax allows asynchronous functions to be written concisely, and the compiler supports error handling to improve stability.

This model is based on Structured Concurrency, and internally utilizes a Coroutine mechanism.

• async: Specifies that a function operates asynchronously.
• await: Indicates a suspension point where the result of an asynchronous function is awaited.

When a task encounters await and is suspended, that task returns control of the thread it was using to the system. The system efficiently uses this thread by assigning it to other tasks, and when it's time for the task to resume, it grants control back to resume it. In this process, continuation helps return to the point where it was suspended.