async await

The async and await keywords in Swift are used to handle asynchronous operations, allowing functions to suspend and resume without blocking the main thread. Here are some key points about async and await from the WWDC sessions:

1. Basic Usage:

   • A function that may suspend is marked with the async keyword.
   • When an async function is called, the await keyword is used to indicate that a suspension can occur on that line.
   • This allows the CPU to be yielded to other tasks while waiting for an asynchronous operation to complete.
   • Example:

     func fetchData() async -> Data {
         let data = await fetchFromNetwork()
         return data
     }
     

2. Concurrency in Swift:

   • Swift's concurrency model includes tasks, async/await, and actors.
   • Actors are reference types that encapsulate shared mutable state and automatically protect their state by serializing accesses.
   • Calls to actor methods from outside the context of the actor are asynchronous and require the await keyword.
   • Example:

     actor UserStore {
         var users: [String] = []
         
         func addUser(_ user: String) {
             users.append(user)
         }
     }
     
     let userStore = UserStore()
     await userStore.addUser("Alice")
     

3. Performance Considerations:

   • Async functions are split into multiple partial functions at runtime to handle suspension points.
   • They keep their local state on a separate stack from the C stack.
   • This design helps in managing memory efficiently and ensures that async tasks do not block precious C threads.
   • Example:

     func processTasks() async {
         for task in tasks {
             let result = await task.perform()
             results.append(result)
         }
     }
     

4. Testing Asynchronous Code:

   • When writing concurrent test code, you can use the same concurrency features in Swift as in production code.
   • await works the same way in tests, allowing other test code to keep the CPU busy while work is pending.
   • Swift provides async overloads for most completion handlers, making it easier to test asynchronous code.
   • Example:

     func testAsyncFunction() async {
         let result = await someAsyncFunction()
         XCTAssertEqual(result, expectedValue)
     }
     

For more detailed information, you can refer to the following sessions from WWDC 2024:

• A Swift Tour: Explore Swift’s features and design
• Explore Swift performance
• Go further with Swift Testing

These sessions cover the basics of async/await, concurrency in Swift, performance considerations, and testing asynchronous code.