Error handling is ad hoc because completion handlers can’t throw errors. For example, this sample code from WWDC’s Meet async/await in Swift downloads data, creates an image from the data, then renders a thumbnail from the image.

This makes it hard to check the code is correct. If a completion handler calls another asynchronous function, and this function has a completion handler, it’s hard to see the happy path in the resulting pyramid of doom. Completion or delegate code that accesses UI elements must be dispatched to the main queue. Methods take a completion handler and call delegate methods.

Methods automatically dispatch to a background queue and immediately return control to the calling code. Swift APIs like URLSession are asynchronous. This lets tasks wait for each other to complete and allows you to effectively manage the overall progress of a task. Swift concurrency provides the necessary tools to carve work up into smaller tasks that can run concurrently. The old GCD concurrency has several problems that make it hard to write apps that safely use concurrency. Good, isn’t it? :]īefore you get to work on the real WaitForIt app, take a high level look at how the new Swift concurrency fixes problems with the old GCD concurrency. OK, that was just a quick check to fix that Xcode glitch and also to show you SwiftUI’s new AsyncImage view.

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