Async-await in Swift is getting more popular as time goes by, but Combine publishers do not have built-in support for it currently. In this blog post, we’ll see how to expand some of the existing publishers.
Async-await supported sink
One case where I have encountered this is when I have wanted to call an async function in sink. Although I could wrap the call with Task within the sink subscriber, it gets unnecessary long if I need to do it in many places. Instead, we can just do it once and add an async-await supported sink subscriber.
Async-await supported map
The Combine framework has map and tryMap for supporting throwing functions, but is lacking something like tryAwaitMap for async throwing functions. Combine has a publisher named Future which supports performing asynchronous work and publishing a value. We can use this to wrap a Task with asynchronous work. Another publisher in Combine is flatMap what is used for turning one kind of publisher to a new kind of publisher. Therefore, we can combine these to turn a downstream publisher to a new publisher of type Future. The first tryAwaitMap below is for a case where the downstream publisher emits errors, and the second one is for the case where the downstream does not emit errors. We need to handle these separately since we need to tell Combine how error types are handled (non-throwing publisher has failure type set to Never).