SE-0296: async/await

[Douglas_Gregor]

@Nevin

If I have a UI application where the user clicks a button to start downloading a file, then in today’s world the button-click function calls a download-file function which takes a completion handler. The button-click function then returns, and control goes back to the UI event loop.

In the proposed async world, where the download-file function is async , how does that work?

We can’t let an await suspend the UI event loop when the download-file function is called.

If I have to manually write Task.runDetached in all my button-click functions, that becomes onerous.

This is where the desire for "asynchronous handles" (@asyncHandler) comes from. It still needs a proper pitch---and probably a better name---but the idea is that it captures the notion of a function that is called synchronously (say, from a UI) but its body is an async task. So in your button-click example you might do something like this:

@asyncHandler()
func onButtonClick(button: Int) {
  let data = await download(url: url)
  myView.image = Image(decoding: data)
}

with is mostly (more-optimizable) syntactic sugar for:

func onButtonClick(button: Int) {
  Task.runDetached {
    let data = await download(url: url)
    myView.image = Image(decoding: data)
  }
}

Doug