Concurrency warning when using WKWebView＃evaluateJavascript and `async let`

Using Swift
4

This is odd to me as well. I thought maybe it had to do with evaluateJavaScript being isolated to @MainActor. Makes sense because results of evaluateJavaScript() are not guaranteed to be Sendable.

But even when we isolated the Task closure to @MainActor it also does not work.

So then I thought maybe we need to isolate the whole function (see below)

@MainActor func asyncLetErrorOnMainActor() async throws {
	async let urlObject = self.webView.evaluateJavaScript("document.location.href")
	async let titleObject = self.webView.evaluateJavaScript("document.title")

	print(try await urlObject)
	print(try await titleObject)
}

I suppose it has to do with how async let is implemented that I just don't understand where it is crossing the boundaries.

I looked at the following posts
(1) Returning non-Sendable object across actor boundaries via sending keyword

(2) https://www.hackingwithswift.com/quick-start/concurrency/how-to-call-an-async-function-using-async-let

(3) Why does `async let` discern "the origin" of its right expression?

To better understand why a value that comes from an @MainActor isolated function within an @MainActor isolated function/closure throws an error. And I couldn't figure it out.

Is this just an bug?

1 Like