On Jun 6, 2016, at 2:49 PM, Michael Peternell <michael.petern...@gmx.at> wrote:
>
> That's really hard to answer in the general case. I think real proposals
> should contain concrete, realistic examples that show the benefit of the
> proposal. It's really hard to argue against a proposal if there is no such
> example. User feedback from a sheet is one of the few examples where
> asynchronous programming makes sense: But I cannot see how a `@required`
> annotation would be useful in that setting.
Quick-n-dirty example, written in Mail. How would you make this synchronous?
import Foundation
enum Result<T> {
case success(T)
case error(ErrorType)
}
enum MyError: ErrorType {
case unknownError
case corruptData
case badStatusCode(Int)
}
struct SomeThing {
init?(data: NSData) {
...
}
}
func getSomethingFromTheNetwork(url: NSURL, completionHandler:
(Result<SomeThing>) -> ()) {
let task = NSURLSession.sharedSession().dataTaskWithURL(url) { data,
response, error in
if let error = error {
completionHandler(.error(error))
return
}
if let httpResponse = response as? NSHTTPURLResponse {
let statusCode = httpResponse.statusCode
if statusCode < 200 || statusCode >= 300 {
completionHandler(.error(MyError.badStatusCode(statusCode)))
return
}
}
guard let data = data else {
completionHandler(.error(MyError.unknownError))
return
}
guard let something = SomeThing(data: data) else {
completionHandler(.error(MyError.corruptData))
return
}
completionHandler(.success(something))
}
task.resume()
}
(disclaimer: yes, we’d probably have to improve the API for NSURLSession a bit
here for @required to be useful.)
How would you make this synchronous:
func getSomethingFromAnotherTask(completionHandler: (Result<SomeThing>) -> ()) {
let message = ...
xpc_send_message_with_reply(self.connection, message,
self.dispatchQueue) { reply in
do {
let something = try
self.turnReplyIntoSomethingSomehow(reply)
completionHandler(.success(something))
} catch {
completionHandler(.error(error))
}
}
}
Or this:
func doSomethingThatNeedsUserInput(completionHandler: (Bool) -> ()) {
let alert = NSAlert()
alert.messageText = “Should we continue?”
alert.addButtonWithTitle(“Continue”)
alert.addButtonWithTitle(“Cancel”)
alert.beginSheetModalForWindow(someWindow) { response in
if response == NSAlertFirstButtonReturn {
completionHandler(true)
}
// uh oh, I forgot to test for other conditions, and now the
completion handler won’t be called if the user clicked “Cancel”.
// Too bad the compiler couldn’t warn me about it.
}
}
There are some tasks which synchronous programming is simply not well-suited
for.
Charles
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