How to make an asynchronous Dart call synchronous?

I'm on the way to evaluate Dart for a German company by porting various Java programs to Dart and compare and analyze the results. In the browser Dart wins hands down. For server software performance seemed to be a serious isssue (see this question of me) but that got mostly defused.

Now I'm in the area of porting some "simple" command-line tools where I did not expect any serious problems at all but there is at least one. Some of the tools do make HTTP requests to collect some data and the stand-alone Dart virtual machine only supports them in an asynchronous fashion. Looking through all I could find it does not seem to be possible to use any asynchronous call in a mostly synchronous software.

I understand that I could restructure the available synchronous software into an asynchronous one. But this would transform a well-designed piece of software into something less readable and more difficult to debug and maintain. For some software pieces this just does not make sense. My question: Is there an (overlooked by me) way to embed an asynchronous call into a synchronously called method?

I imagine that it would not be to difficult to provide a system call, usable only from within the main thread, which just transfers the execution to the whole list of queued asynchronous function calls (without having to end the main thread first) and as soon as the last one got executed returns and continues the main thread.

Something which might look like this:

var synchFunction() {
  var result;
  asyncFunction().then(() { result = ...; });

  resync(); // the system call to move to and wait out all async execution

  return result;
}

Having such a method would simplify the lib APIs as well. Most "sync" calls could be removed because the re-synchronisation call would do the job. It seems to be such a logical idea that I still think it somehow exists and I have missed it. Or is there a serious reason why that would not work?

After thinking about the received answer from lm (see below) for two days I still do not understand why the encapsulation of an asynchronous Dart call into a synchronous one should not be possible. It is done in the "normal" synchronous programing world all the time. Usually you can wait for a resynchronization by either getting a "Done" from the asynchronous routine or if something fails continue after a timeout.

With that in mind my first proposal could be enhanced like that:

var synchFunction() {
  var result;
  asyncFunction()
    .then(() { result = ...; })
    .whenComplete(() { continueResync() }); // the "Done" message

  resync(timeout); // waiting with a timeout as maximum limit

  // Either we arrive here with the [result] filled in or a with a [TimeoutException].
  return result;
}

The resync() does the same that would normally happen after ending the main method of an isolate, it starts executing the queued asynchronous functions (or waits for events to make them executable). As soon as it encounters a continueResync() call a flag is set which stops this asynchronous execution and resync() returns to the main thread. If no continueResync() call is encountered during the given timeout period it too aborts the asynchronous execution and leaves resync() with a TimeoutException.

For some groups of software which benefit from straight synchronous programing (not the client software and not the server software) such a feature would solve lots of problems for the programer who has to deal with asynchrounous-only libraries.

I believe that I have also found a solution for the main argument in lm's argumentation below. Therefore my question still stands with respect to this "enhanced" solution which I proposed: Is there anything which really makes it impossible to implement that in Dart?

The only time that you can wrap an async method in a synchronous one is when you don't need to get a return value.

For example if you want to disable the save button, save results to the server asynchronously and re-enable the save button when the job is done you can write it like this:

Future<bool> save() async {
  // save changes async here
  return true;
}

void saveClicked() {
  saveButton.enabled = false;
  save()
    .then((success) => window.alert(success ? 'Saved' : 'Failed'))
    .catchError((e) => window.alert(e))
    .whenComplete(() { saveButton.enabled = true; });
}

Note that the saveClicked method is fully synchronous, but executes the save method asynchronously.

Note that if you make saveClicked async, not only do you have to call it using the async pattern, but the entire method body will run asynchronously so the save button will not be disabled when the function returns.

For completeness the async version of saveClicked looks like this:

Future<Null> saveClicked() async {
  saveButton.enabled = false;
  try {
    bool success = await save();
    window.alert(success ? 'Saved' : 'Failed');
  }
  catch (e) {
    window.alert(e);
  }
  finally {
    saveButton.enabled = true;
  }
}

The resync function cannot be implemented in Dart's current execution model.

Asynchronous execution is contagious. A synchronous function must return before any other asynchronous events can execute, so there is no way to synchronously wait for asynchronous execution.

Execution in Dart is single-threaded and event based. There is no way for the resync function to block without it also blocking all other execution in the same isolate, so the pending async operations will never happen.

To block the synchronous execution, and continue executing something else, you need to preserve the entire call stack up to that point, and reinstate it later when the synchronous operations have completed. If you have that functionality, then there are probably better ways to do things than Future and Stream :)

Also, waiting for "all async execution" isn't well-defined in an event based system. There might be a broadcast Stream emitting events coming in from the network, a periodic timer, or a receive port getting data from another isolate, or some other source of events that you can't wait for because they come from outside the isolate, or event the process. When the current isolate shuts down, it might send a final shut-down message to another isolate, so effectively the "async execution" isn't over until the isolate dies.

Using the async/await syntax, you won't get synchronous operation, but it will be easier to code the similar asynchronous operation:

function() async {
  var result = await asyncFunction();
  return result;
}

It won't wait for async operations that aren't reflected in the Future returned by asyncFunction, but that's the job of asyncFunction to not complete until its operations are complete.

Yes, this is way late, but I think this is a cool feature new people should know about.

There is a way, but the Dart docs warn against it (and it's somehow "experimental", although the implications aren't really discussed).

The waitFor command.

You basically pass in an asynchronous function that returns a Future, an optional timeout parameter, and the waitFor function will return the result.

For example:

final int number = waitFor<int>(someAsyncThatReturnsInt);

Dart is inherently async. Trying to avoid asynchronity won't work out. There are sync versions of some API calls for example in dart:io and in some situations it might seem simpler to use them instead but because there aren't sync versions for all methods/functions you can't avoid async entirely.

With the recent introduction of the async/await feature programming async become much simpler and the code looks almost like sync code (but it isn't).

If a call went async it stays async. As far as I know there is nothing you can do about it.

来源：https://stackoverflow.com/questions/28238161/how-to-make-an-asynchronous-dart-call-synchronous

标签

asynchronous

dart

synchronous