Dart AppEngine Headers already sent when using Futures - google-app-engine

Question

I am using AppEngine and wrote a server application in Dart. Since the gcloud API is using Futures for pretty much everything, I make use of them as well. The problem however, once I start using Futures, a BadStateException is thrown. See the two examples below.
Working server
runAppEngine((HttpRequest request) {
request.response
..writeln('Hello world')
..statusCode = HttpStatus.OK
..close();
}, port: port);
Output:
Hello world
Not working server: Headers already sent
Future _asyncTask(HttpRequest request) {
return new Future(() {
request.response
..writeln('Hello world')
..statusCode = HttpStatus.OK; // causes BadStateException
}).catchError((e) {
request.response.writeln(e.toString());
}).whenComplete(() {
request.response.close();
});
}
runAppEngine((HttpRequest request) {
return _asyncTask(request);
}, port: port);
Output:
Hello world Bad state: HTTP headers were already sent.
I would like to use Futures but for some reasons, I just cannot get rid of the BadStateException. Can someone point me to what I am doing wrong?

Answer 1

Oh well, seems like I found the bug. I was looking for a fix to this problem all day and now that I ask here, I find the problem five minutes later.
Anyhow, I hope this answer helps people with the same problem.
The problem is that with writing to the response, headers are set implicitly. Hence, once you call writeln(String) you cannot set any headers afterwards anymore.
The reason, that there is no error logged in the first server code is, that there is simply no logic to log errors, but it is as wrong as the second one.
Here is the correct way of doing it:
Future _asyncTask(HttpRequest request) {
return new Future(() {
request.response
..statusCode = HttpStatus.OK
..writeln('Hello world');
}).catchError((e) {
request.response.writeln(e.toString());
}).whenComplete(() {
request.response.close();
});
}
runAppEngine((HttpRequest request) {
return _asyncTask(request);
}, port: port);
As you can see, the problem is fixed by sending the headers first.