async.Future async.Completer - how to "continue" if an error - dart

Question

Some help with the following would be appreciated. I am writing some console test programs, and I want to be able to enter some parameters from the terminal (I don't want to use command line arguments - too many parameters). I have tried some variations, but I cannot find how to accomplish this. The following is the latest version of my test for terminal input. The problem with this program is that if an error is encountered, the Completer closes automatically, and I want to continue from either the Main() or from fGetNumber() function. While I can see why this program doesn't work, it illustrates what I need to achieve - re-enter the number, but I cannot find how to achieve that. If a valid number is entered, there is no problem. If an invalid number is entered, I cannot find out how to re-enter the number.
The code is as follows, and the problem I have is highlighted by "//////////" :
import "dart:async" as async;
import "dart:io";
void main() {
fGetNumber("Enter Nr of Iterations : ", 0, 999999)
.then((int iIters){
print ("In Main : Iterations selected = ${iIters}");
if (iIters == null) {
print ("In Main: Invalid Number of iterations : ${iIters}.");
} else {
fProcessData(iIters);
}
print ("Main Completed");
});
}
async.Future<int> fGetNumber(String sPrompt, int iMin, int iMax) {
print ("In fGetNumber");
int iIters = 0;
async.Completer<int> oCompleter = new async.Completer();
while (!oCompleter.isCompleted) { /////////// This loop does not work ///////
return fGetUserInput(sPrompt).then((String sIters) {
iIters = int.parse(sIters);
if (iIters < iMin || iIters > iMax) throw new Exception("Invalid");
oCompleter.complete(iIters);
return oCompleter.future;
}).catchError((_) => print ("Invalid - number must be from ${iMin} to ${iMax}")
).whenComplete(() => print ("fGetNumber - whenComplete"));// always gets here
}
print ("In fGetNumber (at end of function)"); //// it never gets here
}
async.Future<String> fGetUserInput(String sPrompt) {
print ("In fGetUserInput");
async.Completer<String> oCompleter = new async.Completer();
stdout.write(sPrompt);
async.Stream<String> oStream = stdin.transform(new StringDecoder());
async.StreamSubscription oSub;
oSub = oStream.listen((String sData) {
oCompleter.complete("$sData");
oSub.cancel();
});
return oCompleter.future;
}
void fProcessData(int iIters) {
print ("In fProcessData");
for (int iPos = 1; iPos <= iIters; iPos++ ) {
if (iPos%100 == 0) print ("Processed = ${iPos}");
}
print ("In fProcessData - completed ${iIters}");
}

Answer 1

// This loop does not work
Of course it does - you enter it exactly once, where you immediately return and therefore leave the loop and method.
// always gets here
That's because whenComplete() always gets called, on success or on error.
// it never gets here
Because you already returned out of the method.
So what can be done?
The easiest way would be to not rely on fGetUserInput(). Listen to stdin in fGetNumber and only complete the completer / cancel the subscription if the input is valid:
async.Future<int> fGetNumber(String sPrompt, int iMin, int iMax) {
print ("In fGetNumber");
async.Completer<String> oCompleter = new async.Completer();
stdout.write(sPrompt);
async.Stream<String> oStream = stdin.transform(new StringDecoder());
async.StreamSubscription oSub;
oSub = oStream.listen((String sData) {
try {
int iIters = int.parse(sData);
if (iIters < iMin || iIters > iMax) throw new Exception("Invalid");
oCompleter.complete(iIters);
oSub.cancel();
} catch(e) {
print("Invalid - number must be from ${iMin} to ${iMax}");
stdout.write(sPrompt);
}
});
return oCompleter.future;
}
Are there alternatives?
Of course. There are likely many, many ways to do this. This one for example:
async.Future<int> fGetNumber(String sPrompt, int iMin, int iMax) {
print ("In fGetNumber");
async.Completer<int> oCompleter = new async.Completer();
fGetUserInput(sPrompt, oCompleter, (String sIters) {
try {
int iIters = int.parse(sIters);
if (iIters < iMin || iIters > iMax) throw new Exception("Invalid");
return iIters;
} catch(e) {
print ("Invalid - number must be from ${iMin} to ${iMax}");
stdout.write(sPrompt);
}
return null;
});
return oCompleter.future;
}
void fGetUserInput(String sPrompt, async.Completer oCompleter, dynamic inputValidator(String sData)) {
print ("In fGetUserInput");
stdout.write(sPrompt);
async.Stream<String> oStream = stdin.transform(new StringDecoder());
async.StreamSubscription oSub;
oSub = oStream.listen((String sData) {
var d = inputValidator(sData);
if(d != null) {
oCompleter.complete(d);
oSub.cancel();
}
});
}
If you really feel there should be something addressed by the Dart team, you could write a feature request. But the Completer is designed to only be completed once. Whatever code you write, you can't just loop to complete it again and again.