Page 13 - Dart Language

Chapter 2: Asynchronous Programming

Examples

Returning a Future using a Completer

Future<Results> costlyQuery() {
var completer = new Completer();

database.query("SELECT * FROM giant_table", (results) {
// when complete
completer.complete(results);
}, (error) {
completer.completeException(error);
});

// this returns essentially immediately,
// before query is finished
return completer.future;
}

Async and Await

import 'dart:async';

Future main() async {
var value = await _waitForValue();
print("Here is the value: $value");
//since _waitForValue() returns immediately if you un it without await you won't get the
result
var errorValue = "not finished yet";
_waitForValue();
print("Here is the error value: $value");// not finished yet
}

Future<int> _waitForValue() => new Future((){

var n = 100000000;

// Do some long process
for (var i = 1; i <= n; i++) {
// Print out progress:
if ([n / 2, n / 4, n / 10, n / 20].contains(i)) {
print("Not done yet...");
}

// Return value when done.
if (i == n) {
print("Done.");
return i;
}
}
});

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