asynchronous programming in dart (Futures, await) - asynchronous

Question

what changes should I do to the following code to get the following output:
Task 1 complete
Task 4 complete
Task 2 complete
Task 3 complete with task 2 data
I currently getting the outputs given below:
Task 1 complete
Task 2 complete
Task 3 complete with task 2 data
Task 4 complete
import 'dart:async';
void main() {
performTasks();
}
void performTasks() async {
task1();
String task2Result = await task2();
task3(task2Result);
task4();
}
void task1() {
String result = 'task 1 data';
print('Task 1 complete');
}
Future<String> task2() async {
Duration threeSeconds = Duration(seconds: 3);
String result;
await Future.delayed(threeSeconds, () {
result = 'task 2 data';
print('Task 2 complete');
});
return result;
}
void task3(String task2Data) {
String result = 'task 3 data';
print('Task 3 complete with $task2Data');
}
void task4() {
String result = 'task 4 data';
print('Task 4 complete');
}

Answer 1

Don't call task4 after waiting for task2.
So:
void performTasks() async {
task1();
task4();
String task2Result = await task2();
task3(task2Result);
}
That looks pretty obvious, so I'm assuming your real problem is more complicated, and you can't move test4() around like that.
In that case, you should not use await. The await ensures that everything written after the await also executes after the awaited future has completed.
Instead you can fall back on the Future API:
void performTasks() { // no `async`
task1();
task2().then(task3); // no `await`
task4();
}
This sets up task2() to run, and when that is done, it calls task3 with the result. It doesn't wait for anything, though, and it executes task4 immediately after setting this up.
The then method on futures takes a callback, and eventually calls that with the result of the future. Here task3 takes one argument, so it can directly be used as that callback.
That assumes that task2's result is directly usable as an argument to task3. If not, and you have to capture the result and manipulate it first, you'd do it as:
void performTasks() { // no `async`
task1();
task2().then((result2) { // still no await here!
var argument3 = manipulate(result2);
// ... and whatever else you want to do
// between task2 completing and task3 starting.
task3(argument3);
});
task4();
}

Answer 2

in performTasks(), you should move task4(); right after task1();.