Completer<T>.sync constructor

Completes the future synchronously.

This constructor should be avoided unless the completion of the future is known to be the final result of another asynchronous operation. If in doubt use the default Completer constructor.

Using an normal, asynchronous, completer will never give the wrong behavior, but using a synchronous completer incorrectly can cause otherwise correct programs to break.

A synchronous completer is only intended for optimizing event propagation when one asynchronous event immediately triggers another. It should not be used unless the calls to complete and completeError are guaranteed to occur in places where it won't break Future invariants.

Completing synchronously means that the completer's future will be completed immediately when calling the complete or completeError method on a synchronous completer, which also calls any callbacks registered on that future.

Completing synchronously must not break the rule that when you add a callback on a future, that callback must not be called until the code that added the callback has completed. For that reason, a synchronous completion must only occur at the very end (in "tail position") of another synchronous event, because at that point, completing the future immediately is be equivalent to returning to the event loop and completing the future in the next microtask.

Example:

var completer = Completer.sync();
// The completion is the result of the asynchronous onDone event.
// No other operation is performed after the completion. It is safe
// to use the Completer.sync constructor.
stream.listen(print, onDone: () { completer.complete("done"); });

Bad example. Do not use this code. Only for illustrative purposes:

var completer = Completer.sync();
completer.future.then((_) { bar(); });
// The completion is the result of the asynchronous onDone event.
// However, there is still code executed after the completion. This
// operation is *not* safe.
stream.listen(print, onDone: () {
  completer.complete("done");
  foo();  // In this case, foo() runs after bar().
});