Register callbacks to be called when this future completes.
When this future completes with a value,
the onValue callback will be called with that value.
If this future is already completed, the callback will not be called
immediately, but will be scheduled in a later microtask.
If onError is provided, and this future completes with an error,
the onError callback is called with that error and its stack trace.
The onError callback must accept either one argument or two arguments.
If onError accepts two arguments,
it is called with both the error and the stack trace,
otherwise it is called with just the error object.
Returns a new Future
which is completed with the result of the call to onValue
(if this future completes with a value)
or to onError (if this future completes with an error).
If the invoked callback throws,
the returned future is completed with the thrown error
and a stack trace for the error.
In the case of onError,
if the exception thrown is identical to the error argument to onError,
the throw is considered a rethrow,
and the original stack trace is used instead.
If the callback returns a Future,
the future returned by then will be completed with
the same result as the future returned by the callback.
If onError is not given, and this future completes with an error,
the error is forwarded directly to the returned future.
In most cases, it is more readable to use catchError separately, possibly
with a test parameter, instead of handling both value and error in a
single then call.
Note that futures don't delay reporting of errors until listeners are
added. If the first then or catchError call happens after this future
has completed with an error then the error is reported as unhandled error.
See the description on Future.
Source
Future<S> then<S>(FutureOr<S> onValue(T value), {Function onError});