then<R> method Null safety

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 where the latter is a StackTrace. 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. The onError callback must return a value or future that can be used to complete the returned future, so it must be something assignable to FutureOr<R>.

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.