listen method Null safety

Adds a subscription to this stream.

Returns a StreamSubscription which handles events from this stream using the provided onData, onError and onDone handlers. The handlers can be changed on the subscription, but they start out as the provided functions.

On each data event from this stream, the subscriber's onData handler is called. If onData is null, nothing happens.

On errors from this stream, the onError handler is called with the error object and possibly a stack trace.

The onError callback must be of type void Function(Object error) or void Function(Object error, StackTrace). The function type determines whether onError is invoked with a stack trace argument. The stack trace argument may be StackTrace.empty if this stream received an error without a stack trace.

Otherwise it is called with just the error object. If onError is omitted, any errors on this stream are considered unhandled, and will be passed to the current Zone's error handler. By default unhandled async errors are treated as if they were uncaught top-level errors.

If this stream closes and sends a done event, the onDone handler is called. If onDone is null, nothing happens.

If cancelOnError is true, the subscription is automatically canceled when the first error event is delivered. The default is false.

While a subscription is paused, or when it has been canceled, the subscription doesn't receive events and none of the event handler functions are called.