when<R> method

R when<R>({ required R data(T data, bool isComplete), required R error(Object error, StackTrace? stackTrace), required R loading(), })

Perform actions conditionally based on the state of the AsyncSnapshot.

All cases are required, which allows returning a non-nullable value.

The data callback is called once the AsyncSnapshot has data. The isComplete parameter wiil be true if the connection/stream is closed.

The error callback is called when the AsyncSnapshot has an error.

The loading callback is called when the AsyncSnapshot is still waiting for data.

Example:

StreamBuilder(
  stream: incomingMessagesStream,
  builder: (context, snapshot) {
    snapshot.when(
      data: (data, isComplete) {
        return Column(
          children: [
            Text('Latest Message: $data'),
            if (isComplete) Text('Messages are complete'),
          ]
        );
      },
      error: (error, stackTrace) {
        return Text('We have an error');
      },
      loading: () {
        return CircularProgressIndicator();
      },
    );
  },
);

Implementation

R when<R>({
  required R Function(T data, bool isComplete) data,
  required R Function(Object error, StackTrace? stackTrace) error,
  required R Function() loading,
}) {
  switch (connectionState) {
    case ConnectionState.none:
      if (this.data is T) {
        return data(this.data as T, true);
      } else {
        return loading();
      }
    case ConnectionState.waiting:
      return loading();
    case ConnectionState.active:
      if (hasError) {
        return error(this.error!, stackTrace);
      } else {
        return data(this.data as T, false);
      }
    case ConnectionState.done:
      if (hasError) {
        return error(this.error!, stackTrace!);
      } else {
        return data(this.data as T, true);
      }
  }
}