new_sb_base.dart

import 'dart:async';

import 'package:flutter/material.dart';

abstract class MyStreamBuilderBase<T, S> extends StatefulWidget {
  /// Creates a [MyStreamBuilderBase] connected to the specified [stream].
  const MyStreamBuilderBase({ Key key, this.stream }) : super(key: key);

  /// The asynchronous computation to which this builder is currently connected,
  /// possibly null. When changed, the current summary is updated using
  /// [afterDisconnected], if the previous stream was not null, followed by
  /// [afterConnected], if the new stream is not null.
  final Stream<T> stream;

  /// Returns the initial summary of stream interaction, typically representing
  /// the fact that no interaction has happened at all.
  ///
  /// Sub-classes must override this method to provide the initial value for
  /// the fold computation.
  S initial();

  /// Returns an updated version of the [current] summary reflecting that we
  /// are now connected to a stream.
  ///
  /// The default implementation returns [current] as is.
  S afterConnected(S current) => current;

  /// Returns an updated version of the [current] summary following a data event.
  ///
  /// Sub-classes must override this method to specify how the current summary
  /// is combined with the new data item in the fold computation.
  S afterData(S current, T data);

  /// Returns an updated version of the [current] summary following an error.
  ///
  /// The default implementation returns [current] as is.
  S afterError(S current, Object error) => current;

  /// Returns an updated version of the [current] summary following stream
  /// termination.
  ///
  /// The default implementation returns [current] as is.
  S afterDone(S current) => current;

  /// Returns an updated version of the [current] summary reflecting that we
  /// are no longer connected to a stream.
  ///
  /// The default implementation returns [current] as is.
  S afterDisconnected(S current) => current;

  /// Returns a Widget based on the [currentSummary].
  Widget build(BuildContext context, S currentSummary);

  @override
  State<MyStreamBuilderBase<T, S>> createState() => _MyStreamBuilderBaseState<T, S>();
}

/// State for [MyStreamBuilderBase].
class _MyStreamBuilderBaseState<T, S> extends State<MyStreamBuilderBase<T, S>> {
  StreamSubscription<T> _subscription;
  S _summary;

  @override
  void initState() {
    super.initState();
    _summary = widget.initial();
    _subscribe();
  }

  @override
  void didUpdateWidget(MyStreamBuilderBase<T, S> oldWidget) {
    super.didUpdateWidget(oldWidget);
    if (oldWidget.stream != widget.stream || oldWidget.initial() != widget.initial()) {
      if (_subscription != null) {
        _unsubscribe();
        _summary = widget.afterDisconnected(_summary);
      }
      _subscribe();
    }
  }

  @override
  Widget build(BuildContext context) => widget.build(context, _summary);

  @override
  void dispose() {
    _unsubscribe();
    super.dispose();
  }

  void _subscribe() {
    if (widget.stream != null) {
      _subscription = widget.stream.listen((T data) {
        setState(() {
          _summary = widget.afterData(_summary, data);
        });
      }, onError: (Object error) {
        setState(() {
          _summary = widget.afterError(_summary, error);
        });
      }, onDone: () {
        setState(() {
          _summary = widget.afterDone(_summary);
        });
      });
      _summary = widget.afterConnected(_summary);
    }
  }

  void _unsubscribe() {
    if (_subscription != null) {
      _subscription.cancel();
      _subscription = null;
    }
  }
}