import 'dart:async';

import 'package:flutter/foundation.dart';

import 'store.dart';

extension StoreChangeExtension<S extends Store> on S {
  Future<T> changeFuture<T>(T Function(S) onChange) async {
    /// Setup a completer
    final completer = Completer<T>();

    /// Get the current value for onChange.
    T current = onChange(this);

    /// Setup a listener
    void listener() async {
      final next = onChange(this);

      /// Check if there's a change
      if (next != current) {
        completer.complete(next);
        this.removeListener(listener);
      }
    }

    this.addListener(listener);
    return completer.future;
  }
}




class StoreChangeObserver<T, K extends Store> {
  /// Observe a store and if [observe] changes, fire [onChange]
  StoreChangeObserver({
    @required this.store,
    @required this.observe,
    @required this.onChange,
  });

  /// The store to listen to for changes.
  final K store;

  /// The field on the store to check for changes.
  final T Function(K) observe;

  /// The callback to fire when [observe] changes.
  final void Function(T) onChange;

  Future<void> initState() async {
    /// Setup the change listener
    store.addListener(_listener);

    /// Trigger the first value
    _listener();
  }

  void dispose() {
    /// Remove the change listener
    store.removeListener(_listener);
  }

  /// Store the previous value so we can determine when a change has occurred.
  T _previous;
  bool _hasDispatchedFirstValue = false;

  void _listener() {
    /// Get the current value.
    final next = observe(store);
    final hasChanged = _previous != next;

    /// This is the first value, let's dispatch it and ensure we don't trigger this multiple times.
    if (_hasDispatchedFirstValue == false) {
      _hasDispatchedFirstValue = true;
      _previous = next;
      onChange(next);
    }

    /// There's been an update, let's save the last value so we don't trigger this multiple times.
    else if (hasChanged) {
      _previous = next;
      onChange(next);
    }
  }
}