The WordPair class in the 'Write Your First App' example.

wordpair.dart

import 'package:flutter/cupertino.dart' show Icon, State;

import 'package:flutter/material.dart' show Colors, Icons;

import 'package:english_words/english_words.dart' as w;

class WordPair {
  WordPair(this.state);
  final State state;
  final suggestions = <w.WordPair>[];
  final Set<w.WordPair> saved = <w.WordPair>{};
  int index;

  void build(int i) {
    index = i ~/ 2;
    if (index >= suggestions.length) {
      suggestions.addAll(w.generateWordPairs().take(10));
    }
  }

  String get current => suggestions[index].asPascalCase;

  Icon get icon {
    bool alreadySaved = saved.contains(suggestions[index]);
    return Icon(
      alreadySaved ? Icons.favorite : Icons.favorite_border,
      color: alreadySaved ? Colors.red : null,
    );
  }

  void onTap(int i) => state.setState(() {
    int index = i ~/ 2;
    w.WordPair pair = suggestions[index];
    if (pair == null) return;
    if (saved.contains(suggestions[index])) {
      saved.remove(pair);
    } else {
      saved.add(pair);
    }
  });
}