回答編集履歴
1
Builderを使った方法について追記
    
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                );
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              }
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            }
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            ```
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            ---
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            **追記 :**
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            ボタン部分を`Builder`で囲むという方法もあります。
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            ```dart
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            class CookApp extends StatelessWidget {
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              var titleText = 'cookapp';
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              @override
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              Widget build(BuildContext context) {
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                return MaterialApp(
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                  home: Scaffold(
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                    appBar: AppBar(
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                      leading: Icon(Icons.menu),
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                      title: Text('レシピ一覧'),
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                    ),
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                    body: Container(
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                      child: Center(
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                        child: Column(
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                          children: <Widget>[
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                            Text(titleText),
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                            // ここがポイント
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                            Builder(
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                              builder: (context) {
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                                return RaisedButton(
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                                  child: Text("Button"),
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                                  color: Colors.orange,
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                                  textColor: Colors.white,
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                                  onPressed: () {
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                                    Navigator.push(
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                                      context,
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                                      MaterialPageRoute(builder: (context) => AddRecipe()),
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                                    );
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                                  },
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                                );
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                              },
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                            ),
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                          ],
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                        ),
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                      ),
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                    ),
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                  ),
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                );
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              }
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            }
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            ```
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