전회까지의 복습과 이번에하고 싶은 일

변경점

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter++;
    });
  }

  //--- 追加ここから
  void _decrementCounter() {
    setState(() {
      _counter--;
    });
  }
  //--- 追加ここまで

      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: const Icon(Icons.add),

      //--- 追加ここから
      floatingActionButton: FloatingActionButton(
        onPressed: _decrementCounter,
        tooltip: 'Decrement',
        child: const Icon(Icons.remove),
      //--- 追加ここまで

      floatingActionButton: Column(
        mainAxisSize: MainAxisSize.min,
        children: <Widget>[
          FloatingActionButton(
            backgroundColor: Colors.red,
            onPressed: () {
              _decrementCounter();
            },
            tooltip: 'Decrement',
            child: const Icon(Icons.remove),
          ),
          Container(
            margin: const EdgeInsets.only(),
            child: FloatingActionButton(
              backgroundColor: Colors.blue,
              onPressed: () {
                _incrementCounter();
              },
              tooltip: 'Increment',
              child: const Icon(Icons.add),
            ),
          ),
        ],
      ),