匿名函数内部的 Flutter / Dart 语法

Question

鉴于 Flutter 中的以下代码块，我看到了意想不到的语法错误

FloatingActionButton(
  onPressed: () => {
        //undefined name x, Expected to find ','
        int x = 12; 

        //Unexpected text 'return'
        if (_timer != null && _timer!.isActive)
        {
          return
        }

        _timer =
            Timer.periodic(Duration(milliseconds: 250), (timer) {
          for (var i = 0; i < 6; i++) {
            myKeys[i].currentState?.roll();
          }
        })
      },
  child: const Icon(Icons.check),
),

在 JS（或 c#）中，类似这样的东西工作正常：

const f = ()=> {
  var j = "123";
  console.log(j);
  return;
}

除了“因为它不是那样实现的”之外，为什么这在 Dart 中不起作用？正确的方法是什么？

Answer 1

箭头函数用于单条语句执行，例如，

onPressed: () => Navigator.push(//stuff in here);

而块语句用于执行多个语句，即

onPressed: (){
// Do a lot of stuff in here
},

Answer 2

你的onTap-parameter：要么对多个语句使用块体 () {}，要么对单个语句使用 arrow-function () => print("hello world")

另外不要忘记return关键字后的分号

固定解：

FloatingActionButton(
  onPressed: () {
    int x = 12;

    if (_timer != null && _timer!.isActive) {
      return;
    }

    _timer =
        Timer.periodic(Duration(milliseconds: 250), (timer) {
      for (var i = 0; i < 6; i++) {
        myKeys[i].currentState?.roll();
      }
    });
  },
  child: const Icon(Icons.check),
),

Answer 3

Both Dart and JavaScript support arrow syntax (=>), but they are different. In Dart, arrow syntax is only used when the function contains a single expression or return statement.

dart documentation

匿名函数内部的 Flutter / Dart 语法

Flutter / Dart syntax Inside an anonymous function

flutter

dart