異步返回String，延遲2s

Future<String> _fetchUserOrder() {
    return Future.delayed(Duration(seconds: 2), () {
      return "Large Latte";
    });
  }

//直接返回

String _createOrderMessage() {
    var order = _fetchUserOrder();
    return "Your order is: $order";
  }
main(){
  print("${_createOrderMessage()}");
}
//打印結果
flutter: Your order is: Instance of 'Future<String>'

Future<String> _createAsyncOrderMessage() async {
    var order = await _fetchUserOrder();
    return "Your order is: $order";
  }
main() async {
  print(await _createAsyncOrderMessage())
 }
//2s后打印結果
flutter: Your order is: Large Latte

Future<void> _printOrderMessage() async {
    print('Awaiting user ordr ...');
    var order = await _fetchUserOrder();
    print('your order is : $order');
  }

  _countSeconds(int s) {
    for (var i = 0; i < s; i++) {
      Future.delayed(Duration(seconds: i), () => print(i));
    }
  }
main() {
_printOrderMessage();
_countSeconds(5);
 }
//打印結果
flutter: Awaiting user ordr ...
flutter: 0
flutter: 1
flutter: your order is : Large Latte
flutter: 2
flutter: 3
flutter: 4
main() async {
 await _printOrderMessage();
_countSeconds(5);
 }
flutter: Awaiting user ordr ...
//2s后
flutter: your order is : Large Latte
flutter: 0
flutter: 1
flutter: 2
flutter: 3
flutter: 4