方法'*'是在null上调用的。接收方：null尝试调用:*（null）

Question

    class _BillDetailsWidgetState extends State<BillDetailsWidget> {
  _BillDetailsWidgetState(
      {this.qtyCount,
      this.singleServicePrice,
      this.itemsTotal,
      this.totalPayablePrice});

  int qtyCount = 10;
  int singleServicePrice = 100;
  int itemsTotal;
  int totalPayablePrice = 0;

  String cartPriceTotal() {
    totalPayablePrice = qtyCount * singleServicePrice;
    return totalPayablePrice.toString();
  }

  @override
  Widget build(BuildContext context) {
    return Container(
      margin: EdgeInsets.only(top: 10.0),
      color: Colors.white,
      child: Container(
        color: Colors.white,
        padding:
            EdgeInsets.only(top: 20.0, bottom: 20.0, left: 20.0, right: 20.0),
        child: Column(
          children: [
            Column(
              crossAxisAlignment: CrossAxisAlignment.start,
              children: [
                Text(
                  'Bill Details:',
                  style: kTextStyle,
                ),
                SizedBox(
                  height: 10,
                ),
                Row(
                  mainAxisAlignment: MainAxisAlignment.spaceBetween,
                  children: [
                    Text(
                      'Item(s) Total',
                      style: kOtherTextStyle,
                    ),
                    Text(
                      '249',
                      style: kOtherTextStyle,
                    ),
                  ],
                ),
                Row(
                  mainAxisAlignment: MainAxisAlignment.spaceBetween,
                  children: [
                    Text(
                      'Safety & Conveyance Charges',
                      style: kOtherTextStyle,
                    ),
                    Text(
                      '₹ 80',
                      style: kOtherTextStyle,
                    ),
                  ],
                ),
                SizedBox(
                  height: 10,
                ),
                Container(
                  height: 1,
                  color: Colors.grey,
                ),
                Row(
                  mainAxisAlignment: MainAxisAlignment.end,
                  children: [
                    Text(
                      cartPriceTotal(),
                      style: kOtherTextStyle,
                    ),
                  ],
                ),
              ],
            ),
          ],
        ),
      ),
    );
  }
}

Answer 1

您必须在构造函数方法中而不是字段本身中设置默认值。否则，如果您没有在创建_BillDetailsWidgetState构造函数的地方提供值，这些值将全部设置为null。当然，乘以null值会导致错误。

dart prettyprint-override">class BillDetailsWidget extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => _BillDetailsWidgetState();
}

在您的情况下，实际上就像是像这样调用状态构造函数：

class BillDetailsWidget extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => _BillDetailsWidgetState(
    qtyCount: null,
    singleServicePrice: null,
    itemsTotal: null,
    totalPayablePrice: null,
  );
}

因此，这解决了这个问题：

  _BillDetailsWidgetState(
      {this.qtyCount = 10,
      this.singleServicePrice = 100,
      this.itemsTotal,
      this.totalPayablePrice = 0});


  int qtyCount;
  int singleServicePrice;
  int itemsTotal;
  int totalPayablePrice;
  //... omitted for brevity

class BillDetailsWidget extends StatelessWidget {
  final int qtyCount;
  final int singleServicePrice;

  const BillDetailsWidget({
    Key key,
    @required this.qtyCount,
    @required this.singleServicePrice,
  })  : assert(qtyCount != null),
        assert(singleServicePrice != null),
        super(key: key);

  String cartPriceTotal() {
    return (qtyCount * singleServicePrice).toString();
  }

  @override
  Widget build(BuildContext context) {
    // ... omitted for brevity
  }
}