引數型別“物件？”無法分配給引數型別“num”，因為getter（作為雙精度）不起作用

我正在 Flutter 中制作個人費用跟蹤器應用程式。我想輸出與每周總費用相比的每日費用的百分比。所以，我需要Double通過.fold()方法獲取資料型別中一周的總和。但檢查員顯示“引數型別'物件？' 不能分配給引數型別 'num'。” 我累都as double和as int。

import 'package:flutter/material.dart';
import 'package:flutter_app_module3/models/transaction.dart';
import 'package:intl/intl.dart';
import './chart_bar.dart';

class Chart extends StatelessWidget {

  final List<Transaction> recentTransaction; 
  

  Chart(this.recentTransactions);

  List<Map<String, Object>> get groupedTrandactionValues {
    return List.generate(7, (index) {
      final weekDay = DateTime.now().subtract(
        Duration(days: index),
      );
      var totalSum = 0.0;

      for (var i = 0; i < recentTransactions.length; i  ) {
        if (recentTransactions[i].date.day == weekDay.day &&
            recentTransactions[i].date.month == weekDay.month &&
            recentTransactions[i].date.year == weekDay.year) {
          totalSum  = recentTransactions[i].amount;
        }
      }

      return {
        "Day": DateFormat.E().format(weekDay).substring(0, 1),
        'amount': totalSum,
      };


  double  get totalSpending {
    return groupedTrandactionValues.fold(0, (prev, element) {
     
      return prev   element['amount'];
    });
  }

  @override
  Widget build(BuildContext context) {
    print(groupedTrandactionValues);
    return Card(
      child: Row(),
    );
  }
}

groupTransactionValues 回來：

  [{"Day": "W", "amount": 86.52},
           {"Day": "T", "amount": 45.0},
           {"Day":" M"," amount": 23.0}, 
           {"Day": "S", "amount": 45.0},
           {"Day": "S", "amount": 67.0}, 
           {"Day": "F", "amount": 98.0}, 
           {'Day': "T", "amount": 45.0}];
            });
          }
    

我試過 as double

double  get totalSpending {
    return groupedTrandactionValues.fold(0.0, (prev, element) {
     
      return prev   element['amount'] as double;
    });
  }

還有as int：

int  get totalSpending {
    return groupedTrandactionValues.fold(0, (prev, element) {
    
      return prev   element['amount'] as int;
    });
  }

uj5u.com熱心網友回復：

更改
List<Map<String, Object>> get groupedTrandactionValues
為
List<Map<String, dynamic>> get groupedTrandactionValues

uj5u.com熱心網友回復：

當從地圖中獲取元素時，element['amount']它可以為空，所以這樣做。

return prev   (element['amount'] ?? 0.0);

這意味著如果元素中的數量由于某種原因為空，它將添加 0.0

編輯：這在飛鏢中對我有用

  List<Map<String, Object>> data = [
    {"Day": "W", "amount": 86.52},
    {"Day": "T", "amount": 45.0},
    {"Day": "M", "amount": 23.0},
    {"Day": "S", "amount": 45.0},
    {"Day": "S", "amount": 67.0},
    {"Day": "F", "amount": 98.0},
    {'Day': "T", "amount": 45.0},
  ];

  double totalSpending() {
    return data.fold(0.0, (double prev, Map<String, Object> element) {
      return prev   (element['amount'] as double? ?? 0.0);
    });
  }

  var total = totalSpending();
  print(total);

409.52

(element['amount'] as double? ?? 0.0) 表示將金額作為雙倍或空值，如果它是空值，請使用 0.0

標籤：扑 镖

上一篇：代替資料的模型實體和模型沒有getter長度Flutter

下一篇：如何在Flutter中上傳多個影像以休息API？