TwoWay绑定不适用于Datepicker MVVM

Question

我终于设法开始使用MVVM并实现了INotifyPropertyChanged，并且面临以下问题：即使我将datepicker的月份作为参数传递给查询，但是如果我选择了一个，就不会改变查询的结果不同的月份。

我希望INotifyPropertyChanged会解决这个问题。 与日期选择器选择不同的月份时，如何确保效率发生变化？

谢谢

视图模型

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Location.Model;
using System.Windows.Controls;

namespace Location.ViewModel
{
    public class LocationViewModel: INotifyPropertyChanged
    {


        public LocationViewModel()
        {
            var month = 0;
            int.TryParse(MDate.ToString("MM"), out month);
            var db = new DailyEntities();
            Efficiency = Convert.ToDecimal(db.LocationKPI.Where(a => a.sMonth == month).Select(a => a.Efficiency).FirstOrDefault());
        }

        private DateTime _mDate = DateTime.Now;

        public DateTime MDate
        {
            get { return _mDate; }
            set { _mDate = value; OnPropertyChanged("MDate"); }
        }

        decimal efficiency;

        public decimal Efficiency
        {
            get { return efficiency; }
            set
            {
                efficiency = value;
                OnPropertyChanged("Efficiency");
            }
        }


        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void OnPropertyChanged(string propertyName = null)
        {
            if (PropertyChanged != null) PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

视图

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        DataContext = new LocationViewModel();
    }
}

XAML

<DatePicker x:Name="vDatePick" SelectedDateChanged="vDatePick_SelectedDateChanged" SelectedDate="{Binding MDate, Mode=TwoWay}" Text="Pick date"/>

Answer 1

您需要声明必须在MDate时更新Efficiency

public DateTime MDate
{
   get { return _mDate; }
   set 
   { 
       _mDate = value; 
       OnPropertyChanged("MDate"); 
       OnPropertyChanged("Efficiency"); 
   }
}

我还将把Efficiency查询放在getter中，然后删除setter。 反过来，这意味着您不再需要私有变量efficiency并且可以在构造函数中删除查询。

public decimal Efficiency
{
    get 
    {
        var month = 0;
        int.TryParse(MDate.ToString("MM"), out month);
        return Convert.ToDecimal(
            db.LocationKPI
                .Where(a => a.sMonth == month)
                .Select(a => a.Efficiency)
                .FirstOrDefault());
    }
}

编辑：更新了Efficiency器和设置器

编辑：要注意的是，如果您想在页面加载时看到结果，也应该在构造函数中将MDate设置为默认值。

Answer 2

它不起作用的原因是因为在ViewModel中，您仅执行代码以在构造函数中设置效率。 您需要创建一个方法，然后从构造函数和MDate集中调用它，因此，每次MDate更改时，效率都会更新：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Location.Model;
using System.Windows.Controls;

namespace Location.ViewModel
{
    public class LocationViewModel: INotifyPropertyChanged
    {
        public LocationViewModel()
        {
            SetEfficiency();
        }

        private DateTime _mDate = DateTime.Now;

        public DateTime MDate
        {
            get { return _mDate; }
            set 
            {
                _mDate = value;
                OnPropertyChanged("MDate");
                SetEfficiency();
            }
        }

        decimal efficiency;

        public decimal Efficiency
        {
            get { return efficiency; }
            set
            {
                efficiency = value;
                OnPropertyChanged("Efficiency");
            }
        }

        DailyEntities db = new DailyEntities();

        private void SetEfficiency()
        {
            var month;
            int.TryParse(MDate.ToString("MM"), out month);
            Efficiency = Convert.ToDecimal(db.LocationKPI.Where(a => a.sMonth == month).Select(a => a.Efficiency).FirstOrDefault());
        }

        public event PropertyChangedEventHandler PropertyChanged;

        protected virtual void OnPropertyChanged(string propertyName = null)
        {
            if (PropertyChanged != null) PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}