TwoWay綁定不適用於Datepicker MVVM

Question

我終於設法開始使用MVVM並實現了INotifyPropertyChanged，並且面臨以下問題：即使我將datepicker的月份作為參數傳遞給查詢，但是如果我選擇了一個，就不會改變查詢的結果不同的月份。

我希望INotifyPropertyChanged會解決這個問題。 與日期選擇器選擇不同的月份時，如何確保效率發生變化？

謝謝

視圖模型

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));
        }
    }
}

視圖

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

您需要聲明必須在MDate時更新Efficiency

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

我還將把Efficiency查詢放在getter中，然后刪除setter。 反過來，這意味着您不再需要私有變量efficiency並且可以在構造函數中刪除查詢。

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());
    }
}

編輯：更新了Efficiency器和設置器

編輯：要注意的是，如果您想在頁面加載時看到結果，也應該在構造函數中將MDate設置為默認值。

Answer 2

它不起作用的原因是因為在ViewModel中，您僅執行代碼以在構造函數中設置效率。 您需要創建一個方法，然后從構造函數和MDate集中調用它，因此，每次MDate更改時，效率都會更新：

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));
        }
    }
}