更改ViewModel后如何獲取更新的View

Question

我正在構建WPF應用程序來管理存儲在SharePoint列表中的學生數據庫。 我是使用MVVM的新手，並且已經完成了一些教程。 我已經成功地創建了視圖和模型並將其綁定到數據網格控件。 我想做的是根據組合框的輸出更新視圖中的數據。

這是我的模型：

using System.ComponentModel;

namespace StudentManagement.Model
{
public class Student : INotifyPropertyChanged
{
    private string _Title;
    private string _FullName;

    public string Title
    {
        get { return _Title; }
        set
        {
            if (_Title != value)
            {
                _Title = value;
                RaisePropertyChanged("Title");
            }
        }
    }

    public string FullName
    {
        get { return _FullName; }
        set
        {
            if (_FullName != value)
            {
                _FullName = value;
                RaisePropertyChanged("FullName");
            }

        }

    }



    public event PropertyChangedEventHandler PropertyChanged;

    private void RaisePropertyChanged(string property)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(property));
        }
    } 
  }
}

這是視圖模型：

using System.Collections.ObjectModel;
using StudentManagement.Model;
using SP = Microsoft.SharePoint.Client;
using StudentManagement.publicClasses;

namespace StudentManagement.ViewModel
{
    public class StudentViewModel
    {
        public ObservableCollection<Student> Students { get; set; }

        public void LoadStudents(string query)
        {


           ObservableCollection<Student> _students = new 
        ObservableCollection<Student>();

        SP.ClientContext ctx = clientContext._clientContext;

        SP.CamlQuery qry = new SP.CamlQuery();
        qry.ViewXml = query;
        SP.ListItemCollection splStudents = 
 ctx.Web.Lists.GetByTitle("Students").GetItems(qry);
        ctx.Load(splStudents);
        ctx.ExecuteQuery();

        foreach (SP.ListItem s in splStudents)
        {
            _students.Add(new Student { Title = (string)s["Title"], FullName = (string)s["FullName"] });
        }
        Students = _students;

        }
    }
}

這是我的XAML

 <UserControl x:Class="StudentManagement.Views.StudentsView" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" xmlns:d="http://schemas.microsoft.com/expression/blend/2008" mc:Ignorable="d" xmlns:local="clr-namespace:StudentManagement.Views" > <Grid> <StackPanel HorizontalAlignment="Left" Width="200" > <TextBox Name="txtSearch" AcceptsReturn="True" ></TextBox> <ComboBox Name="cmbStatus" SelectionChanged="cmbStatus_SelectionChanged" SelectedIndex="0"> <ComboBoxItem>Active</ComboBoxItem> <ComboBoxItem>Inquiring</ComboBoxItem> <ComboBoxItem>Inactive</ComboBoxItem> <ComboBoxItem>Monitoring</ComboBoxItem> </ComboBox> <DataGrid Name="dgStudentList" ItemsSource="{Binding Path=Students}" AutoGenerateColumns="False"> <DataGrid.Columns> <DataGridTextColumn Header="Name" Binding="{Binding Title, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" Width="100"/> <DataGridTextColumn Header="Parent" Binding="{Binding FullName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" Width="100" /> </DataGrid.Columns> </DataGrid> </StackPanel> </Grid> </UserControl> 

...以及該視圖后面的代碼：

using System.Windows.Controls;
using StudentManagement.ViewModel;

namespace StudentManagement.Views
{
    /// <summary>
    /// Interaction logic for StudentsView.xaml
    /// </summary>
    public partial class StudentsView : UserControl
    {
        private StudentViewModel _viewModel = new 
StudentViewModel();

    public StudentsView()
    {
            InitializeComponent();
            DataContext = _viewModel;
        }

        private void cmbStatus_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            string combotext = ((sender as ComboBox).SelectedItem as ComboBoxItem).Content as string;   
            string qry = @"<View>  
                        <Query> 
                            <Where><Eq><FieldRef Name='Current_x0020_Status' /><Value Type='Choice'>" + combotext + @"</Value></Eq></Where> 
                        </Query> 
                       </View>";

            _viewModel.LoadStudents(qry);

        }
    }
}

從目前的情況來看，學生可以很好地加載到數據網格中。 當事件cmbStatus_SelectionChanged觸發時，我已經完成了測試，並且可以看到LoadStudents函數觸發並返回正確數量的條目，但是在datagrid上沒有任何更新。

我敢肯定這是一個菜鳥錯誤，我想念一些基本的東西，但是我正在努力，我將不勝感激。

Answer 1

由於StudentViewModel.LoadStudents()改變的價值Students屬性，視圖模型需要通知認為，這改變了。 您可以通過讓StudentViewModel實現INotifyPropertyChanged來做到這一點（就像Student一樣）。 DataGrid將預訂PropertyChanged事件，並在觸發該事件時更新其內容。

Answer 2

如果組合框的選擇發生更改，則每次都在初始化學生集合。

ObservableCollection<Student> _students = new ObservableCollection<Student>();

您不應在ViewModel中使用綁定集合來執行此操作。 您可以像這樣清除集合並添加新項目。

public class StudentViewModel
{
    public ObservableCollection<Student> Students { get; set; } = new ObservableCollection<Student>();

    public void LoadStudents(string query)
    {
        Students.Clear();

        SP.ClientContext ctx = clientContext._clientContext;

        SP.CamlQuery qry = new SP.CamlQuery();
        qry.ViewXml = query;
        SP.ListItemCollection splStudents =  ctx.Web.Lists.GetByTitle("Students").GetItems(qry);
        ctx.Load(splStudents);
        ctx.ExecuteQuery();

        foreach (SP.ListItem s in splStudents)
        {
            Students.Add(new Student { Title = (string)s["Title"], FullName = (string)s["FullName"] });
        }
    }
}