如何实时跟踪Student模型IsSelected属性变化并维护选中ID列表

问题背景 #

现有代码已实现学生列表展示与勾选功能，需在勾选/取消勾选CheckBox时，实时将对应学生的Id添加/移除到SelectedIds列表中。

解决方案 #

步骤1：初始化SelectedIds列表

在ViewModel构造函数中完成SelectedIds的初始化，避免空引用：

public YourViewModel()
{
    SelectedIds = new List<Guid>();
}

步骤2：监听Student的IsSelected属性变化

在ViewModel的OnActivatedAsync方法加载数据后，给每个Student对象订阅PropertyChanged事件，监听IsSelected的变更：

public override async Task OnActivatedAsync()
{
    var results = await _service.GetAllStudents(take: 100);
    Students.ReplaceRange(results);
    
    // 为每个学生订阅属性变更事件
    foreach (var student in Students)
    {
        student.PropertyChanged += Student_PropertyChanged;
    }
}

// 处理IsSelected变更的事件逻辑
private void Student_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
    if (e.PropertyName != nameof(Student.IsSelected)) return;
    
    var student = sender as Student;
    if (student == null) return;
    
    if (student.IsSelected)
    {
        if (!SelectedIds.Contains(student.Id))
        {
            SelectedIds.Add(student.Id);
        }
    }
    else
    {
        SelectedIds.Remove(student.Id);
    }
}

步骤3：清理事件订阅（推荐）

为避免内存泄漏，在ViewModel的OnDeactivatedAsync方法中取消事件订阅：

public override async Task OnDeactivatedAsync()
{
    foreach (var student in Students)
    {
        student.PropertyChanged -= Student_PropertyChanged;
    }
    await base.OnDeactivatedAsync();
}

备选方案：自定义IsSelected变更事件

若不想依赖PropertyChanged事件，可在Student模型中添加自定义事件：

public class Student : BindableBase
{
    public Guid Id { get; set; }
    public string Fullname { get; set; }
    
    // 自定义选中状态变更事件
    public event Action<Student, bool> IsSelectedChanged;
    
    private bool _isSelected;
    public bool IsSelected 
    { 
        get => _isSelected; 
        set 
        {
            if (SetProperty(ref _isSelected, value))
            {
                IsSelectedChanged?.Invoke(this, value);
            }
        }
    }
}

ViewModel中订阅该自定义事件：

public override async Task OnActivatedAsync()
{
    var results = await _service.GetAllStudents(take: 100);
    Students.ReplaceRange(results);
    
    foreach (var student in Students)
    {
        student.IsSelectedChanged += Student_IsSelectedChanged;
    }
}

private void Student_IsSelectedChanged(Student student, bool isSelected)
{
    if (isSelected)
    {
        if (!SelectedIds.Contains(student.Id))
        {
            SelectedIds.Add(student.Id);
        }
    }
    else
    {
        SelectedIds.Remove(student.Id);
    }
}

同样在OnDeactivatedAsync中取消订阅：

public override async Task OnDeactivatedAsync()
{
    foreach (var student in Students)
    {
        student.IsSelectedChanged -= Student_IsSelectedChanged;
    }
    await base.OnDeactivatedAsync();
}

额外提示 #

• 若Students集合后续会动态增删元素，需监听集合的CollectionChanged事件，为新增学生订阅事件、为移除的学生取消订阅。
• 若需UI实时绑定选中ID列表，可将SelectedIds改为ObservableCollection<Guid>。

内容的提问来源于stack exchange，提问作者John