ObservableCollection不更新视图 [英] ObservableCollection not updating View

问题描述

我只是从MVVM开始，遇到了障碍，希望有人可以帮助我.我正在尝试创建一个带有2个列表框的简单视图.从第一个列表框选择的内容将填充第二个列表框.我创建了一个类，用于存储要绑定的信息.

I am just starting with MVVM and have hit a hurdle that I hope someone can help me with. I am trying to create a simple View with 2 listboxes. A selection from the first listbox will populate the second list box. I have a class created that stores the information I want to bind to.

MyObject类(可观察对象只是实现INotifyPopertyChanged的基类)

MyObject Class (Observable Object is just a base class that implements INotifyPopertyChanged)

public class MyObject : ObservableObject
{
    String _name = String.Empty;
    ObservableCollection<MyObject> _subcategories;

    public ObservableCollection<MyObject> SubCategories
    {
        get { return _subcategories; }

        set
        {
            _subcategories = value;
            RaisePropertyChanged("SubCategories");
        }
    }

    public String Name
    {
        get { return _name; }
        set
        {
            _name = value;
            RaisePropertyChanged("Name");
        }
    }


    public MyObject()
    {
        _subcategories = new ObservableCollection<EMSMenuItem>();
    }
}

在我的视图模型中，我创建了两个ObservableCollections

In my viewmodel I have two ObservableCollections created

public ObservableCollection<EMSMenuItem> Level1MenuItems { get; set; }
public ObservableCollection<EMSMenuItem> Level2MenuItems { get; set; }

在ViewModel的构造函数中，我有:

In my constructor of the ViewModel I have:

this.Level1MenuItems = new ObservableCollection<EMSMenuItem>();
this.Level2MenuItems = new ObservableCollection<EMSMenuItem>();
this.Level1MenuItems = LoadEMSMenuItems("Sample.Xml");

这对于Level1项效果很好，并且可以在视图中正确显示.但是，我有一个命令，当用户单击列表框中的项目时会调用该命令，该命令具有以下内容:

That works fine for the Level1 items and they correctly show in the View. However I have a command that gets called when the user clicks an item in the listbox, which has the following:

Level2MenuItems = ClickedItem.SubCategories;

由于某些原因，这不会更新第二个列表框的UI.如果在此位置放置一个断点，我可以看到Level2MenuItems中存储了正确的信息.如果我编写了一个foreach循环并将其分别添加到Level2MenuItems集合中，则它可以正确显示.

For some reason this does not update the UI of the second listbox. If I put a breakpoint at this location I can see that Level2MenuItems has the correct information stored in it. If I write a foreach loop and add them individually to the Level2MenuItems collection then it does display correctly.

作为测试，我还向构造函数添加了以下内容:

Also as a test I added the following to the constructor:

Level2MenuItems = Level1MenuItems[0].SubCategories;

并且更新正确.

那么，为什么代码在构造函数中或在循环浏览时能按预期工作，而在用户单击列表框中的项目时却不能运行?

So why would the code work as expected in the constructor, or when looping through, but not when a user clicks on an item in the listbox?

推荐答案

您需要在Level2MenuItems属性上引发更改通知.

You need to raise the change notification on the Level2MenuItems property.

代替

public ObservableCollection<EMSMenuItem> Level2MenuItems { get; set; }

您需要

private ObservableCollection<EMSMenuItem> _level2MenuItems;
public ObservableCollection<EMSMenuItem> Level2MenuItems
{
    get { return _level2MenuItems; }
    set 
     {
        _level2MenuItems = value; 
        RaisePropertyChanged("Level2MenuItems");
     }
 }

前者在构造函数中起作用的原因是绑定尚未发生.但是，由于您是通过绑定后发生的命令执行来更改引用的，因此需要告诉视图它已更改

The reason the former works in the constructor is that the Binding has not taken place yet. However since you are changing the reference via a command execute which happens after the binding you need to tell view that it changed

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