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

问题描述

我刚刚开始使用 MVVM，遇到了一个障碍，希望有人能帮助我.我正在尝试使用 2 个列表框创建一个简单的视图.从第一个列表框中选择的内容将填充第二个列表框.我创建了一个类来存储我想要绑定的信息.

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(Observable Object 只是一个实现 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; }

在我的 ViewModel 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");

这适用于 Level1 项目并且它们正确显示在视图中.但是，当用户单击列表框中的项目时，我有一个命令会被调用，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;

出于某种原因，这不会更新第二个列表框的 UI.如果我在这个位置放置一个断点，我可以看到 Level2MenuItems 中存储了正确的信息.如果我编写了一个 foreach 循环并将它们单独添加到 Level2MenuItems 集合中，那么它确实可以正确显示.

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.

同样作为测试，我在构造函数中添加了以下内容:

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

Level2MenuItems = Level1MenuItems[0].SubCategories;

而且更新正确.

那么为什么代码在构造函数中或在循环时会按预期工作，而当用户单击列表框中的项目时却不会?

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.

取而代之的是

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

你需要

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，由于您正在通过绑定后发生的命令执行更改引用，因此您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

这篇关于ObservableCollection 不更新视图的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！