所以我的情况是:我希望能够使用MongoDB将MVVM与我的WPF应用程序一起使用。我对MVVM很陌生(我对它知之甚少),但我有一些使用.NET和WPF的经验。
我有一个用于调用MongoDB集合的名称空间,其中Model组件作为一个名为"User"的类存储在那里
模型(在单独的命名空间中):
public class User
{
[BsonElement("_id")]
public ObjectId Id { get; set; }
public string name { get; set; }
// other methods listed here
public async static Task<List<User>> getUserList()
{
// allows me to get a list of users
var col = MongoDBServer<User>.openMongoDB("Users");
var filter = Builders<User>.Filter.Exists("name");
List<User> userList = await col.Find(filter).ToListAsync();
return userList;
}
}
我创建了一个非常基本的ViewModelBase(抽象ViewModelBase):
public abstract class ViewModelBase : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged(string propertyName)
{
PropertyChangedEventHandler handler = this.PropertyChanged;
if(handler == null)
{
var e = new PropertyChangedEventArgs(propertyName);
handler(this, e);
}
}
}
以及用于处理用户列表(ViewModel)的派生类:
public class UserListViewModel : ViewModelBase
{
private User _user;
private ObservableCollection<User> _userList;
public User user
{
get { return _user; }
set
{
_user = value;
OnPropertyChanged("user");
}
}
public ObservableCollection<User> userList
{
get { return _userList; }
set
{
_userList = value;
OnPropertyChanged("userList");
}
}
public UserListViewModel()
{
user = new User();
this.userList = new ObservableCollection<User>();
// since MongoDB operations are asyncrhonous, the async method "getUserList()" is used to fill the observable collection
getUserList().Wait();
}
public async Task getUserList()
{
var UserListRaw = await User.getUserList();
this.userList = new ObservableCollection<User>(UserListRaw);
}
}
视图组件是一个带有列表框的简单窗口,如下所示(视图):
<Window x:Class="UserManagementMVVM.UsersWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:UserManagementMVVM"
mc:Ignorable="d"
Title="UsersWindow" Height="300" Width="300">
<Window.Resources>
<local:UserListViewModel x:Key="ViewModel"/>
<!-- Receiving error for this XAML block saying "Object reference not set to instance of an object -->
</Window.Resources>
<Grid DataContext="{Binding ViewModel}">
<ListBox Margin="5" ItemsSource="{Binding userList}"/>
</Grid>
</Window>
App.Xaml及其代码索引保持不变,View的代码索引也是如此。
当我运行程序时,什么都不显示(即:窗口启动,但ListBox是空的,即使有数据)。我将很快添加一些按钮功能,这些功能将使用MongoDB执行原子操作。
近两周来,我一直在尝试制作自己的MVVM程序,但没有成功。如有任何协助,我们将不胜感激。
您没有将getUserList()返回值放入变量中
我想你的意思是做以下
Task.Run(async ()=>this.userList = await getUserList());
这将起作用——你应该考虑是否要等待任务完成,然后在它之后放置.Wait()
。
您的另一个问题可能是在应该使用StaticResource而不是绑定的上下文中绑定ViewModel的方式
像这样:
<Grid DataContext="{StaticResource ViewModel}">
<Window x:Class="UserManagementMVVM.UsersWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:UserManagementMVVM"
mc:Ignorable="d"
Title="UsersWindow" Height="300" Width="300">
<Window.DataContext>
<!--You have to set the DataContext -->
<local:UserListViewModel x:Key="ViewModel"/>
</Window.DataContext>
<Grid>
<ListBox Margin="5" ItemsSource="{Binding userList}"/>
</Grid>
</Window>
您必须正确设置DataContext。我换了你的xaml。但我更喜欢在Codebehin或app.xaml.cs.中为主窗口设置DataContext
例如:app.xaml.cs
protected override void OnStartup(StartupEventArgs e)
{
var data = new MainWindowViewmodel();
this.MainWindow = new MainWindow(data);
this.MainWindow.Show();
}
我的视图的所有其他DataContext都是用ResourceDictionary 中的DataTemplates完成的
<DataTemplate DataType="{x:Type local:MyOtherViewmodel}">
<local::MyOtherViewmodelView />
</DataTemplate>
我想赞扬gilMishal和blindmes为我指明了正确的方向。你的两个答案都有帮助。这是我更新的(和功能!)代码:
App.xaml.cs已修改如下(归功于blindmeis):
public partial class App : Application
{
protected override void OnStartup(StartupEventArgs e)
{
base.OnStartup(e);
UsersWindow window = new UsersWindow();
var ViewModel = new UserListViewModel();
window.DataContext = ViewModel;
window.Show();
}
}
ViewModel已更新:
public class UserListViewModel : ViewModelBase
{
private User _user;
private ObservableCollection<string> _userList; // changed from "User" class to string
public User user
{
get { return _user; }
set
{
_user = value;
OnPropertyChanged("user");
}
}
public ObservableCollection<string> userList
{
get { return _userList; }
set
{
_userList = value;
OnPropertyChanged("userList");
}
}
public UserListViewModel()
{
userList = new ObservableCollection<string>();
Task.Run(async () => this.userList = await getUserList()); // Credit to gilMishal
}
public async Task<ObservableCollection<string>> getUserList()
{
var UserListRaw = await User.getUserList();
var userListOC = new ObservableCollection<string>();
foreach (var doc in UserListRaw) // extracting the "name" property from each "User" object
{
userListOC.Add(doc.name);
}
return userListOC;
}
}
和观点:
<Window x:Class="UserManagementMVVM.UsersWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:UserManagementMVVM"
mc:Ignorable="d"
Title="UsersWindow" Height="300" Width="300">
<Window.Resources>
<local:UserListViewModel x:Key="ViewModel"/>
</Window.Resources>
<Grid> <!-- data context removed from here, credit blindmeis -->
<ListBox Margin="5" ItemsSource="{Binding userList}"/>
</Grid>
</Window>