WPF 将 ComboBox 绑定到我的 ViewModel

WPF Binding ComboBox to my ViewModel

提问by pixel

I am trying to bind my ViewModel to my ComboBox. I have ViewModel class defined like this:

我正在尝试将我的 ViewModel 绑定到我的 ComboBox。我有这样定义的 ViewModel 类：

class ViewModel
{
    public ViewModel()
    {
        this.Car= "VW";
    }

    public string Car{ get; set; }
}

I set this ViewModel as DataContext in Window_Load like:

我在 Window_Load 中将此 ViewModel 设置为 DataContext ，例如：

   private void Window_Loaded(object sender, RoutedEventArgs e)
    {
        this.DataContext = new CarModel();
    }

Then in my xaml, I do this to bind my ComboBox to this ViewModel. I want to show the "VW" as selected by default in my ComboBox:

然后在我的 xaml 中，我这样做是为了将我的 ComboBox 绑定到这个 ViewModel。我想在我的 ComboBox 中显示默认选择的“VW”：

        <ComboBox Name="cbCar" SelectedItem="{Binding Car, UpdateSourceTrigger=PropertyChanged}">
            <ComboBoxItem Tag="Mazda">Mazda</ComboBoxItem>
            <ComboBoxItem Tag="VW">VW</ComboBoxItem>
            <ComboBoxItem Tag="Audi">Audi</ComboBoxItem>
        </ComboBox>

I have 2 questions:

我有两个问题：

1. How do I set default value selected in Combo Box to "VW" (once form loads, it should show "VW" in combo box).
2. Instead of setting ComboBoxItems like above in xaml, how to I set it in my ViewModel and then load these in ComboBox?

1. 如何将组合框中选择的默认值设置为“VW”（一旦表单加载，它应该在组合框中显示“VW”）。
2. 不是在 xaml 中像上面那样设置 ComboBoxItems，如何在我的 ViewModel 中设置它，然后在 ComboBox 中加载它们？

Thanks,

谢谢，

UPDATE: So far, I manage to implement this but I get error as below in the ViewModel c-tor:

更新：到目前为止，我设法实现了这一点，但在 ViewModel c-tor 中出现如下错误：

namespace MyData
{
    class ViewModel
    {
        public ViewModel()
        {
            this.Make = "";
            this.Year = 1;

            this.DefaultCar = "VW"; //this is where I get error 'No string allowed'
        }

        public IEnumerable<Car> Cars
        {
            get
            {
                var cars = new Car[] { new Car{Model="Mazda"}, new Car{Model="VW"}, new Car{Model="Audi"} };
                DefaultCar = cars.FirstOrDefault(car => car.Model == "VW"); 
            }
        }

        public string Make { get; set; }
        public int Year { get; set; }
        public Car DefaultCar { get; set; }
    }

    class Car
    {
        public string Model { get; set; }
    }
}

回答by E-Bat

As you are goint to implement MVVM it will be a lot better if you start to think in objects to represent Cars in your application:

当您将要实现 MVVM 时，如果您开始考虑在应用程序中使用对象来表示汽车，那将会好得多：

    public class ViewModel
    {    
            public Car SelectedCar{ get; set; }
            public IEnumerable<Car> Cars{ 
                get  {
                    var cars = YOUR_DATA_STORE.Cars.ToList();
                    SelectedCar = cars.FirstOrDefault(car=>car.Model == "VW");
                    return cars;
                }
            }
    }

    public class Car 
    {
        public string Model {get;set;}
        public string Make { get; set; }
        public int Year { get; set; }
    }

Your Xaml:

你的 Xml：

<ComboBox SelectedItem="{Binding SelectedCar}", ItemsSource="{Binding Cars}" 
        UpdateSourceTrigger=PropertyChanged}"/>

回答by FUR10N

1. Default Value: If you set viewModel.Car = "VW", then it should auto-select that item in the combo box. For this to work you will need to either implement INotifyPropertyChangedor set Carbefore you set DataContext.

1. 默认值：如果您设置了viewModel.Car = "VW"，则它应该在组合框中自动选择该项目。为此，您需要在INotifyPropertyChanged设置Car之前实现或设置DataContext。

INotifyPropertyChanged implementation might look like:

INotifyPropertyChanged 实现可能如下所示：

class ViewModel : INotifyPropertyChanged
{
    private string car;

    public ViewModel()
    {
        this.Car = "VW";
        this.Cars = new ObservableCollection<string>() { "Mazda", "VW", "Audi" };
    }

    public string Car
    {
        get
        {
            return this.car;
        }
        set
        {
            if (this.car != value)
            {
                this.car = value;
                OnPropertyChanged();
            }
        }
    }

    public ObservableCollection<string> Cars { get; }

    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

2. Bind ItemsSourceand SelectedItem.

2. 绑定ItemsSource和SelectedItem。

<ComboBox ItemsSource="{Binding Cars}"
          SelectedItem="{Binding Car, Mode=TwoWay}">
</ComboBox>

You can also set ComboBox.ItemTemplateif your source or view is more complex than just displaying a string:

您还可以设置ComboBox.ItemTemplate源或视图是否比仅显示字符串更复杂：

    <ComboBox.ItemTemplate>
        <DataTemplate>
            <TextBlock Text="{Binding}" />
        </DataTemplate>
    </ComboBox.ItemTemplate>

In the view model just add a list property:

在视图模型中只需添加一个列表属性：

public ObservableCollection<string> Cars { get; set; }

It doesn't have to be ObservableCollectionbut that type will auto-update the UI whenever you change the collection.

它不一定是，ObservableCollection但只要您更改集合，该类型就会自动更新 UI。