Xamarin.Forms - 未调用扩展器命令

Question

I really don't understand what I am doing wrong, I have using toolkits expander and I am trying to call a method when the expander header is being tapped. According to their docs:

Command, of type ICommand, which is executed when the Expander header is tapped.

so i tried this:

 <xct:Expander Command="{Binding GetMathSubCatgories}">
                            <xct:Expander.Header>
                                <Frame Padding="10" Margin="10" HasShadow="False" BorderColor="LightGray" VerticalOptions="CenterAndExpand">
                                    <StackLayout Orientation="Horizontal">
                                        <Image Source="{Binding icon}" WidthRequest="25" HeightRequest="25"></Image>
                                        <Label Text="{Binding name}" TextColor="{Binding textColor}" FontSize="Large" FontAttributes="Bold" HeightRequest="35" VerticalOptions="CenterAndExpand"></Label>
                                    </StackLayout>
                                </Frame>
                            </xct:Expander.Header>
                            <Grid Padding="10">
                                <Grid.ColumnDefinitions>
                                    <ColumnDefinition Width="Auto" />
                                </Grid.ColumnDefinitions>
                                <ListView x:Name="SubCategories" ItemsSource="{Binding subCategories}" ItemSelected="SubCategories_ItemSelected">
                                    <ListView.ItemTemplate>
                                        <DataTemplate>
                                            <ViewCell>
                                                <StackLayout>
                                                    <Label Text="{Binding name}" TextColor="#02cc9d" FontAttributes="Bold" HeightRequest="35" VerticalOptions="CenterAndExpand"></Label>
                                                </StackLayout>
                                            </ViewCell>
                                        </DataTemplate>
                                    </ListView.ItemTemplate>
                                </ListView>
                            </Grid>
                        </xct:Expander>

with this in my code behind:

public Command GetMathSubCatgories
        {
            get
            {
                return new Command((obj) =>
                {
                    Console.Write("Here");
                });
            }
        }

But its not being called, what am I doing wrong?

Here is my full code behind:

public partial class AssignTaskPage : ContentPage
{

    

    public AssignTaskPage()
    {
        InitializeComponent();

        GetMathSubCatgories = new Command(() => MathSubCatgoriesCommand());

    }

    public ICommand GetMathSubCatgories { get; private set; }
    void MathSubCatgoriesCommand()
    {
        Console.Write("Here");
    }

}

Answer 1

You could use the code below.

 public class CatgoryViewModel : INotifyPropertyChanged
 {
    public CatgoryViewModel()
    {           
        GetMathSubCatgories = new Command(() => MathSubCatgoriesCommand());            
     }
      public ICommand GetMathSubCatgories { get; private set; }   
      void MathSubCatgoriesCommand()
      {
        Console.Write("Here");
      }
 }

OutPut: https://imgur.com/Vda9p2p

Update:

If you do not do that in viewmodel, you could try the code below for code behind.

 public partial class Page3 : ContentPage
{        
    public ObservableCollection<Catgory> catgories { get; set; }
    public Page3()
    {
        InitializeComponent();
        catgories = new ObservableCollection<Catgory>()
        {
            new Catgory{ icon="cactus_24px.png", name="A", textColor="Red"},
            new Catgory{ icon="cactus_24px.png", name="B", textColor="Green"},
            new Catgory{ icon="cactus_24px.png", name="C", textColor="Red"},
            new Catgory{ icon="cactus_24px.png", name="D", textColor="Green"},
            new Catgory{ icon="cactus_24px.png", name="E", textColor="Red"},
            new Catgory{ icon="cactus_24px.png", name="F", textColor="Green"},
            new Catgory{ icon="cactus_24px.png", name="G", textColor="Red"},
            new Catgory{ icon="cactus_24px.png", name="H", textColor="Green"},
            new Catgory{ icon="cactus_24px.png", name="I", textColor="Red"},
        };
        GetMathSubCatgories = new Command(() => MathSubCatgoriesCommand());
        this.BindingContext = this;
    }
    public ICommand GetMathSubCatgories { get; private set; }
    void MathSubCatgoriesCommand()
    {
        Console.Write("Here");
    }

    private void SubCategories_ItemSelected(object sender, SelectedItemChangedEventArgs e)
    {

    }
}
public class Catgory
{
    public string icon { get; set; }
    public string name { get; set; }
    public string textColor { get; set; }

}

Answer 2

you need to assign a BindingContext in order for binding to work

public AssignTaskPage()
{
    InitializeComponent();

    GetMathSubCatgories = new Command(() => MathSubCatgoriesCommand());

    this.BindingContext = this;
}