以编程方式添加控件，UI不更新

Question

我只是在和.Net毛伊玩。我希望从Rest服务中检索信息，并根据结果以编程方式向VerticalStackLayout添加按钮。调试解决方案时，按钮将添加到VerticalStackLayout中，但UI不会更新。

在这里，代码片段

var  btn  = new Button();
btn.Text = "Button " + count + " added";
btn.Clicked += OnCounterClicked;
btn.HorizontalOptions = LayoutOptions.Center;
VLayout.Add(btn);

这里的XAML

  <ScrollView>
    <VerticalStackLayout x:Name="VLayout"
        Spacing="25" 
        Padding="30,0" 
        VerticalOptions="Center">
        <Image
            Source="dotnet_bot.png"
            SemanticProperties.Description="Cute dot net bot waving hi to you!"
            HeightRequest="200"
            HorizontalOptions="Center" />            
        <Label 
            Text="Hello, World!"
            SemanticProperties.HeadingLevel="Level1"
            FontSize="32"
            HorizontalOptions="Center" />        
        <Label 
            Text="Welcome to .NET Multi-platform App UI"
            SemanticProperties.HeadingLevel="Level2"
            SemanticProperties.Description="Welcome to dot net Multi platform App U I"
            FontSize="18"
            HorizontalOptions="Center" />
            <Entry x:Name="entry"
       Placeholder="Enter text"
       TextChanged="OnTextChanged"
       Completed="OnTextCompleted" />           
        <Button 
            x:Name="KommenBtn"
            Text="Kommen"
            SemanticProperties.Hint="Counts the number of times you click"
            Clicked="OnCounterClicked"
            HorizontalOptions="Center" />
        <Button 
            x:Name="GehenBtn"
            Text="Gehen"
            SemanticProperties.Hint="Counts the number of times you click"
            Clicked="OnCounterClicked"
            HorizontalOptions="Center" />
    </VerticalStackLayout>
</ScrollView>

提前感谢您的提示和帮助！带宽

Answer 1

尝试更新主线程上的UI，您可以将代码包装到Application.Current.Dispatcher.Dispatch中。

样本代码

Application.Current.Dispatcher.Dispatch(() =>
{
    var  btn  = new Button();
    btn.Text = "Button " + count + " added";
    btn.Clicked += OnCounterClicked;
    btn.HorizontalOptions = LayoutOptions.Center;
    VLayout.Add(btn);
    
});

Answer 2

若要在影响控件的层次结构(或位置)的更改后更新UI：

    (VLayout as IView).InvalidateArrange();

注:这大致相当于Xamarin.Forms layout.ForceLayout();

如果尚未在UI MainThread上运行的代码中，请将其包装在Dispatch中：

Dispatcher.Dispatch(() =>
    (VLayout as IView).InvalidateArrange());