Expander and ForceUpdateSizeCommand

Question

Could anyone please describe the ForceUpdateSizeCommand attribute for the Expander?

I have this:
<StackLayout BindableLayout.ItemsSource="{Binding VARIABLE, Mode=TwoWay}">
<Expander>
<Expander.Header>....</Expander.header>
... more stacklayouts with bindable.layout which iterates over elements inside VARIABLE.

The single example I've seen regarding ForceUpdateSizeCommand adds this to the view-model:
public ICommand UpdateSizeCommand { get; set; }

And then perform UpdateSizeCommand.Execute(null) when a given variable is (for example ViewModel.VARIABLE[0].IsExpanded). That seems to work, but if I try to execute the command upon another triggered command or via Message.Send / Message.Subscribe it fails without notice - the Expander does not change its size. I tried the Message.Send/Subscribe thing because my VARIABLE is a composite of 2 different classes and those classes do not have access to the UpdateSizeCommand.

The problem I am trying to solve: The expander does not auto-resize when elements are hidden via IsVisible={Binding Foobar}. I've tried to convert to Grid etc but finally found out that the expander is the culprit.

Answer 1

Hello,​

Welcome to our Microsoft Q&A platform!

The expander does not auto-resize when elements are hidden via IsVisible={Binding Foobar}

To resize the Expander, try calling the ForceUpdateSize method to update the size of the expander. For example, the size of the font is changed, it's necessary to call the method update the size.

Check the code:

   <Expander x:Name="expander">  
       <Expander.Header>  
           <Label Text="Testing for Expander"  
               FontAttributes="Bold"  
               FontSize="Medium" />  
       </Expander.Header>  
       <Grid Padding="10">  
           <Grid.ColumnDefinitions>  
               <ColumnDefinition Width="Auto" />  
               <ColumnDefinition Width="Auto" />  
           </Grid.ColumnDefinitions>  
           <Image Source="https://upload.wikimedia.org/wikipedia/commons/thumb/8/87/Golden_lion_tamarin_portrait3.jpg/220px-Golden_lion_tamarin_portrait3.jpg"   
                   Aspect="AspectFill"  
                   HeightRequest="120"   
                   WidthRequest="120" />  
           <Label  
                   Text="The details of the expander's content,The details of the expander's content,The details of the expander's content,The details of the expander's content,The details of the expander's content,The details of the expander's content,The details of the expander's content"  
                   FontAttributes="Italic">  
               <Label.GestureRecognizers>  
                   <TapGestureRecognizer Tapped="TapGestureRecognizer_Tapped" />  
               </Label.GestureRecognizers>  
           </Label>  
       </Grid>  
   </Expander>  

Tap gesture event:

   private void TapGestureRecognizer_Tapped(object sender, EventArgs e)  
   {  
       var label = sender as Label;  
     
       if (label.FontSize == Device.GetNamedSize(NamedSize.Default, label))  
       {  
           label.FontSize = Device.GetNamedSize(NamedSize.Large, label);  
       }  
       else  
       {  
           label.FontSize = Device.GetNamedSize(NamedSize.Default, label);  
       }  
     
       expander.ForceUpdateSize();  
   }  

Effect gif: https://us.v-cdn.net/5019960/uploads/editor/3b/85d2wpveijbb.gif

Check the link:
https://learn.microsoft.com/en-us/xamarin/xamarin-forms/user-interface/expander#resize-an-expander-at-runtime

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Answer 2

Worked. My solution:

The ViewModel:
public bool IsExpanded
{
get {
return _isExpanded;
}
set {
_isExpanded = value;
OnpropertyChanged();
MessagingCenter.Send(this, "RefreshView");
}
}

Then in min Page-code-behind:
MessagingCenter.Subscribe<RiskSet>(this, "RefreshView", (sender) =>
{
foreach(Expander myExpander in MainStackLayout.Children)
{
myExpander.ForceUpdateSize();
}
});

As the expanders were created dynamically I couldnt give them I name so I had to fetch them from the MainStackLayout via Children().