DataTemplate with TemplateSelector - bind to parent command

MJoehl

Hi all

I have a ListView with a TemplateSelector and two different DataTemplates (ViewCells). Behind the ListView is a ViewModel for the ConentPage with an ObservableCollection which contains some item-models. The Binding to this Item-Model in my templates work like a charm.

What I want: My ContentPage-ViewModel has some commands. The DataTemplate should use this command. How can I bind to this commands?

I know, that I can make the commands static and reference with x:Static to it. Or I can add the Command to my Item-Model. But both are no Option for my solution.

Thanks in advance

MJoehl

I have found a way to achive this. I made a Basic class for my templates and added a BindableProperty to it:

public class BaseTemplate : ViewCell
{
    public static BindableProperty ParentBindingContextProperty = BindableProperty.Create(nameof(ParentBindingContext), typeof(object), typeof(BaseTemplate), null);

    public object ParentBindingContext
    {
        get { return GetValue(ParentBindingContextProperty); }
        set { SetValue(ParentBindingContextProperty, value); }
    }
}

And in my TemplateSelector I do the following:

protected override DataTemplate OnSelectTemplate(object item, BindableObject container)
{
    var mytemplate = new DataTemplate(typeof (MyTemplateType));
    mytemplate .SetValue(BaseTemplate.ParentBindingContextProperty, container.BindingContext);

    return mytemplate;
}

Now I can Access the parent ViewModel with the property ParentBindingContext.

Is there a better way to do this?

MJoehl

I have found a way to achive this. I made a Basic class for my templates and added a BindableProperty to it:

public class BaseTemplate : ViewCell
{
    public static BindableProperty ParentBindingContextProperty = BindableProperty.Create(nameof(ParentBindingContext), typeof(object), typeof(BaseTemplate), null);

    public object ParentBindingContext
    {
        get { return GetValue(ParentBindingContextProperty); }
        set { SetValue(ParentBindingContextProperty, value); }
    }
}

And in my TemplateSelector I do the following:

protected override DataTemplate OnSelectTemplate(object item, BindableObject container)
{
    var mytemplate = new DataTemplate(typeof (MyTemplateType));
    mytemplate .SetValue(BaseTemplate.ParentBindingContextProperty, container.BindingContext);

    return mytemplate;
}

Now I can Access the parent ViewModel with the property ParentBindingContext.

Is there a better way to do this?

jiangzhiyi

thanks for @MichaelJhl,you are great!

RianDutra

@MJoehl how can I do it with XAML?

I tried the code below but it didn't work:

<Button Text="Test"
      Command="{Binding Path=ParentBindingContext.OpenCommand, Source={x:Reference MainPage}}"
      CommandParameter="{Binding .}" />

Lippel

It does work, thanks alot!

But I found one downside: You cant use CachingStrategy="RecycleElement" on the listview if you use this custom view cell

Any thoughts on that?

StinkyTowel

@MichaelJhl, did you inherit from BaseTemplate for your datatemplates? When I do, I get Error CS0260: Missing partial modifier on declaration of type 'SomeDataTemplate'; another partial declaration of this type exists.

StinkyTowel

I was able to get this going and it works very well (code behind and xaml), thanks @MichaelJhl for sharing!

Liger_Jerome

Hi !

Thanks for this tip, in theory I'm agree with it... but impossible to make it works on my side :-(

First, in your override "DataTemplate OnSelectTemplate", you can't create a new DataTemplate for each row... because on Android you will reach the "23 template count limit"... and I don't use RecycleElement as @Lippel suggested

So this is my code

'public class CheckItem_DatatemplateSelector : DataTemplateSelector
{
DataTemplate Acceptedtemplate { get; set; }
DataTemplate Refusedtemplate { get; set; }
DataTemplate Uncheckedtemplate { get; set; }

    protected override DataTemplate OnSelectTemplate(object item, BindableObject container)
    {
        if (((CheckablePacketItem)item).IsChecked)
        {
            if (((CheckablePacketItem)item).IsAccepted)
            {
                if (Acceptedtemplate == null)
                {
                    Acceptedtemplate = new DataTemplate(typeof(CkeckItem_accepted));
                }
                Acceptedtemplate.SetValue(BaseItem.ParentBindingContextProperty, container.BindingContext);
                return Acceptedtemplate;
            }
            else
            {
                if (Refusedtemplate == null)
                {
                    Refusedtemplate = new DataTemplate(typeof(CheckItem_refused));
                }
                Refusedtemplate.SetValue(BaseItem.ParentBindingContextProperty, container.BindingContext);
                return Refusedtemplate;
            }
        }
        else
        {
            if (Uncheckedtemplate == null)
            {
                Uncheckedtemplate = new DataTemplate(typeof(CheckItem_unchecked));
            }
            Uncheckedtemplate.SetValue(BaseItem.ParentBindingContextProperty, container.BindingContext);
            return Uncheckedtemplate;
        }
    }'

The binding seems to work.

But in my "CkeckItem_accepted" (or in the two other templates too), I try to bind a button with my "ParentBindingContext.m_CallExpCommand" and I don't know why but the command is never executed...

''

Does anybody that has made this tips working, can post a complete sample ? (or help if you see errors...)

Thanks in advance

KlimovGleb

@RianDutra Insted of ParentBindingContext u should use BindingContext.

UnreachableCode

Is RecycleElementAndDataTemplate a suitable fix to the issue @Lippel and @Liger_Jerome mention?

"The RecycleElementAndDataTemplate caching strategy builds on the RecycleElement caching strategy by additionally ensuring that when a ListView uses a DataTemplateSelector to select a DataTemplate, DataTemplates are cached by the type of item in the list. Therefore, DataTemplates are selected once per item type, instead of once per item instance."

Lippel

@UnreachableCode
Quite some time passed since I added my comment.
I believe RecycleElementAndDataTemplate didnt exist at that time. You should give it a try, it seems very promising.

Liger_Jerome

Hum... yes there was some time passed ...

But I remember that I finally got this working (and this is in production since months)

Opening the source-code (my memory is failing), I can see that :

• My Listview CachingStrategy is set to "RecycleElement"
• My Listview ItemTemplate is set to my DataTemplateSelector
• All my differents DataTemplates (used in my datatemplateSelector) inherits from a "BaseViewCell" where I have created a Bindable Property (to keep the "parentBindingContext of the cell/myPageViewModel" for triggering a Command (of my PageViewModel) on an item button click(with a property of the item as parameter of the command)

If you want some sample code, MP-me

UnreachableCode

Hm, I'm getting a pretty big performance hit when set to RecycleElementAndDataTemplate. I'm not sure I can use this. Will I have problems if I leave it set to RetainElement?