[Bug] [UWP] LayoutChildren method is not called in custom control after upgrade Xamarin.Forms 5.0

[bharathirajauk]

Description

I have created a custom control which is extended from ContentView in Xamarin.Forms. I have layout the Content for some scenario using LayoutChildren override method in Forms. This is working fine up to XF version 4.8. After upgrade to Xamarin.Forms 5.0, this LayouChildren override method is not called and Content also not displayed. I suspect that in Xamarin.Forms 5.0, layout process is not performed for Content.

Note: If i add the same custom control within Grid, custom control's Content is displayed.

1. I simply created a small custom control to reproduce my issue. Created custom control in Xamarin.Forms

public class CustomContentView: ContentView
    {
        public CustomContentView()
        {
           
        }

        protected override SizeRequest OnMeasure(double widthConstraint, double heightConstraint)
        {
            return base.OnMeasure(widthConstraint, heightConstraint);
        }
        protected override void LayoutChildren(double x, double y, double width, double height)
        {
            base.LayoutChildren(x, y, width, height);
        }
    }

2. Created custom renderer for this Forms control in UWP renderer project. Native UWP control is extended from ContentPresenter. I have converted Forms content to native and set to content control's content. Then assigned content control to Native control.

Native control

  public class CustomView : ContentPresenter
    {
        public CustomView()
        {
        }
    }

Renderer class

public class CustomRenderer : VisualElementRenderer<CustomControl.CustomContentView, CustomView>
    {
        private CustomView customView;
        public CustomRenderer()
        {
            this.AutoPackage = false;
        }
        protected override void OnElementChanged(ElementChangedEventArgs<CustomContentView> e)
        {
            if (e.NewElement == null)
            {
                return;
            }

            this.customView = new CustomView();
            this.customView.DataContext = this.Element.BindingContext;
            this.SetNativeControl(this.customView);

            if (Element.Content != null)
            {
                var renderer = Element.Content.GetOrCreateRenderer();
                var contentControl = new ContentControl();

                contentControl.Content = renderer.ContainerElement;
                this.customView.Content = contentControl;
            }
            base.OnElementChanged(e);
        }
    }

Steps to Reproduce

1. Run the attached sample (Sample link given in Reproduction Link)
2. Added Button control as content in my custom control. But it is not displayed.

Expected Behavior

Content (Button) should be displayed

Actual Behavior

Display with blank page.

Basic Information

• UWP: All SDK version
• NuGet Packages: Xamarin.Forms: 5.0.0.1905

Reproduction Link

Sample link with custom control: https://github.com/bharathirajauk/MySamples/blob/master/CustomControl_XF5.0.zip?raw=true

[PureWeen]

@hartez it looks like this is a regression from

#12017

It looks like because the Layout is no longer marshaled to the UI Thread the layout code never gets beyond this point

Xamarin.Forms/Xamarin.Forms.Core/Layout.cs

Line 473 in 51d840a

return false;

I'm guessing before when it was all being executed from Device.BeginInvoke that the renderers were getting attached before the layout code processed