Set Min and Max height of items in ItemsControl

Question

I am using an ItemsControl to display a list of 1 - 10 items (usually 2 - 4). I am trying to satisfy all these requirements:

All rows must be the same height
All rows should be displayed at a height of 300 maximum if possible.
If there is not enough room to display all rows at 300 high, then display at the largest possible height.
If the largest possible height is less than 150, then display at maxsize and use a scrollbar
If the rows do not fill the page, then it must be vertically aligned at the top

This is what I have so far:

This is what it currently looks like with 1 item:

and this is what it should look like:

2 or 3 items display as expected:

For 4+ items, the scrollbar appears correctly but the items are all sized to 150, rather than 300:

Question

How do I align the content to top when there is only 1 item? (without breaking the other functionality obviously)

Bonus question: How do I get the items to resize to maxheight instead of minheight when there are 4+ items?

Alex.Wei · Accepted Answer

During the WPF layout process, measuring and arranging will be done in order. In most cast, if an UIElement has variable size, it will return minimum required as result. But if any layout alignment has been set to Stretch, UIElement will take as possible as it can in that direction in arranging. In your case, UniFormGrid will always return 160(which is Border.MinHeight + Border.Margin.Top + Border.Margin.Bottom) * the count of items as desired height in measuring result(which will stored in DesiredSize.DesiredSize.Height). But it will take ItemsControl.ActualHeight as arranged height since it has Stretch VerticalAlignment. So, if UniFormGrid.DesiredSize.Height was less then ItemsControl.ActualHeight, UniFormGrid and any child has Stretch VerticalAlignment will be stretch in vertically, until it encountered its MaxHeight. This is why your 1 item test resulted in the center. If you change UniFormGrid.VerticalAlignment or Border.VerticalAlignment to Top, you will get a 160 height item in the top of ItemsContorl.

The most simple solution to both questions is override the measuring result base on maximum row height and minimum row height. I write the codes in below and had done some basic tests, it seems to work just fine.

namespace WpfApp1
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
    }

    public class MyScrollViewer : ScrollViewer
    {
        public double DesiredViewportHeight;

        public MyScrollViewer() : base() { }

        protected override Size MeasureOverride(Size constraint)
        {
            // record viewport's height for late calculation 
            DesiredViewportHeight = constraint.Height;

            var result = base.MeasureOverride(constraint);

            // make sure that `ComputedVerticalScrollBarVisibility` will get correct value 
            if (ComputedVerticalScrollBarVisibility == Visibility.Visible && ExtentHeight <= ViewportHeight)
                result = base.MeasureOverride(constraint);

            return result;
        }
    }

    public class MyUniformGrid : UniformGrid
    {
        private MyScrollViewer hostSV;
        private ItemsControl hostIC;

        public MyUniFormGrid() : base() { }

        public double MaxRowHeight { get; set; }
        public double MinRowHeight { get; set; }

        protected override Size MeasureOverride(Size constraint)
        {
            if (hostSV == null)
            {
                hostSV = VisualTreeHelperEx.GetAncestor(this);
                hostSV.SizeChanged += (s, e) =>
                {
                    if (e.HeightChanged)
                    {
                        // need to redo layout pass after the height of host had changed.  
                        this.InvalidateMeasure();
                    }
                };
            }

            if (hostIC == null)
                hostIC = VisualTreeHelperEx.GetAncestor(this);

            var viewportHeight = hostSV.DesiredViewportHeight;
            var rows = hostIC.Items.Count;
            var rowHeight = viewportHeight / rows;
            double desiredHeight = 0;

            // calculate the correct height
            if (rowHeight > MaxRowHeight || rowHeight < MinRowHeight)
                desiredHeight = MaxRowHeight * rows;
            else
                desiredHeight = viewportHeight;

            var result = base.MeasureOverride(constraint);

            return new Size(result.Width, desiredHeight);
        }
    }

    public class VisualTreeHelperEx
    {
        public static T GetAncestor(DependencyObject reference, int level = 1) where T : DependencyObject
        {
            if (level < 1)
                throw new ArgumentOutOfRangeException(nameof(level));

            return GetAncestorInternal(reference, level);
        }

        private static T GetAncestorInternal(DependencyObject reference, int level) where T : DependencyObject
        {
            var parent = VisualTreeHelper.GetParent(reference);

            if (parent == null)
                return null;

            if (parent is T && --level == 0)
                return (T)parent;

            return GetAncestorInternal(parent, level);
        }
    }
}

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Set Min and Max height of items in ItemsControl

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