WPF - How can I center all items in a WrapPanel?

Question

I'm using a WrapPanel as the ItemsPanel of an ItemsControl. Right now, the items in the control wrap like this:

|1234567  |
|890      |

I'd like them to wrap like this:

| 1234567 |
|   890   |

Conceptually, the layout process should align each line of items such that it's centered within the WrapPanel's bounds.

Can someone explain how this is possible please?

Solution 1

The built in WrapPanel will not allow you to align its content - only itself. Here is a technique that allows you to set HorizontalContentAlignment:

using System;
using System.Windows;
using System.Windows.Controls;

public class AlignableWrapPanel : Panel
{
    public HorizontalAlignment HorizontalContentAlignment
    {
        get { return (HorizontalAlignment)GetValue(HorizontalContentAlignmentProperty); }
        set { SetValue(HorizontalContentAlignmentProperty, value); }
    }

    public static readonly DependencyProperty HorizontalContentAlignmentProperty =
        DependencyProperty.Register("HorizontalContentAlignment", typeof(HorizontalAlignment), typeof(AlignableWrapPanel), new FrameworkPropertyMetadata(HorizontalAlignment.Left, FrameworkPropertyMetadataOptions.AffectsArrange));

    protected override Size MeasureOverride(Size constraint)
    {
        Size curLineSize = new Size();
        Size panelSize = new Size();

        UIElementCollection children = base.InternalChildren;

        for (int i = 0; i < children.Count; i++)
        {
            UIElement child = children[i] as UIElement;

            // Flow passes its own constraint to children
            child.Measure(constraint);
            Size sz = child.DesiredSize;

            if (curLineSize.Width + sz.Width > constraint.Width) //need to switch to another line
            {
                panelSize.Width = Math.Max(curLineSize.Width, panelSize.Width);
                panelSize.Height += curLineSize.Height;
                curLineSize = sz;

                if (sz.Width > constraint.Width) // if the element is wider then the constraint - give it a separate line                    
                {
                    panelSize.Width = Math.Max(sz.Width, panelSize.Width);
                    panelSize.Height += sz.Height;
                    curLineSize = new Size();
                }
            }
            else //continue to accumulate a line
            {
                curLineSize.Width += sz.Width;
                curLineSize.Height = Math.Max(sz.Height, curLineSize.Height);
            }
        }

        // the last line size, if any need to be added
        panelSize.Width = Math.Max(curLineSize.Width, panelSize.Width);
        panelSize.Height += curLineSize.Height;

        return panelSize;
    }

    protected override Size ArrangeOverride(Size arrangeBounds)
    {
        int firstInLine = 0;
        Size curLineSize = new Size();
        double accumulatedHeight = 0;
        UIElementCollection children = this.InternalChildren;

        for (int i = 0; i < children.Count; i++)
        {
            Size sz = children[i].DesiredSize;

            if (curLineSize.Width + sz.Width > arrangeBounds.Width) //need to switch to another line
            {
                ArrangeLine(accumulatedHeight, curLineSize, arrangeBounds.Width, firstInLine, i);

                accumulatedHeight += curLineSize.Height;
                curLineSize = sz;

                if (sz.Width > arrangeBounds.Width) //the element is wider then the constraint - give it a separate line                    
                {
                    ArrangeLine(accumulatedHeight, sz, arrangeBounds.Width, i, ++i);
                    accumulatedHeight += sz.Height;
                    curLineSize = new Size();
                }
                firstInLine = i;
            }
            else //continue to accumulate a line
            {
                curLineSize.Width += sz.Width;
                curLineSize.Height = Math.Max(sz.Height, curLineSize.Height);
            }
        }

        if (firstInLine < children.Count)
            ArrangeLine(accumulatedHeight, curLineSize, arrangeBounds.Width, firstInLine, children.Count);

        return arrangeBounds;
    }

    private void ArrangeLine(double y, Size lineSize, double boundsWidth, int start, int end)
    {
        double x = 0;
        if (this.HorizontalContentAlignment == HorizontalAlignment.Center)
        {
            x = (boundsWidth - lineSize.Width) / 2;
        }
        else if (this.HorizontalContentAlignment == HorizontalAlignment.Right)
        {
            x = (boundsWidth - lineSize.Width);
        }

        UIElementCollection children = InternalChildren;
        for (int i = start; i < end; i++)
        {
            UIElement child = children[i];
            child.Arrange(new Rect(x, y, child.DesiredSize.Width, lineSize.Height));
            x += child.DesiredSize.Width;
        }
    }
}

Solution 2

Unfortunately the stock WrapPanel will not do what you're asking for, because its intention is to fill all available space horizontally (or vertically) before wrapping. You will probably have to design your own WrapPanel to handle this case. You could start with this sample that shows how to create your own WrapPanel.

Solution 3

Version with VerticalContentAlignment (for vertical panels):

public class AlignableWrapPanel : Panel {
    public AlignableWrapPanel() {
        _orientation = Orientation.Horizontal;
    }

    private static bool IsWidthHeightValid(object value) {
        var v = (double)value;
        return (double.IsNaN(v)) || (v >= 0.0d && !double.IsPositiveInfinity(v));
    }

    public static readonly DependencyProperty ItemWidthProperty = DependencyProperty.Register("ItemWidth", typeof(double),
            typeof(AlignableWrapPanel), new FrameworkPropertyMetadata(double.NaN, FrameworkPropertyMetadataOptions.AffectsMeasure),
            IsWidthHeightValid);

    [TypeConverter(typeof(LengthConverter))]
    public double ItemWidth {
        get { return (double)GetValue(ItemWidthProperty); }
        set { SetValue(ItemWidthProperty, value); }
    }

    public static readonly DependencyProperty ItemHeightProperty = DependencyProperty.Register("ItemHeight", typeof(double),
            typeof(AlignableWrapPanel), new FrameworkPropertyMetadata(double.NaN, FrameworkPropertyMetadataOptions.AffectsMeasure),
            IsWidthHeightValid);

    [TypeConverter(typeof(LengthConverter))]
    public double ItemHeight {
        get { return (double)GetValue(ItemHeightProperty); }
        set { SetValue(ItemHeightProperty, value); }
    }

    public static readonly DependencyProperty OrientationProperty = StackPanel.OrientationProperty.AddOwner(typeof(AlignableWrapPanel),
            new FrameworkPropertyMetadata(Orientation.Horizontal, FrameworkPropertyMetadataOptions.AffectsMeasure, OnOrientationChanged));

    public Orientation Orientation {
        get { return _orientation; }
        set { SetValue(OrientationProperty, value); }
    }

    private static void OnOrientationChanged(DependencyObject d, DependencyPropertyChangedEventArgs e) {
        var p = (AlignableWrapPanel)d;
        p._orientation = (Orientation)e.NewValue;
    }

    private Orientation _orientation;

    private struct UvSize {
        internal UvSize(Orientation orientation, double width, double height) {
            U = V = 0d;
            _orientation = orientation;
            Width = width;
            Height = height;
        }

        internal UvSize(Orientation orientation) {
            U = V = 0d;
            _orientation = orientation;
        }

        internal double U;
        internal double V;
        private readonly Orientation _orientation;

        internal double Width {
            get { return (_orientation == Orientation.Horizontal ? U : V); }
            private set { if (_orientation == Orientation.Horizontal) U = value; else V = value; }
        }

        internal double Height {
            get { return (_orientation == Orientation.Horizontal ? V : U); }
            private set { if (_orientation == Orientation.Horizontal) V = value; else U = value; }
        }
    }

    protected override Size MeasureOverride(Size constraint) {
        var curLineSize = new UvSize(Orientation);
        var panelSize = new UvSize(Orientation);
        var uvConstraint = new UvSize(Orientation, constraint.Width, constraint.Height);
        var itemWidth = ItemWidth;
        var itemHeight = ItemHeight;
        var itemWidthSet = !double.IsNaN(itemWidth);
        var itemHeightSet = !double.IsNaN(itemHeight);

        var childConstraint = new Size(
                (itemWidthSet ? itemWidth : constraint.Width),
                (itemHeightSet ? itemHeight : constraint.Height));

        var children = InternalChildren;

        for (int i = 0, count = children.Count; i < count; i++) {
            var child = children[i];
            if (child == null) continue;

            //Flow passes its own constrint to children 
            child.Measure(childConstraint);

            //this is the size of the child in UV space 
            var sz = new UvSize(
                    Orientation,
                    (itemWidthSet ? itemWidth : child.DesiredSize.Width),
                    (itemHeightSet ? itemHeight : child.DesiredSize.Height));

            if (curLineSize.U + sz.U > uvConstraint.U) {
                //need to switch to another line 
                panelSize.U = Math.Max(curLineSize.U, panelSize.U);
                panelSize.V += curLineSize.V;
                curLineSize = sz;

                if (!(sz.U > uvConstraint.U)) continue;
                //the element is wider then the constrint - give it a separate line
                panelSize.U = Math.Max(sz.U, panelSize.U);
                panelSize.V += sz.V;
                curLineSize = new UvSize(Orientation);
            } else {
                //continue to accumulate a line
                curLineSize.U += sz.U;
                curLineSize.V = Math.Max(sz.V, curLineSize.V);
            }
        }

        //the last line size, if any should be added 
        panelSize.U = Math.Max(curLineSize.U, panelSize.U);
        panelSize.V += curLineSize.V;

        //go from UV space to W/H space
        return new Size(panelSize.Width, panelSize.Height);
    }

    protected override Size ArrangeOverride(Size finalSize) {
        var firstInLine = 0;
        var itemWidth = ItemWidth;
        var itemHeight = ItemHeight;
        double accumulatedV = 0;
        var itemU = (Orientation == Orientation.Horizontal ? itemWidth : itemHeight);
        var curLineSize = new UvSize(Orientation);
        var uvFinalSize = new UvSize(Orientation, finalSize.Width, finalSize.Height);
        var itemWidthSet = !double.IsNaN(itemWidth);
        var itemHeightSet = !double.IsNaN(itemHeight);
        var useItemU = (Orientation == Orientation.Horizontal ? itemWidthSet : itemHeightSet);

        var children = InternalChildren;

        for (int i = 0, count = children.Count; i < count; i++) {
            var child = children[i];
            if (child == null) continue;

            var sz = new UvSize(
                    Orientation,
                    (itemWidthSet ? itemWidth : child.DesiredSize.Width),
                    (itemHeightSet ? itemHeight : child.DesiredSize.Height));

            if (curLineSize.U + sz.U > uvFinalSize.U) {
                //need to switch to another line 
                ArrangeLine(finalSize, accumulatedV, curLineSize, firstInLine, i, useItemU, itemU);

                accumulatedV += curLineSize.V;
                curLineSize = sz;

                if (sz.U > uvFinalSize.U) {
                    //the element is wider then the constraint - give it a separate line 
                    //switch to next line which only contain one element 
                    ArrangeLine(finalSize, accumulatedV, sz, i, ++i, useItemU, itemU);

                    accumulatedV += sz.V;
                    curLineSize = new UvSize(Orientation);
                }

                firstInLine = i;
            } else {
                //continue to accumulate a line
                curLineSize.U += sz.U;
                curLineSize.V = Math.Max(sz.V, curLineSize.V);
            }
        }

        //arrange the last line, if any
        if (firstInLine < children.Count) {
            ArrangeLine(finalSize, accumulatedV, curLineSize, firstInLine, children.Count, useItemU, itemU);
        }

        return finalSize;
    }

    private void ArrangeLine(Size finalSize, double v, UvSize line, int start, int end, bool useItemU, double itemU) {
        double u;
        var isHorizontal = Orientation == Orientation.Horizontal;

        if (_orientation == Orientation.Vertical) {
            switch (VerticalContentAlignment) {
                case VerticalAlignment.Center:
                    u = (finalSize.Height - line.U) / 2;
                    break;
                case VerticalAlignment.Bottom:
                    u = finalSize.Height - line.U;
                    break;
                default:
                    u = 0;
                    break;
            }
        } else {
            switch (HorizontalContentAlignment) {
                case HorizontalAlignment.Center:
                    u = (finalSize.Width - line.U) / 2;
                    break;
                case HorizontalAlignment.Right:
                    u = finalSize.Width - line.U;
                    break;
                default:
                    u = 0;
                    break;
            }
        }

        var children = InternalChildren;
        for (var i = start; i < end; i++) {
            var child = children[i];
            if (child == null) continue;
            var childSize = new UvSize(Orientation, child.DesiredSize.Width, child.DesiredSize.Height);
            var layoutSlotU = (useItemU ? itemU : childSize.U);
            child.Arrange(new Rect(
                    isHorizontal ? u : v,
                    isHorizontal ? v : u,
                    isHorizontal ? layoutSlotU : line.V,
                    isHorizontal ? line.V : layoutSlotU));
            u += layoutSlotU;
        }
    }

    public static readonly DependencyProperty HorizontalContentAlignmentProperty = DependencyProperty.Register(nameof(HorizontalContentAlignment), typeof(HorizontalAlignment),
            typeof(AlignableWrapPanel), new FrameworkPropertyMetadata(HorizontalAlignment.Left, FrameworkPropertyMetadataOptions.AffectsArrange));

    public HorizontalAlignment HorizontalContentAlignment {
        get { return (HorizontalAlignment)GetValue(HorizontalContentAlignmentProperty); }
        set { SetValue(HorizontalContentAlignmentProperty, value); }
    }

    public static readonly DependencyProperty VerticalContentAlignmentProperty = DependencyProperty.Register(nameof(VerticalContentAlignment), typeof(VerticalAlignment),
            typeof(AlignableWrapPanel), new FrameworkPropertyMetadata(VerticalAlignment.Top, FrameworkPropertyMetadataOptions.AffectsArrange));

    public VerticalAlignment VerticalContentAlignment {
        get { return (VerticalAlignment)GetValue(VerticalContentAlignmentProperty); }
        set { SetValue(VerticalContentAlignmentProperty, value); }
    }
}

Solution 4

The simple answer is that you can't center align the contents of a WrapPanel. You can center align the panel itself, but the last line will still be left aligned within the panel.

Alterate suggestions:

Use Grid with rows and columns. If you are not adding items to the collection dynamically, this could work nicely.

Create your own version of the WrapPanel that works the way you need it to. This MSDN document describes how panels work and includes a section on creating custom panels. It also has a link to a sample custom panel.

Solution 5

Here is the Silverlight version

Special Thanks to @DTig,

using System.Windows.Controls;
using System.Windows;
using Telerik.Windows;
using System;
using System.Linq;

public class AlignableWrapPanel : Panel
{
    public HorizontalAlignment HorizontalContentAlignment
    {
        get { return (HorizontalAlignment)GetValue(HorizontalContentAlignmentProperty); }
        set { SetValue(HorizontalContentAlignmentProperty, value); }
    }

    public static readonly DependencyProperty HorizontalContentAlignmentProperty =
        DependencyProperty.Register("HorizontalContentAlignment", typeof(HorizontalAlignment), typeof(AlignableWrapPanel), new FrameworkPropertyMetadata(HorizontalAlignment.Left, FrameworkPropertyMetadataOptions.AffectsArrange));

    protected override Size MeasureOverride(Size constraint)
    {
        Size curLineSize = new Size();
        Size panelSize = new Size();

        UIElementCollection children = base.Children;

        for (int i = 0; i < children.Count; i++)
        {
            UIElement child = children[i] as UIElement;

            // Flow passes its own constraint to children
            child.Measure(constraint);
            Size sz = child.DesiredSize;

            if (curLineSize.Width + sz.Width > constraint.Width) //need to switch to another line
            {
                panelSize.Width = Math.Max(curLineSize.Width, panelSize.Width);
                panelSize.Height += curLineSize.Height;
                curLineSize = sz;

                if (sz.Width > constraint.Width) // if the element is wider then the constraint - give it a separate line                    
                {
                    panelSize.Width = Math.Max(sz.Width, panelSize.Width);
                    panelSize.Height += sz.Height;
                    curLineSize = new Size();
                }
            }
            else //continue to accumulate a line
            {
                curLineSize.Width += sz.Width;
                curLineSize.Height = Math.Max(sz.Height, curLineSize.Height);
            }
        }

        // the last line size, if any need to be added
        panelSize.Width = Math.Max(curLineSize.Width, panelSize.Width);
        panelSize.Height += curLineSize.Height;

        return panelSize;
    }

    protected override Size ArrangeOverride(Size arrangeBounds)
    {
        int firstInLine = 0;
        Size curLineSize = new Size();
        double accumulatedHeight = 0;
        UIElementCollection children = this.Children;

        for (int i = 0; i < children.Count; i++)
        {
            Size sz = children[i].DesiredSize;

            if (curLineSize.Width + sz.Width > arrangeBounds.Width) //need to switch to another line
            {
                ArrangeLine(accumulatedHeight, curLineSize, arrangeBounds.Width, firstInLine, i);

                accumulatedHeight += curLineSize.Height;
                curLineSize = sz;

                if (sz.Width > arrangeBounds.Width) //the element is wider then the constraint - give it a separate line                    
                {
                    ArrangeLine(accumulatedHeight, sz, arrangeBounds.Width, i, ++i);
                    accumulatedHeight += sz.Height;
                    curLineSize = new Size();
                }
                firstInLine = i;
            }
            else //continue to accumulate a line
            {
                curLineSize.Width += sz.Width;
                curLineSize.Height = Math.Max(sz.Height, curLineSize.Height);
            }
        }

        if (firstInLine < children.Count)
            ArrangeLine(accumulatedHeight, curLineSize, arrangeBounds.Width, firstInLine, children.Count);

        return arrangeBounds;
    }

    private void ArrangeLine(double y, Size lineSize, double boundsWidth, int start, int end)
    {
        double x = 0;
        if (this.HorizontalContentAlignment == HorizontalAlignment.Center)
        {
            x = (boundsWidth - lineSize.Width) / 2;
        }
        else if (this.HorizontalContentAlignment == HorizontalAlignment.Right)
        {
            x = (boundsWidth - lineSize.Width);
        }

        UIElementCollection children = Children;
        for (int i = start; i < end; i++)
        {
            UIElement child = children[i];
            var rect = new System.Windows.Rect(x, y, child.DesiredSize.Width, lineSize.Height);
            child.Arrange(rect);
            x += child.DesiredSize.Width;
        }
    }
}

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Comments

[Gman]

Nice class, thanks! I've adapted it to respect the value of FrameworkElement.HorizontalAlignment, if the child has one: gist.github.com/gmanny/7450651 (only ArrangeLine method is changed).

[Rover]

For stretch aligment: if (i == end - 1 && HorizontalContentAlignment == HorizontalAlignment.Stretch) { child.Arrange(new Rect(x, y, boundsWidth - x, lineSize.Height)); } else { child.Arrange(new Rect(x, y, child.DesiredSize.Width, lineSize.Height)); x += child.DesiredSize.Width; }

[Admin]

For some reason this solution works, but the accepted answer doesn't. Thank you for this.

[Abdul Saleem]

Perfect one. Thanks

[Jackie Jones]

Is it possible to extract from this a new method that, when called, will insert a linebreak? I could only get it to do so when I did some hacky code to trick the MeasureOverride into thinking it needed to wrap but I was hoping to have it work normally in a side method.

[Aeiddius]

I'am sorry. I'm a complete noob. How do i got about using this?