Avalonia/src/Avalonia.Base/Layout/LayoutHelper.cs

using System;
using System.Diagnostics.CodeAnalysis;
using System.Runtime.CompilerServices;
using Avalonia.Utilities;
using Avalonia.VisualTree;

namespace Avalonia.Layout
{
    /// <summary>
    /// Provides helper methods needed for layout.
    /// </summary>
    public static class LayoutHelper
    {
        /// <summary>
        /// Epsilon value used for certain layout calculations.
        /// Based on the value in WPF LayoutDoubleUtil.
        /// </summary>
        public static double LayoutEpsilon { get; } = 0.00000153;

        /// <summary>
        /// Calculates a control's size based on its <see cref="Layoutable.Width"/>,
        /// <see cref="Layoutable.Height"/>, <see cref="Layoutable.MinWidth"/>,
        /// <see cref="Layoutable.MaxWidth"/>, <see cref="Layoutable.MinHeight"/> and
        /// <see cref="Layoutable.MaxHeight"/>.
        /// </summary>
        /// <param name="control">The control.</param>
        /// <param name="constraints">The space available for the control.</param>
        /// <returns>The control's size.</returns>
        public static Size ApplyLayoutConstraints(Layoutable control, Size constraints)
            => ApplyLayoutConstraints(new MinMax(control), constraints);

        internal static Size ApplyLayoutConstraints(MinMax minMax, Size constraints)
            => new(
                MathUtilities.Clamp(constraints.Width, minMax.MinWidth, minMax.MaxWidth),
                MathUtilities.Clamp(constraints.Height, minMax.MinHeight, minMax.MaxHeight));

        public static Size MeasureChild(Layoutable? control, Size availableSize, Thickness padding,
            Thickness borderThickness)
        {
            if (IsParentLayoutRounded(control, out double scale))
            {
                padding = RoundLayoutThickness(padding, scale);
                borderThickness = RoundLayoutThickness(borderThickness, scale);
            }

            if (control != null)
            {
                control.Measure(availableSize.Deflate(padding + borderThickness));
                return control.DesiredSize.Inflate(padding + borderThickness);
            }

            return new Size().Inflate(padding + borderThickness);
        }

        public static Size MeasureChild(Layoutable? control, Size availableSize, Thickness padding)
        {
            if (IsParentLayoutRounded(control, out double scale))
            {
                padding = RoundLayoutThickness(padding, scale);
            }

            if (control != null)
            {
                control.Measure(availableSize.Deflate(padding));
                return control.DesiredSize.Inflate(padding);
            }

            return new Size(padding.Left + padding.Right, padding.Bottom + padding.Top);
        }

        public static Size ArrangeChild(Layoutable? child, Size availableSize, Thickness padding, Thickness borderThickness)
        {
            if (IsParentLayoutRounded(child, out double scale))
            {
                padding = RoundLayoutThickness(padding, scale);
                borderThickness = RoundLayoutThickness(borderThickness, scale);
            }

            return ArrangeChildInternal(child, availableSize, padding + borderThickness);
        }

        public static Size ArrangeChild(Layoutable? child, Size availableSize, Thickness padding)
        {
            if(IsParentLayoutRounded(child, out double scale))
                padding = RoundLayoutThickness(padding, scale);

            return ArrangeChildInternal(child, availableSize, padding);
        }

        private static Size ArrangeChildInternal(Layoutable? child, Size availableSize, Thickness padding)
        {
            child?.Arrange(new Rect(availableSize).Deflate(padding));

            return availableSize;
        }

        private static bool IsParentLayoutRounded(Layoutable? child, out double scale)
        {
            var layoutableParent = (child as Visual)?.GetVisualParent() as Layoutable;

            if (layoutableParent == null || !layoutableParent.UseLayoutRounding)
            {
                scale = 1.0;
                return false;
            }

            scale = GetLayoutScale(layoutableParent);
            return true;
        }

        /// <summary>
        /// Invalidates measure for given control and all visual children recursively.
        /// </summary>
        public static void InvalidateSelfAndChildrenMeasure(Layoutable control)
        {
            void InnerInvalidateMeasure(Visual target)
            {
                if (target is Layoutable targetLayoutable)
                {
                    targetLayoutable.InvalidateMeasure();
                }

                var visualChildren = target.VisualChildren;
                var visualChildrenCount = visualChildren.Count;

                for (int i = 0; i < visualChildrenCount; i++)
                {
                    Visual child = visualChildren[i];

                    InnerInvalidateMeasure(child);
                }
            }

            if (control is Visual v)
                InnerInvalidateMeasure(v);
        }

        /// <summary>
        /// Obtains layout scale of the given control.
        /// </summary>
        /// <param name="control">The control.</param>
        /// <exception cref="Exception">Thrown when control has no root or returned layout scaling is invalid.</exception>
        public static double GetLayoutScale(Layoutable control)
            => control.VisualRoot is ILayoutRoot layoutRoot ? layoutRoot.LayoutScaling : 1.0;

        /// <summary>
        /// Rounds a size to integer values for layout purposes, compensating for high DPI screen
        /// coordinates by rounding the size up to the nearest pixel.
        /// </summary>
        /// <param name="size">Input size.</param>
        /// <param name="dpiScaleX">DPI along x-dimension.</param>
        /// <param name="dpiScaleY">DPI along y-dimension.</param>
        /// <returns>Value of size that will be rounded under screen DPI.</returns>
        /// <remarks>
        /// This is a layout helper method. It takes DPI into account and also does not return
        /// the rounded value if it is unacceptable for layout, e.g. Infinity or NaN. It's a helper
        /// associated with the UseLayoutRounding property and should not be used as a general rounding
        /// utility.
        /// </remarks>
        public static Size RoundLayoutSizeUp(Size size, double dpiScaleX, double dpiScaleY)
        {
            return new Size(RoundLayoutValueUp(size.Width, dpiScaleX), RoundLayoutValueUp(size.Height, dpiScaleY));
        }

        [SuppressMessage("ReSharper", "CompareOfFloatsByEqualityOperator", Justification = "The DPI scale should have been normalized.")]
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        internal static Size RoundLayoutSizeUp(Size size, double dpiScale)
        {
            // If DPI == 1, don't use DPI-aware rounding.
            return dpiScale == 1.0 ?
                new Size(
                    Math.Ceiling(size.Width),
                    Math.Ceiling(size.Height)) :
                new Size(
                    Math.Ceiling(RoundTo8Digits(size.Width) * dpiScale) / dpiScale,
                    Math.Ceiling(RoundTo8Digits(size.Height) * dpiScale) / dpiScale);
        }

        /// <summary>
        /// Rounds a thickness to integer values for layout purposes, compensating for high DPI screen
        /// coordinates.
        /// </summary>
        /// <param name="thickness">Input thickness.</param>
        /// <param name="dpiScaleX">DPI along x-dimension.</param>
        /// <param name="dpiScaleY">DPI along y-dimension.</param>
        /// <returns>Value of thickness that will be rounded under screen DPI.</returns>
        /// <remarks>
        /// This is a layout helper method. It takes DPI into account and also does not return
        /// the rounded value if it is unacceptable for layout, e.g. Infinity or NaN. It's a helper
        /// associated with the UseLayoutRounding property and should not be used as a general rounding
        /// utility.
        /// </remarks>
        public static Thickness RoundLayoutThickness(Thickness thickness, double dpiScaleX, double dpiScaleY)
        {
            return new Thickness(
                RoundLayoutValue(thickness.Left, dpiScaleX),
                RoundLayoutValue(thickness.Top, dpiScaleY),
                RoundLayoutValue(thickness.Right, dpiScaleX),
                RoundLayoutValue(thickness.Bottom, dpiScaleY)
            );
        }

        [SuppressMessage("ReSharper", "CompareOfFloatsByEqualityOperator", Justification = "The DPI scale should have been normalized.")]
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        internal static Thickness RoundLayoutThickness(Thickness thickness, double dpiScale)
        {
            // If DPI == 1, don't use DPI-aware rounding.
            return dpiScale == 1.0 ?
                new Thickness(
                    Math.Round(thickness.Left),
                    Math.Round(thickness.Top),
                    Math.Round(thickness.Right),
                    Math.Round(thickness.Bottom)) :
                new Thickness(
                    Math.Round(thickness.Left * dpiScale) / dpiScale,
                    Math.Round(thickness.Top * dpiScale) / dpiScale,
                    Math.Round(thickness.Right * dpiScale) / dpiScale,
                    Math.Round(thickness.Bottom * dpiScale) / dpiScale);
        }

        [SuppressMessage("ReSharper", "CompareOfFloatsByEqualityOperator", Justification = "The DPI scale should have been normalized.")]
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        internal static Point RoundLayoutPoint(Point point, double dpiScale)
        {
            // If DPI == 1, don't use DPI-aware rounding.
            return dpiScale == 1.0 ?
                new Point(
                    Math.Round(point.X),
                    Math.Round(point.Y)) :
                new Point(
                    Math.Round(point.X * dpiScale) / dpiScale,
                    Math.Round(point.Y * dpiScale) / dpiScale);
        }

        /// <summary>
        /// Calculates the value to be used for layout rounding at high DPI by rounding the value
        /// up or down to the nearest pixel.
        /// </summary>
        /// <param name="value">Input value to be rounded.</param>
        /// <param name="dpiScale">Ratio of screen's DPI to layout DPI</param>
        /// <returns>Adjusted value that will produce layout rounding on screen at high dpi.</returns>
        /// <remarks>
        /// This is a layout helper method. It takes DPI into account and also does not return
        /// the rounded value if it is unacceptable for layout, e.g. Infinity or NaN. It's a helper
        /// associated with the UseLayoutRounding property and should not be used as a general rounding
        /// utility.
        /// </remarks>
        public static double RoundLayoutValue(double value, double dpiScale)
        {
            // If DPI == 1, don't use DPI-aware rounding.
            return MathUtilities.IsOne(dpiScale) ?
                Math.Round(value) :
                Math.Round(value * dpiScale) / dpiScale;
        }

        /// <summary>
        /// Calculates the value to be used for layout rounding at high DPI by rounding the value up
        /// to the nearest pixel.
        /// </summary>
        /// <param name="value">Input value to be rounded.</param>
        /// <param name="dpiScale">Ratio of screen's DPI to layout DPI</param>
        /// <returns>Adjusted value that will produce layout rounding on screen at high dpi.</returns>
        /// <remarks>
        /// This is a layout helper method. It takes DPI into account and also does not return
        /// the rounded value if it is unacceptable for layout, e.g. Infinity or NaN. It's a helper
        /// associated with the UseLayoutRounding property and should not be used as a general rounding
        /// utility.
        /// </remarks>
        public static double RoundLayoutValueUp(double value, double dpiScale)
        {
            // If DPI == 1, don't use DPI-aware rounding.
            return MathUtilities.IsOne(dpiScale) ?
                Math.Ceiling(value) :
                Math.Ceiling(RoundTo8Digits(value) * dpiScale) / dpiScale;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static double RoundTo8Digits(double value)
        {
            // Round the value to avoid FP errors. This is needed because if `value` has a floating
            // point precision error (e.g. 79.333333333333343) then when it's multiplied by
            // `dpiScale` and rounded up, it will be rounded up to a value one greater than it
            // should be.
#if NET6_0_OR_GREATER
            return Math.Round(value, 8, MidpointRounding.ToZero);
#else
            // MidpointRounding.ToZero isn't available in netstandard2.0.
            return Math.Truncate(value * 1e8) / 1e8;
#endif
        }
    }
}