Improves multiline labels measure

samples/ViewModelsSamples/Axes/LabelsRotation/ViewModel.cs

@@ -1,4 +1,5 @@
-using System.Collections.ObjectModel;
+using System;
+using System.Collections.ObjectModel;
 using CommunityToolkit.Mvvm.ComponentModel;
 using LiveChartsCore;
 using LiveChartsCore.SkiaSharpView;
@@ -17,6 +18,10 @@ public partial class ViewModel
         new LineSeries<double>
         {
             Values = new ObservableCollection<double> { 200, 558, 458, 249, 457, 339, 587 },
+            TooltipLabelFormatter = (point) =>
+                $"This is {Environment.NewLine}" +
+                $"A multi-line label {Environment.NewLine}" +
+                $"With a value of {Environment.NewLine}" + point.PrimaryValue,
         }
     };
 
@@ -26,15 +31,18 @@ public partial class ViewModel
         {
             // Use the Label property to indicate the format of the labels in the axis
             // The Labeler takes the value of the label as parameter and must return it as string
-            Labeler = (value) => "Day " + value,
+            Labeler = (value) =>
+                $"This is {Environment.NewLine}" +
+                $"A multi-line label {Environment.NewLine}" +
+                $"With a value of {Environment.NewLine}" +value * 100,
 
             // The MinStep property lets you define the minimum separation (in chart values scale)
             // between every axis separator, in this case we don't want decimals,
             // so lets force it to be greater or equals than 1
             MinStep = 1,
 
             // labels rotations is in degrees (0 - 360)
-            LabelsRotation = 0,
+            LabelsRotation = 45,
 
             SeparatorsPaint = new SolidColorPaint(SKColors.LightGray, 2)
         }

src/LiveChartsCore/Drawing/ILabelGeometry.cs

@@ -38,6 +38,14 @@ public interface ILabelGeometry<TDrawingContext> : IGeometry<TDrawingContext>
     /// </value>
     Padding Padding { get; set; }
 
+    /// <summary>
+    /// Gets or sets the line height [in times the text height].
+    /// </summary>
+    /// <value>
+    /// The padding.
+    /// </value>
+    float LineHeight { get; set; }
+
     /// <summary>
     /// Gets or sets the vertical align.
     /// </summary>

src/skiasharp/LiveChartsCore.SkiaSharp/Drawing/Geometries/LabelGeometry.cs

@@ -74,6 +74,9 @@ public LabelGeometry()
     /// <inheritdoc cref="ILabelGeometry{TDrawingContext}.Padding" />
     public Padding Padding { get; set; } = new();
 
+    /// <inheritdoc cref="ILabelGeometry{TDrawingContext}.LineHeight" />
+    public float LineHeight { get; set; } = 1.75f;
+
     /// <inheritdoc cref="Geometry.OnDraw(SkiaSharpDrawingContext, SKPaint)" />
     public override void OnDraw(SkiaSharpDrawingContext context, SKPaint paint)
     {
@@ -92,9 +95,13 @@ public override void OnDraw(SkiaSharpDrawingContext context, SKPaint paint)
         var lines = GetLines(Text);
         double linesCount = lines.Length;
         var lineNumber = 0;
+        var lhd = (GetActualLineHeight(paint) - GetRawLineHeight(paint)) * 0.5f;
+
         foreach (var line in lines)
         {
-            DrawLine(line, -size.Height + (float)(++lineNumber / linesCount * size.Height), context, paint);
+            var ph = (float)(++lineNumber / linesCount) * size.Height;
+            var yLine = ph - size.Height;
+            DrawLine(line, yLine - lhd, context, paint);
         }
     }
 
@@ -178,25 +185,35 @@ private void DrawLine(string content, float yLine, SkiaSharpDrawingContext conte
     private LvcSize MeasureLines(SKPaint paint)
     {
         float w = 0f, h = 0f;
+        var lineHeight = GetActualLineHeight(paint);
 
         foreach (var line in GetLines(Text))
         {
             var bounds = new SKRect();
-            var boundsH = new SKRect();
 
             _ = paint.MeasureText(line, ref bounds);
 
-            // measure "|": hack to force the same max height?
-            // otherwise strings like "___" and "|||" will result in ---||| if the alignment is middle
-            _ = paint.MeasureText("|", ref boundsH);
-
             if (bounds.Width > w) w = bounds.Width;
-            h += boundsH.Height;
+            h += lineHeight;
         }
 
         return new LvcSize(w, h);
     }
 
+    private float GetActualLineHeight(SKPaint paint)
+    {
+        var boundsH = new SKRect();
+        _ = paint.MeasureText("█", ref boundsH);
+        return LineHeight * boundsH.Height;
+    }
+
+    private float GetRawLineHeight(SKPaint paint)
+    {
+        var boundsH = new SKRect();
+        _ = paint.MeasureText("█", ref boundsH);
+        return boundsH.Height;
+    }
+
     private string[] GetLines(string multiLineText)
     {
         return string.IsNullOrEmpty(multiLineText)

src/skiasharp/LiveChartsCore.SkiaSharp/VisualElements/LabelVisual.cs

@@ -24,7 +24,6 @@
 using LiveChartsCore.Drawing;
 using LiveChartsCore.Kernel;
 using LiveChartsCore.Measure;
-using LiveChartsCore.Motion;
 using LiveChartsCore.SkiaSharpView.Drawing;
 using LiveChartsCore.SkiaSharpView.Drawing.Geometries;
 using LiveChartsCore.VisualElements;
@@ -46,6 +45,7 @@ public class LabelVisual : VisualElement<SkiaSharpDrawingContext>
     internal LvcColor _backgroundColor;
     internal Padding _padding = new(0);
     internal double _rotation;
+    internal float _lineHeight = 1.75f;
     internal LvcPoint _translate = new();
     private LvcSize _actualSize = new();
     private LvcPoint _targetPosition = new();
@@ -99,6 +99,11 @@ public class LabelVisual : VisualElement<SkiaSharpDrawingContext>
     /// </summary>
     public Padding Padding { get => _padding; set { _padding = value; OnPropertyChanged(); } }
 
+    /// <summary>
+    /// Gets or sets the line height [times the text measured height].
+    /// </summary>
+    public float LineHeight { get => _lineHeight; set { _lineHeight = value; OnPropertyChanged(); } }
+
     internal override IPaint<SkiaSharpDrawingContext>?[] GetPaintTasks()
     {
         return new[] { _paint };
@@ -162,6 +167,7 @@ protected internal override void OnInvalidated(Chart<SkiaSharpDrawingContext> ch
         _labelGeometry.HorizontalAlign = HorizontalAlignment;
         _labelGeometry.Background = BackgroundColor;
         _labelGeometry.Padding = Padding;
+        _labelGeometry.LineHeight = LineHeight;
 
         var drawing = chart.Canvas.Draw();
         if (Paint is not null) _ = drawing.SelectPaint(Paint).Draw(_labelGeometry);

tests/LiveChartsCore.UnitTesting/LabelsMeasureTest.cs

@@ -0,0 +1,131 @@
+// The MIT License(MIT)
+//
+// Copyright(c) 2021 Alberto Rodriguez Orozco & LiveCharts Contributors
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+using System;
+using System.Collections.Generic;
+using LiveChartsCore.Kernel;
+using LiveChartsCore.SkiaSharpView;
+using LiveChartsCore.SkiaSharpView.Drawing;
+using LiveChartsCore.SkiaSharpView.Painting;
+using LiveChartsCore.SkiaSharpView.SKCharts;
+using LiveChartsCore.SkiaSharpView.VisualElements;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using SkiaSharp;
+
+namespace LiveChartsCore.UnitTesting;
+
+[TestClass]
+public class LabelsMeasureTest
+{
+    [TestMethod]
+    public void BasicCase()
+    {
+        var visuls = new List<ChartElement<SkiaSharpDrawingContext>>();
+
+        var y = 10;
+        var h = 20;
+        for (var i = 0; i < 10; i++)
+        {
+            visuls.Add(new LabelVisual
+            {
+                X = 10,
+                Y = y,
+                LocationUnit = Measure.MeasureUnit.Pixels,
+                VerticalAlignment = Drawing.Align.Start,
+                HorizontalAlignment = Drawing.Align.Start,
+                TextSize = h,
+                Text = "X-█-X",
+                Paint = new SolidColorPaint(SKColors.Black),
+                BackgroundColor = new Drawing.LvcColor(0, 0, 0, 50)
+            });
+
+            y += h + 10;
+            h += 20;
+        }
+
+        var chart = new SKCartesianChart
+        {
+            Width = 600,
+            Height = y + 10,
+            Series = Array.Empty<ISeries>(),
+            XAxes = new[] { new Axis { IsVisible = false } },
+            YAxes = new[] { new Axis { IsVisible = false } },
+            VisualElements = visuls
+        };
+
+        //_ = chart.GetImage();
+        chart.SaveImage("test.png"); // use this method to see the actual tested image
+    }
+
+    [TestMethod]
+    public void MultiLine()
+    {
+        var visuls = new List<ChartElement<SkiaSharpDrawingContext>>();
+
+        var y = 10f;
+        var h = 25;
+        var lines = 5;
+        var lineHeight = 0.1f;
+
+        for (var i = 0; i < 20; i++)
+        {
+            var t = "";
+            for (var j = 0; j < lines; j++)
+            {
+                t += $"{(j % 2 == 0 ? "-" : "")}█-{lineHeight:N2}-█{(j != lines - 1 ? Environment.NewLine : "")}";
+            }
+
+            var th = h * lines;
+
+            visuls.Add(new LabelVisual
+            {
+                X = 10,
+                Y = y,
+                LocationUnit = Measure.MeasureUnit.Pixels,
+                VerticalAlignment = Drawing.Align.Start,
+                HorizontalAlignment = Drawing.Align.Start,
+                TextSize = 25,
+                Text = t,
+                Paint = new SolidColorPaint(SKColors.Black),
+                BackgroundColor = new Drawing.LvcColor(0, 0, 0, 50),
+                LineHeight = lineHeight
+            });
+
+            y += th * lineHeight + 10;
+            lineHeight += 1f;
+        }
+
+        var chart = new SKCartesianChart
+        {
+            Width = 900,
+            Height = (int)(y + 10),
+            Series = Array.Empty<ISeries>(),
+            XAxes = new[] { new Axis { IsVisible = false } },
+            YAxes = new[] { new Axis { IsVisible = false } },
+            VisualElements = visuls
+        };
+
+        //_ = chart.GetImage();
+        chart.SaveImage("multi line labels test.png"); // use this method to see the actual tested image
+    }
+}
+