QuestPDF/QuestPDF.Previewer/InteractiveCanvas.cs

using System.Diagnostics;
using Avalonia;
using Avalonia.Platform;
using Avalonia.Rendering.SceneGraph;
using Avalonia.Skia;
using QuestPDF.Helpers;
using SkiaSharp;

namespace QuestPDF.Previewer;

class InteractiveCanvas : ICustomDrawOperation
{
    public Rect Bounds { get; set; }
    public ICollection<RenderedPageInfo> Pages { get; set; }

    private float Width => (float)Bounds.Width;
    private float Height => (float)Bounds.Height;
    
    public float Scale { get; private set; } = 1;
    public float TranslateX { get; private set; }
    public float TranslateY { get; private set; }

    private const float MinScale = 0.1f;
    private const float MaxScale = 10f;
    
    private const float PageSpacing = 50f;
    private const float SafeZone = 50f;

    private void LimitScale()
    {
        Scale = Math.Max(Scale, MinScale);
        Scale = Math.Min(Scale, MaxScale);
    }
    
    private void LimitTranslate()
    {
        var totalHeight = Pages.Sum(x => x.Size.Height) + (Pages.Count - 1) * PageSpacing;
        var maxWidth = Pages.Max(x => x.Size.Width);

        var maxTranslateY = SafeZone / Scale;
        var minTranslateY = (Height - SafeZone) / Scale - totalHeight;
        
        TranslateY = Math.Min(TranslateY, maxTranslateY);
        TranslateY = Math.Max(TranslateY, minTranslateY);
        
        if (maxWidth < Width / Scale)
        {
            TranslateX = 0;
        }
        else
        {
            var maxTranslateX = (Width / 2 - SafeZone) / Scale - maxWidth / 2;
        
            TranslateX = Math.Min(TranslateX, -maxTranslateX);
            TranslateX = Math.Max(TranslateX, maxTranslateX);
        }
    }

    public void TranslateWithCurrentScale(float x, float y)
    {
        TranslateX += x / Scale;
        TranslateY += y / Scale;

        LimitTranslate();
    }
    
    public void ZoomToPoint(float x, float y, float factor)
    {
        var oldScale = Scale;
        Scale *= factor;
                
        LimitScale();
                
        factor = Scale / oldScale;
        
        TranslateX -= x / (oldScale * factor) - x / oldScale;
        TranslateY -= y / (oldScale * factor) - y / oldScale;

        LimitTranslate();
    }
    
    public void Render(IDrawingContextImpl context)
    {
        if (Pages.Count <= 0)
            return;

        LimitScale();
        LimitTranslate();

        
        var canvas = (context as ISkiaDrawingContextImpl)?.SkCanvas;
        
        if (canvas == null)
            throw new InvalidOperationException($"Context needs to be ISkiaDrawingContextImpl but got {nameof(context)}");

        var originalMatrix = canvas.TotalMatrix;

        
        canvas.Translate(Width / 2, Height / 2);
        
        canvas.Scale(Scale);
        canvas.Translate(TranslateX, TranslateY);

        foreach (var page in Pages)
        {
            canvas.Translate(-page.Size.Width / 2f, 0);
            DrawPageShadow(canvas, page.Size);
            canvas.DrawPicture(page.Picture);
            canvas.Translate(page.Size.Width / 2f, page.Size.Height + PageSpacing);
        }

        canvas.SetMatrix(originalMatrix);
        
        if (TranslateY < 0)
            DrawInnerGradient(canvas);
    }
    
    public void Dispose() { }
    public bool Equals(ICustomDrawOperation? other) => false;
    public bool HitTest(Point p) => true;

    #region page shadow

    private static readonly SKImageFilter PageShadow1 = SKImageFilter.CreateDropShadowOnly(0, 6, 6, 6, SKColors.Black.WithAlpha(64));
    private static readonly SKImageFilter PageShadow2 = SKImageFilter.CreateDropShadowOnly(0, 10, 14, 14, SKColors.Black.WithAlpha(32));
    
    private static SKPaint PageShadowPaint = new SKPaint
    {
        ImageFilter = SKImageFilter.CreateBlendMode(SKBlendMode.Overlay, PageShadow1, PageShadow2)
    };
    
    private void DrawPageShadow(SKCanvas canvas, QuestPDF.Infrastructure.Size size)
    {
        canvas.DrawRect(0, 0, size.Width, size.Height, PageShadowPaint);
    }
    
    #endregion

    #region inner viewport gradient

    private const float InnerGradientSize = 24f;
    private static readonly SKColor InnerGradientColor = SKColor.Parse("#666");
    
    private void DrawInnerGradient(SKCanvas canvas)
    {
        // gamma correction
        var colors = Enumerable
            .Range(0, 17)
            .Select(x => 1f - x / 16f)
            .Select(x => Math.Pow(x, 2f))
            .Select(x => (byte)(x * 255))
            .Select(x => InnerGradientColor.WithAlpha(x))
            .ToArray();
        
        using var fogPaint = new SKPaint
        {
            Shader = SKShader.CreateLinearGradient(
                new SKPoint(0, 0),
                new SKPoint(0, InnerGradientSize), 
                colors,
                SKShaderTileMode.Clamp)
        };
        
        canvas.DrawRect(0, 0, Width, InnerGradientSize, fogPaint);
    }

    #endregion
}