在SKCanvas中进行绘制时，C＃：AccessViolationException

Question

介绍

我正在创建我的第一个Xamarin应用程序（它将首先针对UWP，然后针对Android，最后可能针对iOS）。

基本上，该应用应检测到多个手指，并且圆圈将在每个手指上弹出并跟随它们。

问题

尝试从MR.gestures的 onPanning方法上使用静态SKCanvas时遇到System.AccessViolationException 。

屏幕截图

细节

System.AccessViolationException的最大产品
HResult = 0x80004003 Message =尝试读取或写入受保护的内存。 这通常表明其他内存已损坏。
来源=应用程序树状结构：àSkiaSharp.SkiaApi.sk_canvas_draw_circle（IntPtr t，Single cx，Single cy，Single radius，IntPtr paint）àSkiaSharp.SKCanvas.DrawCircle（Single cx，Single cy，Single radius，SKPaint paint）à App1.MainPage.drawCircleOnCanvas（PointpointerPos，Int32半径）dans d：\\ Profiles \\ qpollet \\ Documents \\ Visual Studio 2017 \\ Projects \\ App1 \\ App1 \\ App1 \\ MainPage.xaml.cs：ligne 70àApp1.MainPage.onPanning（Object发送者PanEventArgs e）dans d：\\ Profiles \\ qpollet \\ Documents \\ Visual Studio 2017 \\ Projects \\ App1 \\ App1 \\ App1 \\ MainPage.xaml.cs：ligne 54àMR.Gestures.GestureHandler。<> c__DisplayClass115_0`1.b__0（ ）Xamarin.Forms.Platform.UWP.WindowsBasePlatformServices。<> c__DisplayClass2_0.b__0（）

编码

MainPage.xaml.cs

using SkiaSharp;
using SkiaSharp.Views.Forms;
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Xamarin.Forms;
using MR.Gestures;

namespace App1
{
    public partial class MainPage : Xamarin.Forms.ContentPage
    {
        private static SKCanvas canvas;

        public MainPage()
        {
            InitializeComponent();
        }

        private void OnPainting(object sender, SKPaintSurfaceEventArgs e)
        {
            canvas = e.Surface.Canvas;

            Point pointerPos = new Point
            {
                X = 200,
                Y = 400
            };
            drawCircleOnCanvas(pointerPos, 50);

            Point pointerPos1 = new Point
            {
                X = 1000,
                Y = 600
            };
            drawCircleOnCanvas(pointerPos1, 50);
        }

        private void onPanning(object sender, PanEventArgs e)
        {
            Debug.WriteLine("MR Panning !!!!!");

            try
            {
                List<Point> touches = e.Touches.ToList();
                //foreach (Point touch in touches)
                for(int i = 0; i <= touches.Count; i++)
                {
                    Point touch = touches[i];
                    Debug.WriteLine("index " + i + " : " + touch.X + " " + touch.Y);
                    drawCircleOnCanvas(touch, 50);
                }
            }
            catch (ArgumentNullException) { }
        }

        internal void drawCircleOnCanvas(Point pointerPos, int radius)
        {
            Debug.WriteLine(pointerPos.X + " " + pointerPos.Y);

            SKPaint circleFill = new SKPaint
            {
                IsAntialias = true,
                Style = SKPaintStyle.Fill,
                Color = SKColors.Red
            };
            canvas.DrawCircle(((float) pointerPos.X - radius), ((float) pointerPos.Y - radius), radius, circleFill);
        }
    }
}

MainPage.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:App1"
             xmlns:views="clr-namespace:SkiaSharp.Views.Forms;assembly=SkiaSharp.Views.Forms"
             xmlns:mr="clr-namespace:MR.Gestures;assembly=MR.Gestures"
             x:Class="App1.MainPage">

    <Label Text="Ooooh, you touch my tralala" 
           VerticalOptions="Center" 
           HorizontalOptions="Center" />

    <RelativeLayout>

        <mr:AbsoluteLayout
            RelativeLayout.XConstraint="{ConstraintExpression Type=Constant, Constant=0}"
            RelativeLayout.YConstraint="{ConstraintExpression Type=Constant, Constant=0}"
            RelativeLayout.WidthConstraint="{ConstraintExpression Type=RelativeToParent, Property=Width}"
            RelativeLayout.HeightConstraint="{ConstraintExpression Type=RelativeToParent, Property=Height}"

            BackgroundColor="Blue"

            x:Name="touchLayout"
            Panning="onPanning">

        </mr:AbsoluteLayout>

        <views:SKCanvasView
            RelativeLayout.XConstraint="{ConstraintExpression Type=Constant, Constant=0}"
            RelativeLayout.YConstraint="{ConstraintExpression Type=Constant, Constant=0}"
            RelativeLayout.WidthConstraint="{ConstraintExpression Type=RelativeToParent, Property=Width}"
            RelativeLayout.HeightConstraint="{ConstraintExpression Type=RelativeToParent, Property=Height}"

            PaintSurface="OnPainting"/>

    </RelativeLayout>
</ContentPage>

结果

任何想法 ？

Answer 1

您将收到该错误，因为您正在保存画布。 在每个帧渲染上都会重新创建画布，因此您无法保留其手柄。

正确的方法是仅在paint方法内部引用画布。 然后，在填充方法中，只需使用InvalidateSurface使表面InvalidateSurface 。

这将触发重新绘制，然后您可以在新位置绘制项目。

class MainPage {
    List<Point> touches;
    void onPan() {
        touches = GetTouchPoints();
        view.InvalidateSurface();
    }
    void onPaint() {
        var canvas = ...;
        canvas.Draw(...);
    }
}