Xamarin/C# 扩展现有类

Question

I've been learning C# for a while most recently MVVM but I've failed to grasp something regarding extending classes, obviously ...

In the example below, SkiaSharp is a Xamarin vector graphics package, I've been supplied some vector icons and I'm looking to have a reusable class to scale and color them at various points in the app

I figured if I subclassed 'SKCanvasView' then I could pass in the variables (icon, color) - all works fine when the code is in the related view but I wanted it in shared code, so I reference it like so

         <ContentView xmlns="http://xamarin.com/schemas/2014/forms" 
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         xmlns:d="http://xamarin.com/schemas/2014/forms/design"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
         mc:Ignorable="d"
         xmlns:local="clr-namespace:Custom"
         x:Class="NavigationIcons">

        <local:RenderSVG StyleId="F9001A"
                         Grid.Row="0"
                         Grid.Column="1"
                         HorizontalOptions="CenterAndExpand"
                         VerticalOptions="CenterAndExpand"
                         ClassId="vector.svg" 
                         PaintSurface="OnCanvasViewPaintSurface">

        </local:RenderSVG>

It builds, but then it complains the method for 'PaintSurface' isn't in the view code-behind file, which of course I didn't think it needed to be if it's referencing another class

I would have assumed that "local:SVGRender" would mean that all code would be run from that shared class, rather than the class file behind the XAML view, but it appears not?

public class RenderSVG : SKCanvasView
{


    void OnCanvasViewPaintSurface(object sender, SKPaintSurfaceEventArgs args)
    {
        try
        { 
            // rendering code is here

        }
    }

}

The error that's being thrown is "EventHandler "Custom.RenderSVG.OnCanvasViewPaintSurface" with correct signature not found in type "NavigationIcons", so it's not even trying to look at the derived class I made

Answer 1

As @FreakyAli said PaintSurface is an event. When using SKCanvasView inside a ContentView , you bind the event to a method (event handler) of the container. SKCanvasView will fire the event and the container method gets called.

But when you subclass SKCanvasView you have to override OnPaintSurface . Alternatively you can at some point attach the OnCanvasViewPaintSurface event handler to PaintSurface event through code (eg at construction) and at some point you will also need to detach it (before destruction). Overriding OnPaintSurface is the preferred way. For example:

public class RenderSVG : SKCanvasView
{
    protected override void OnPaintSurface(SKPaintSurfaceEventArgs args)
    {
        try
        { 
            // rendering code is here
        }
    }
}

Of course you need to remove PaintSurface="OnCanvasViewPaintSurface" from your XAML file:

<ContentView xmlns="http://xamarin.com/schemas/2014/forms" 
     xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
     xmlns:d="http://xamarin.com/schemas/2014/forms/design"
     xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
     mc:Ignorable="d"
     xmlns:local="clr-namespace:Custom"
     x:Class="NavigationIcons">

    <local:RenderSVG StyleId="F9001A"
                     Grid.Row="0"
                     Grid.Column="1"
                     HorizontalOptions="CenterAndExpand"
                     VerticalOptions="CenterAndExpand"
                     ClassId="vector.svg">
    </local:RenderSVG>