Win2D 学习系列（二）： CanvasControl

　　在第一篇Hello Win2D中，我们初步使用CanvasControl进行了文字的绘制，CanvasControl作为win2d中最重要的一个类，提供的功能自然也不仅仅是绘制文字，那么今天就来深入学习一下CanvasControl的前世今生。

　　在api文档中可以看到CanvasControl的继承关系：

  public sealed class CanvasControl : UserControl, ICanvasControl,
            ICanvasResourceCreatorWithDpi, ICanvasResourceCreator

　　熟悉XAML的同学都知道，继承自UserControl的自定义控件和普通XAML控件一样，可以添加进XAML界面布局中，也拥有普通XAML控件同样的事件和属性。这样就很好理解上一篇中在XAML中添加CanvasControl并作为绘制的入口。那么Draw事件又是从何而来呢？且来看ICanvasControl：

 internal interface ICanvasControl : ICanvasResourceCreatorWithDpi, ICanvasResourceCreator
 {
     Color ClearColor { get; set; }
     bool ReadyToDraw { get; }

     event TypedEventHandler<CanvasControl, CanvasCreateResourcesEventArgs> CreateResources;
     event TypedEventHandler<CanvasControl, CanvasDrawEventArgs> Draw;

     void Invalidate();
 }

　　ICanvasControl同样继承自ICanvasResourceCreatorWithDpi和ICanvasResourceCreator，关于这两个接口我在后面会讲。这里只看ICanvasControl的定义。首先来看ClearColor属性，来指定CanvasControl进行重绘时的背景颜色，在Demo中新建Page2,在Page2.xaml中添加CanvasControl，指定ClearColor属性：

<Page
    x:Class="App1.Page2"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:App1"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:canvas="using:Microsoft.Graphics.Canvas"
    mc:Ignorable="d">

    <Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
        <canvas:CanvasControl Name="canvas" ClearColor="Blue"/>
    </Grid>
</Page>

　　然后 ReadyToDraw的返回值标记是否创建了绘图设备（Win2D使用GPU进行绘图，后面会单独写一篇文章来说，这里不深入了解）， CreateResources和Draw是实现ICanvasControl接口的控件独有的事件，CreateResources事件在CanvasControl创建完成后触发，Draw事件中负责进行绘制。和普通XAML控件不一样的地方在于，CanvasControl的Loaded事件并不会初始化绘图设备，所以使用CanvasControl绘图使用的资源需要放在CreateResources事件中创建和初始化。来看下面的代码：

public Page2()
{
    this.InitializeComponent();
    canvas.Loaded += canvas_Loaded;
    canvas.CreateResources += canvas_CreateResources;
}
private async void canvas_Loaded(object sender, RoutedEventArgs e)
{
    if (canvas.ReadyToDraw)
    {
        MessageDialog dialog = new MessageDialog("CanvasControl绘图设备在Loaded事件中创建");
        await dialog.ShowAsync();
    }
}
private async void canvas_CreateResources(CanvasControl sender, CanvasCreateResourcesEventArgs args)
{
    if (canvas.ReadyToDraw)
    {
        MessageDialog dialog = new MessageDialog("CanvasControl绘图设备在CreateResources事件中创建");
        await dialog.ShowAsync();
    }
}

运行结果：

　　最后，我们来看一下Invalidate方法。Invalidate方法会重新调用Draw事件对CanvasControl的内容进行重绘。Demo代码如下：

 <Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
     <canvas:CanvasControl Name="canvas" ClearColor="Blue"/>
     <Button Content="Invalidate" HorizontalAlignment="Center" Click="Button_Click" />
 </Grid>

　　在Page2.xaml.cs中注册CanvasControl的Draw事件，在Button的Clieck的事件中调用Invalidate方法：

int Index = 0;
public Page2()
{
    this.InitializeComponent();
    canvas.Loaded += canvas_Loaded;
    canvas.CreateResources += canvas_CreateResources;
    canvas.Draw += canvas_Draw;
}
private void canvas_Draw(CanvasControl sender, CanvasDrawEventArgs args)
{
    Index++;
    CanvasDrawingSession ds = args.DrawingSession;
    ds.DrawText(Index.ToString(), new System.Numerics.Vector2(180, 180), Colors.White);
}
private void Button_Click(object sender, RoutedEventArgs e)
{
    canvas.Invalidate();
}

　  Demo下载：https://github.com/leandro-lua/Win2dStudyDemo

　　附录：

　　1，微软win2D团队博客：Win2D Team Blog

　　2，api文档：Win2D Documentation

　　3，Win2D源码可以在Github下载：https://github.com/Microsoft/Win2D