1. 前言
之前用PointLight做了一個番茄鐘，效果還不錯，具體可見這篇文章：
[UWP]使用PointLight并實作影片效果
后來試玩了Win2D，這次就用Win2D實作文字的鏤空效果，配合PointLight做一個內斂不張揚的番茄鐘，
實作鏤空文字的核心思想是使用CanvasGeometry.CreateText從TextLayout獲取一個Geometry，然后使用DrawGeometry將它畫到DrawingSurface，這篇文章介紹了具體的實作步驟，
2. 參考例子
Win2D Gallery提供了大量Win2D的Sample，這次就參考了其中的文字鏤空效果例子，地址和運行效果如下：
https://github.com/microsoft/Win2D-Samples/blob/master/ExampleGallery/TextOutlines.xaml.cs
3. 實作步驟
Sample的代碼量雖多，其實核心并不復雜，下面講講需要用到的API：
3.1 CanvasDevice.GetSharedDevice
因為要用到Win2D，所以首先要參考Win2D.uwp nuget包，因為我的目標不是輸出到CanvasControl上，而是想要輸出到一個SpriteVisual上，所以使用CanvasDevice：
var canvasDevice = CanvasDevice.GetSharedDevice();
3.2 CanvasComposition.CreateCompositionGraphicsDevice
然后創建一個Compositor，并將這個Compositor和CanvasDevice關聯起來，這里需要使用 CanvasComposition 創建 GraphicsDevice：
var compositor = ElementCompositionPreview.GetElementVisual(this).Compositor;
var graphicsDevice = CanvasComposition.CreateCompositionGraphicsDevice(compositor, canvasDevice);
3.3 CompositionGraphicsDevice.CreateDrawingSurface
然后使用CompositionGraphicsDevice.CreateDrawingSurface創建一個CompositionDrawingSurface物件，它是用來繪畫內容的表面：
var drawingSurface = graphicsDevice.CreateDrawingSurface(e.NewSize, DirectXPixelFormat.B8G8R8A8UIntNormalized, DirectXAlphaMode.Premultiplied);
3.4 Compositor.CreateSurfaceBrush
使用Compositor.CreateSurfaceBrush創建一個CompositionSurfaceBrush，它的作用是使用像素繪制SpriteVisual，簡單來說它就是一張位圖，然后輸出到SpriteVisual上：
var maskSurfaceBrush = compositor.CreateSurfaceBrush(drawingSurface);
spriteTextVisual.Brush = maskSurfaceBrush;
3.5 CanvasComposition.CreateDrawingSession
有了CompositionDrawingSurface就可以為所欲為了，將這個DrawingSurface作為引數，呼叫CanvasComposition.CreateDrawingSession創建DrawingSession，DrawingSession提供了多個函式，可以自由地在DrawingSurface上畫文字、形狀、圖片甚至SVG，
using (var session = CanvasComposition.CreateDrawingSession(drawingSurface))
{
}
3.6 CanvasTextFormat和CanvasTextLayout
要再DrawingSurface上寫字，需要CanvasTextLayout，而CanvasTextLayout中的文字大小、格式等則由CanvasTextFormat定義：
using (var textFormat = new CanvasTextFormat()
{
FontSize = (float)FontSize,
Direction = CanvasTextDirection.LeftToRightThenTopToBottom,
VerticalAlignment = CanvasVerticalAlignment.Center,
HorizontalAlignment = CanvasHorizontalAlignment.Center,
})
{
using (var textLayout = new CanvasTextLayout(session, Text, textFormat, width, height))
{
Color fontColor = FontColor;
session.DrawTextLayout(textLayout, 0, 0, fontColor);
}
}
3.7 CanvasGeometry.CreateText
因為我的目標是鏤空的文字，所以不能直接使用DrawTextLayout，這里需要使用CanvasGeometry.CreateText從TextLayout獲取一個Geometry，然后使用DrawGeometry將它畫到DrawingSurface，CanvasStrokeStyle是可選的，它控制邊框的虛線，
using (var textGeometry = CanvasGeometry.CreateText(textLayout))
{
var dashedStroke = new CanvasStrokeStyle()
{
DashStyle = DashStyle
};
session.DrawGeometry(textGeometry, OutlineColor, (float)StrokeWidth, dashedStroke);
}
4. 封裝為控制元件
將上面的代碼總結一下，封裝為一個OutlineTextControl 控制元件，它提供了Text、OutlineColor、FontColor等屬性，在控制元件SizeChanged時，或者各個屬性改變時呼叫DrawText重新在CompositionDrawingSurface上繪制文字，代碼大致如下：
public class OutlineTextControl : Control
{
private CompositionDrawingSurface _drawingSurface;
public OutlineTextControl()
{
var compositor = ElementCompositionPreview.GetElementVisual(this).Compositor;
var graphicsDevice = CanvasComposition.CreateCompositionGraphicsDevice(compositor, CanvasDevice.GetSharedDevice());
var spriteTextVisual = compositor.CreateSpriteVisual();
ElementCompositionPreview.SetElementChildVisual(this, spriteTextVisual);
SizeChanged += (s, e) =>
{
_drawingSurface = graphicsDevice.CreateDrawingSurface(e.NewSize, DirectXPixelFormat.B8G8R8A8UIntNormalized, DirectXAlphaMode.Premultiplied);
DrawText();
var maskSurfaceBrush = compositor.CreateSurfaceBrush(_drawingSurface);
spriteTextVisual.Brush = maskSurfaceBrush;
spriteTextVisual.Size = e.NewSize.ToVector2();
};
RegisterPropertyChangedCallback(FontSizeProperty, new DependencyPropertyChangedCallback((s, e) =>
{
DrawText();
}));
}
private void DrawText()
{
if (ActualHeight == 0 || ActualWidth == 0 || string.IsNullOrWhiteSpace(Text) || _drawingSurface == null)
return;
var width = (float)ActualWidth;
var height = (float)ActualHeight;
using (var session = CanvasComposition.CreateDrawingSession(_drawingSurface))
{
session.Clear(Colors.Transparent);
using (var textFormat = new CanvasTextFormat()
{
FontSize = (float)FontSize,
Direction = CanvasTextDirection.LeftToRightThenTopToBottom,
VerticalAlignment = CanvasVerticalAlignment.Center,
HorizontalAlignment = CanvasHorizontalAlignment.Center,
})
{
using (var textLayout = new CanvasTextLayout(session, Text, textFormat, width, height))
{
if (ShowNonOutlineText)
{
session.DrawTextLayout(textLayout, 0, 0, FontColor);
}
using (var textGeometry = CanvasGeometry.CreateText(textLayout))
{
var dashedStroke = new CanvasStrokeStyle()
{
DashStyle = DashStyle
};
session.DrawGeometry(textGeometry, OutlineColor, (float)StrokeWidth, dashedStroke);
}
}
}
}
}
//SOME CODE AND PROPERTIES
}
5. 結語
文章開頭的那個番茄鐘原始碼可以在這里查看：
OnePomodoro_OutlineTextView.xaml at master
也可以安裝我的番茄鐘應用試玩一下，安裝地址：
一個番茄鐘
6. 參考
CanvasComposition Class
CanvasDrawingSession Class
CanvasGeometry Class
CompositionGraphicsDevice Class (Windows.UI.Composition) - Windows UWP applications _ Microsoft Docs
CompositionDrawingSurface Class (Windows.UI.Composition) - Windows UWP applications _ Microsoft Docs
CompositionGraphicsDevice Class (Windows.UI.Composition) - Windows UWP applications _ Microsoft Docs
CompositionSurfaceBrush Class (Windows.UI.Composition) - Windows UWP applications _ Microsoft Docs
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標籤：UWP