[UWP]用Win2D實現鏤空文字
1. 前言
之前用PointLight做了一個番茄鍾，效果還不錯，具體可見這篇文章：
[UWP]使用PointLight並實現動畫效果
後來試玩了Win2D，這次就用Win2D實現文字的鏤空效果，配合PointLight做一個內斂不張揚的番茄鍾。
實現鏤空文字的核心思想是使用CanvasGeometry.CreateText從TextLayout獲取一個Geometry，然後使用DrawGeometry將它畫到DrawingSurface。這篇文章介紹了具體的實現步驟。
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，地址和執行效果如下：
https://github.com/microsoft/Win2D-Samples/blob/master/ExampleGallery/TextOutlines.xaml.cs
3. 實現步驟
Sample的程式碼量雖多，其實核心並不複雜，下面講講需要用到的API：
3.1 CanvasDevice.GetSharedDevice
因為要用到Win2D，所以首先要引用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
然後建立一個Compositor，並將這個Compositor和CanvasDevice關聯起來，這裡需要使用 CanvasComposition 建立 GraphicsDevice：
var compositor = ElementCompositionPreview.GetElementVisual(this).Compositor; var graphicsDevice = CanvasComposition.CreateCompositionGraphicsDevice(compositor, canvasDevice);
3.3 CompositionGraphicsDevice.CreateDrawingSurface
然後使用CompositionGraphicsDevice.CreateDrawingSurface建立一個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建立一個CompositionSurfaceBrush，它的作用是使用畫素繪製SpriteVisual，簡單來說它就是一張點陣圖，然後輸出到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建立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。這裡需要使用CanvasGeometry.CreateText從TextLayout獲取一個Geometry，然後使用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
將上面的程式碼總結一下，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上繪製文字。程式碼大致如下：
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鍾原始碼可以在這裡檢視：
OnePomodoro_OutlineTextView.xaml at master
也可以安裝我的番茄鍾應用試玩一下，安裝地址：
一個番茄鍾
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 D