!18165 新增OffscreenCanvas文档

Merge pull request !18165 from limeng/OffscreenCanvas

zh-cn/application-dev/reference/arkui-ts/ts-components-offscreencanvas.md

+#  OffscreenCanvas
+
+OffscreenCanvas组件用于自定义绘制图形。
+
+使用[Canvas](ts-components-canvas-canvas.md)组件或[Canvas API](ts-canvasrenderingcontext2d.md)时，渲染、动画和用户交互通常发生在应用程序的主线程上，与画布动画和渲染相关的计算可能会影响应用程序性能。OffscreenCanvas提供了一个可以在屏幕外渲染的画布，这样可以在单独的线程中运行一些任务，从而避免影响应用程序主线程性能。
+
+> **说明：** 
+>
+> 该组件从API Version 8开始支持。后续版本如有新增内容，则采用上角标单独标记该内容的起始版本。
+
+## 子组件
+
+不支持。
+
+## 接口
+
+OffscreenCanvas(width: number, height: number)
+
+从API version 9开始，该接口支持在ArkTS卡片中使用。
+
+**参数：**
+
+| 参数名 | 参数类型 | 必填 | 默认值 | 参数描述                              |
+| ------ | -------- | ---- | ------ | ------------------------------------- |
+| width  | number   | 是   | 0      | OffscreenCanvas组件的宽度，单位为vp。 |
+| height | number   | 是   | 0      | OffscreenCanvas组件的高度，单位为vp。 |
+
+## 属性
+
+除支持[通用属性](ts-universal-attributes-size.md)外，还支持以下属性：
+
+| 名称   | 类型   | 默认值 | 描述                                                         |
+| ------ | ------ | ------ | ------------------------------------------------------------ |
+| width  | number | 0      | OffscreenCanvas组件的宽度，单位为vp。从API version 9开始，该接口支持在ArkTS卡片中使用。 |
+| height | number | 0      | OffscreenCanvas组件的高度，单位为vp。从API version 9开始，该接口支持在ArkTS卡片中使用。 |
+
+### width
+
+```ts
+// xxx.ets
+@Entry
+@Component
+struct OffscreenCanvasPage {
+  private settings: RenderingContextSettings = new RenderingContextSettings(true);
+  private context: CanvasRenderingContext2D = new CanvasRenderingContext2D(this.settings);
+  private offCanvas: OffscreenCanvas = new OffscreenCanvas(200, 300)
+
+  build() {
+    Flex({ direction: FlexDirection.Row, alignItems: ItemAlign.Start, justifyContent: FlexAlign.Start }) {
+      Column() {
+        Canvas(this.context)
+          .width('100%')
+          .height('100%')
+          .borderWidth(5)
+          .borderColor('#00FF00')
+          .backgroundColor('#FFFFFF')
+          .onReady(() => {
+            var offContext = this.offCanvas.getContext("2d", this.settings)
+            offContext.fillStyle = '#CDCDCD'
+            offContext.fillRect(0, 0, this.offCanvas.width, 150)
+            var image = this.offCanvas.transferToImageBitmap()
+            this.context.setTransform(1, 0, 0, 1, 50, 200)
+            this.context.transferFromImageBitmap(image)
+          })
+      }
+    }.width('100%').height('100%')
+  }
+}
+```
+
+![zh-cn_image_0000001194032666](figures/offscreen_canvas_width.png)
+
+### height
+
+```ts
+// xxx.ets
+@Entry
+@Component
+struct OffscreenCanvasPage {
+  private settings: RenderingContextSettings = new RenderingContextSettings(true);
+  private context: CanvasRenderingContext2D = new CanvasRenderingContext2D(this.settings);
+  private offCanvas: OffscreenCanvas = new OffscreenCanvas(200, 300)
+
+  build() {
+    Flex({ direction: FlexDirection.Row, alignItems: ItemAlign.Start, justifyContent: FlexAlign.Start }) {
+      Column() {
+        Canvas(this.context)
+          .width('100%')
+          .height('100%')
+          .borderWidth(5)
+          .borderColor('#00FF00')
+          .backgroundColor('#FFFFFF')
+          .onReady(() => {
+            var offContext = this.offCanvas.getContext("2d", this.settings)
+            offContext.fillStyle = '#CDCDCD'
+            offContext.fillRect(0, 0, 100, this.offCanvas.height)
+            var image = this.offCanvas.transferToImageBitmap()
+            this.context.setTransform(1, 0, 0, 1, 50, 200)
+            this.context.transferFromImageBitmap(image)
+          })
+      }
+    }.width('100%').height('100%')
+  }
+}
+```
+
+![zh-cn_image_0000001194032666](figures/offscreen_canvas_height.png)
+
+## 方法
+
+### transferToImageBitmap
+
+transferToImageBitmap(): ImageBitmap
+
+从OffscreenCanvas组件中最近渲染的图像创建一个ImageBitmap对象。
+
+从API version 9开始，该接口支持在ArkTS卡片中使用。
+
+**返回值：**
+
+| 类型                                               | 描述                    |
+| -------------------------------------------------- | ----------------------- |
+| [ImageBitmap](ts-components-canvas-imagebitmap.md) | 创建的ImageBitmap对象。 |
+
+**示例：**
+
+```ts
+// xxx.ets
+@Entry
+@Component
+struct OffscreenCanvasPage {
+  private settings: RenderingContextSettings = new RenderingContextSettings(true)
+  private context: CanvasRenderingContext2D = new CanvasRenderingContext2D(this.settings)
+  private offCanvas: OffscreenCanvas = new OffscreenCanvas(300, 500)
+
+  build() {
+    Flex({ direction: FlexDirection.Column, alignItems: ItemAlign.Center, justifyContent: FlexAlign.Center }) {
+      Canvas(this.context)
+        .width('100%')
+        .height('100%')
+        .borderWidth(5)
+        .borderColor('#00FF00')
+        .backgroundColor('#FFFFFF')
+        .onReady(() => {
+          var offContext = this.offCanvas.getContext("2d", this.settings)
+          offContext.fillStyle = '#CDCDCD'
+          offContext.fillRect(0, 0, 300, 500)
+          offContext.fillStyle = '#000000'
+          offContext.font = '70px serif bold'
+          offContext.fillText("Offscreen : Hello World!", 20, 60)
+          var image = this.offCanvas.transferToImageBitmap()
+          this.context.transferFromImageBitmap(image)
+        })
+    }
+    .width('100%')
+    .height('100%')
+  }
+}
+```
+
+![zh-cn_image_0000001194032666](figures/offscreen_canvas_transferToImageBitmap.png)
+
+### getContext<sup>10+</sup>
+
+getContext(contextType: "2d", option?: RenderingContextSettings): OffscreenCanvasRenderingContext2D
+
+返回OffscreenCanvas组件的绘图上下文。
+
+**参数：**
+
+| 参数        | 类型                                                         | 必填 | 默认值 | 说明                                                         |
+| ----------- | ------------------------------------------------------------ | ---- | ------ | ------------------------------------------------------------ |
+| contextType | string                                                       | 是   | "2d"   | OffscreenCanvas组件绘图上下文的类型。                        |
+| option      | [RenderingContextSettings](ts-canvasrenderingcontext2d.md#renderingcontextsettings) | 否   | -      | 见[RenderingContextSettings](ts-canvasrenderingcontext2d.md#renderingcontextsettings)。 |
+
+**返回值：**
+
+| 类型                                                         | 描述                              |
+| ------------------------------------------------------------ | --------------------------------- |
+| [OffscreenCanvasRenderingContext2D](ts-offscreencanvasrenderingcontext2d.md) | OffscreenCanvas组件的绘图上下文。 |
+
+**示例：**
+
+```ts
+@Entry
+@Component
+struct OffscreenCanvasExamplePage {
+  private settings: RenderingContextSettings = new RenderingContextSettings(true);
+  private context: CanvasRenderingContext2D = new CanvasRenderingContext2D(this.settings);
+  private offscreenCanvas: OffscreenCanvas = new OffscreenCanvas(600, 800)
+
+  build() {
+    Flex({ direction: FlexDirection.Row, alignItems: ItemAlign.Start, justifyContent: FlexAlign.Start }) {
+      Column() {
+        Canvas(this.context)
+          .width('100%')
+          .height('100%')
+          .backgroundColor('#FFFFFF')
+          .onReady(() => {
+            var offContext = this.offscreenCanvas.getContext("2d", this.settings)
+            offContext.font = '70px sans-serif'
+            offContext.fillText("Offscreen : Hello World!", 20, 60)
+            offContext.fillStyle = "#0000ff"
+            offContext.fillRect(230, 350, 50, 50)
+            offContext.fillStyle = "#EE0077"
+            offContext.translate(70, 70)
+            offContext.fillRect(230, 350, 50, 50)
+            offContext.fillStyle = "#77EE0077"
+            offContext.translate(-70, -70)
+            offContext.fillStyle = "#00ffff"
+            offContext.rotate(45 * Math.PI / 180);
+            offContext.fillRect(180, 120, 50, 50);
+            offContext.rotate(-45 * Math.PI / 180);
+            offContext.beginPath()
+            offContext.moveTo(10, 150)
+            offContext.bezierCurveTo(20, 100, 200, 100, 200, 20)
+            offContext.stroke()
+            offContext.fillStyle = '#FF00FF'
+            offContext.fillRect(100, 100, 60, 60)
+            var imageData = this.offscreenCanvas.transferToImageBitmap()
+            this.context.transferFromImageBitmap(imageData)
+          })
+      }.width('100%').height('100%')
+    }
+    .width('100%')
+    .height('100%')
+  }
+}
+```
+
+![zh-cn_image_0000001194032666](figures/offscreen_canvas.png)
+