#  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(() => {
            let offContext = this.offCanvas.getContext("2d", this.settings)
            offContext.fillStyle = '#CDCDCD'
            offContext.fillRect(0, 0, this.offCanvas.width, 150)
            let 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(() => {
            let offContext = this.offCanvas.getContext("2d", this.settings)
            offContext.fillStyle = '#CDCDCD'
            offContext.fillRect(0, 0, 100, this.offCanvas.height)
            let 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(() => {
          let 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)
          let 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，当前仅支持"2d"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。如果getContext方法的入参contextType为"2d"以外类型（包括null或者undefined），返回null。 |

**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(() => {
            let 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)
            let imageData = this.offscreenCanvas.transferToImageBitmap()
            this.context.transferFromImageBitmap(imageData)
          })
      }.width('100%').height('100%')
    }
    .width('100%')
    .height('100%')
  }
}
```

![zh-cn_image_0000001194032666](figures/offscreen_canvas.png)