OffscreenCanvas

OffscreenCanvas provides an offscreen canvas for drawing.

When using Canvas or Canvas API, rendering, animations, and user interactions generally occur on the main thread of an application. The computation relating to canvas animations and rendering may affect application performance. OffscreenCanvas allows for rendering off the screen. This means that some tasks can be run in a separate thread to reduce the load on the main thread.

NOTE

The APIs of this module are supported since API version 8. Updates will be marked with a superscript to indicate their earliest API version.

Child Components

Not supported

APIs

OffscreenCanvas(width: number, height: number)

Since API version 9, this API is supported in ArkTS widgets.

Parameters

Name	Type	Mandatory	Default Value	Description
width	number	Yes	0	Width of the offscreen canvas, in vp.
height	number	Yes	0	Height of the offscreen canvas, in vp.

Attributes

In addition to the universal attributes, the following attributes are supported.

Name	Type	Default Value	Description
width	number	0	Width of the offscreen canvas, in vp. Since API version 9, this API is supported in ArkTS widgets.
height	number	0	Height of the offscreen canvas, in vp. Since API version 9, this API is supported in ArkTS widgets.

width

// 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('#057D02')
          .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%')
  }
}

height

// 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('#057D02')
          .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%')
  }
}

Methods

transferToImageBitmap

transferToImageBitmap(): ImageBitmap

Creates an ImageBitmap object from the most recently rendered image of the offscreen canvas.

Since API version 9, this API is supported in ArkTS widgets.

Return value

Type	Description
ImageBitmap	ImageBitmap object created.

Example

// 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('#057D02')
        .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%')
  }
}

getContext¹⁰⁺

getContext(contextType: "2d", option?: RenderingContextSettings): OffscreenCanvasRenderingContext2D

Obtains the drawing context of the offscreen canvas.

Parameters

Name	Type	Mandatory	Default Value	Description
contextType	string	Yes	"2d"	Type of the drawing context of the offscreen canvas. The value can only be "2d".
option	RenderingContextSettings	No	-	For details, see RenderingContextSettings.

Return value

Type	Description
OffscreenCanvasRenderingContext2D	Drawing context of the offscreen canvas. If contextType in getContext is set to a value other than "2d" (including null and undefined), null is returned.

Example

@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%')
  }
}

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