【图片】【编缉】图片增加水印（通过组件的Overlay方法增加水印）

对图片增加水印经常在分享，版权保护等场景使用，在鸿蒙系统中提供了多种方，本篇介绍通过组件的Overlay方法增加水印的方法，是对组件进行的水印操作。代码如下，主要分为两步

第一步，实现增加水印的组件，该组件实现水印内容的绘制

@Component
export struct Watermark {
  private settings: RenderingContextSettings = new RenderingContextSettings(true);
  private context: CanvasRenderingContext2D = new CanvasRenderingContext2D(this.settings);
  @Prop watermarkWidth: number = 120;
  @Prop watermarkHeight: number = 120;
  @Prop watermarkText: string = "水印";
  @Prop rotationAngle: number = -30;
  @Prop fillColor: string | number | CanvasGradient | CanvasPattern = '#10000000';
  @Prop font: string = '16vp';

  draw() {
    this.context.fillStyle = this.fillColor;
    this.context.font = this.font;
    const colCount = Math.ceil(this.context.width / this.watermarkWidth);
    const rowCount = Math.ceil(this.context.height / this.watermarkHeight);
    for (let col = 0; col <= colCount; col++) {
      let row = 0;
      for (; row <= rowCount; row++) {
        const angle = this.rotationAngle * Math.PI / 180;
        this.context.rotate(angle);
        const positionX = this.rotationAngle > 0 ? this.watermarkHeight * Math.tan(angle) : 0;
        const positionY = this.rotationAngle > 0 ? 0 : this.watermarkWidth * Math.tan(-angle);
        this.context.fillText(this.watermarkText, positionX, positionY);
        this.context.rotate(-angle);
        this.context.translate(0, this.watermarkHeight);
      }
      this.context.translate(0, -this.watermarkHeight * row);
      this.context.translate(this.watermarkWidth, 0);
    }
  }

  build() {
    Canvas(this.context)
      .width('100%')
      .height('100%')
      .hitTestBehavior(HitTestMode.Transparent)
      .onReady(() => this.draw())
  }
}

第二步，对组件增加水印Builder，在调用的页面类实现

struct OverlayWatermarkPage {
  // 开始Builder
  @Builder
  watermarkBuilder() {
    Column() {
      Watermark()
    }
  }
  // 结束Builder
}

使用代码

Image($r('app.media.image'))
  .width(110)
  .height(88)
  .overlay(this.watermarkBuilder())