HarmonyOS Canvas绘制曲线相关

参考示例：

@Entry
@Component
struct IndexView {
  highTemperature: number[] = [21, 16, 17, 21, 21, 22, 24, 22, 25, 23, 24, 26, 25, 24, 24];
  lowTemperature: number[] = [14, 13, 12, 13, 15, 13, 17, 15, 16, 18, 18, 20, 22, 20, 20];
  settings: RenderingContextSettings = new RenderingContextSettings(true)
  context: CanvasRenderingContext2D = new CanvasRenderingContext2D(this.settings)
  canvasHeight: number = 120;
  itemWidth: number = 50;
  build() {
    Row() {
      Column({ space: 15 }) {
        Text('温度曲线图')
          .fontSize(18)
          .fontWeight(FontWeight.Bold)
        Scroll() {
          Canvas(this.context)
            .width(this.highTemperature.length * this.itemWidth + this.itemWidth)
            .height(this.canvasHeight)
            .backgroundColor('#f1f3f5')
            .onReady(() => {
              // 找出高温曲线的最大值，和低温曲线的最小值，用于后续确定曲线竖直方向的比例
              const maxTemperature: number = Math.max(...this.highTemperature);
              const minTemperature: number = Math.min(...this.lowTemperature);
              // 此处要求绘制的点的上下限之间的差值完全等于画布的高度的70%，也就是max ~ min之间的温度要均匀分布在canvasHeight * 70% = 84之间。
              // step 表示每改变 1℃，纵坐标改变的高度
              const step: number = this.canvasHeight * 0.7 / (maxTemperature - minTemperature);
              // curveChartMargin 表示曲线图大小极值与画布上下边的距离
              const curveChartMargin: number = this.canvasHeight * 0.15;
              // 设置曲线样式
              this.context.lineWidth = 1;
              this.context.font = 'normal bold 35px';
              this.context.fillStyle = '#008000';
              this.context.strokeStyle = '#008000';
              this.context.globalAlpha = 1;
              // 存放坐标的数组
              let xPos: number[] = [];
              let highYPos: number[] = [];
              let lowYPos: number[] = [];
              for (let i: number = 0; i < this.highTemperature.length; i++) {
                // 确定每个点的坐标，包括高温和低温，其中，高温和低温坐标的横坐标都是一致的
                let x: number = (i + 1) * this.itemWidth;
                let yHeight: number = this.canvasHeight - (curveChartMargin + (this.highTemperature[i] - minTemperature) * step);
                let yLow: number = this.canvasHeight - (curveChartMargin + (this.lowTemperature[i] - minTemperature) * step);
                // 存放数据
                xPos.push(x);
                highYPos.push(yHeight);
                lowYPos.push(yLow);
                // 给每个点画出一个圆并填充颜色，这里设置圆的半径为2
                let region: Path2D = new Path2D();
                region.ellipse(x, yHeight, 2, 2, 0, 0, Math.PI * 2);
                region.ellipse(x, yLow, 2, 2, 0, 0, Math.PI * 2);
                this.context.fill(region);
                // 准备气温文字
                let highTemperatureStr: string = `${this.highTemperature[i]}℃`;
                let lowTemperatureStr: string = `${this.lowTemperature[i]}℃`;
                // 绘制高温点文字
                this.context.fillText(highTemperatureStr, x - this.context.measureText(highTemperatureStr)
                  .width / 2, yHeight - 6);
                // 绘制低温点文字
                this.context.fillText(lowTemperatureStr, x - this.context.measureText(lowTemperatureStr)
                  .width / 2, yLow + 15);
              }
              // 绘制高温曲线
              // 首先设置初始点
              this.context.beginPath();
              this.context.moveTo(xPos[0], highYPos[0]);
              for (let i: number = 1; i < xPos.length; i++) {
                let x0: number = i * this.itemWidth;
                let y0: number = this.canvasHeight - (curveChartMargin + (this.highTemperature[i - 1] - minTemperature) * step);
                // 使用三次贝塞尔曲线绘制
                this.context.bezierCurveTo(x0 + (xPos[i] - x0) * 0.3, y0, xPos[i] - (xPos[i] - x0) * 0.3, highYPos[i], xPos[i], highYPos[i]);
              }
              this.context.stroke();
              // 绘制低温曲线
              // 记得首先要设置初始点
              this.context.beginPath();
              this.context.moveTo(xPos[0], lowYPos[0]);
              for (let i: number = 1; i < xPos.length; i++) {
                let x0: number = i * 50;
                let y0: number = this.canvasHeight - (curveChartMargin + (this.lowTemperature[i - 1] - minTemperature) * step);
                // 使用三次贝塞尔曲线
                this.context.bezierCurveTo(x0 + (xPos[i] - x0) * 0.3, y0, xPos[i] - (xPos[i] - x0) * 0.3, lowYPos[i], xPos[i], lowYPos[i]);
              }
              this.context.stroke();
              // 制作闭合图像
              this.context.lineTo(this.highTemperature.length * this.itemWidth + this.itemWidth, this.canvasHeight)
              this.context.lineTo(0, this.canvasHeight + 0)
              // 为了确保线不会显示在页面上，我们将线的宽度设为0，颜色设为完全透明
              this.context.lineWidth = 0;
              this.context.strokeStyle = '#00000000';
              this.context.stroke();
              this.context.fill()
            })
        }
        .scrollable(ScrollDirection.Horizontal)
        .scrollBar(BarState.Off)
      }
      .width('100%')
    }
    .height('100%')
    .border({ width: { bottom: 20 }, color: Color.Black })
  }
}
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