给我写一份微信小程序大转盘前端代码，要求：每个人只能参与一次，前端可以控制奖品的抽奖率

以下是微信小程序大转盘前端代码示例，包含每个人只能参与一次和控制奖品抽奖率的功能。

HTML部分：

<view class="container">
  <canvas canvas-id="canvas" class="canvas"></canvas>
  <view class="btn" bindtap="startLottery">开始抽奖</view>
</view>

CSS部分：

.container {
  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  height: 100vh;
  background-color: #f0f0f0;
}

.canvas {
  width: 80vw;
  height: 80vw;
  border-radius: 50%;
  background-color: #fff;
}

.btn {
  margin-top: 20px;
  padding: 10px 20px;
  font-size: 24px;
  background-color: #f00;
  color: #fff;
  border: none;
  border-radius: 5px;
}

.btn:hover {
  cursor: pointer;
}

JS部分：

const app = getApp();
const prizes = [
  {
    name: "奖品1",
    rate: 0.1
  },
  {
    name: "奖品2",
    rate: 0.2
  },
  {
    name: "奖品3",
    rate: 0.3
  },
  {
    name: "奖品4",
    rate: 0.4
  }
];
const totalRate = prizes.reduce((acc, prize) => acc + prize.rate, 0);

Page({
  data: {
    canLottery: true,
    prizeIndex: -1
  },

  onLoad: function () {
    this.initCanvas();
  },

  initCanvas: function () {
    const ctx = wx.createCanvasContext("canvas");
    const width = app.globalData.systemInfo.windowWidth;
    const height = app.globalData.systemInfo.windowHeight;
    const radius = width * 0.4;
    const centerX = width / 2;
    const centerY = height * 0.4;
    const angle = (2 * Math.PI) / prizes.length;
    const colors = ["#ffc107", "#e91e63", "#9c27b0", "#2196f3"];

    ctx.translate(centerX, centerY);
    ctx.rotate(-Math.PI / 2);

    prizes.forEach((prize, index) => {
      const startAngle = index * angle;
      const endAngle = (index + 1) * angle;
      const gradient = ctx.createCircularGradient(0, 0, radius);
      gradient.addColorStop(0, colors[index]);
      gradient.addColorStop(1, "#fff");

      ctx.beginPath();
      ctx.moveTo(0, 0);
      ctx.arc(0, 0, radius, startAngle, endAngle);
      ctx.setFillStyle(gradient);
      ctx.fill();
      ctx.closePath();

      ctx.save();
      ctx.beginPath();
      ctx.rotate(startAngle + angle / 2);
      ctx.setFillStyle("#000");
      ctx.setFontSize(20);
      ctx.fillText(prize.name, -ctx.measureText(prize.name).width / 2, -radius * 0.8);
      ctx.closePath();
      ctx.restore();
    });

    ctx.draw();
  },

  startLottery: function () {
    if (!this.data.canLottery) {
      wx.showToast({
        title: "您已参与过抽奖",
        icon: "none"
      });
      return;
    }

    const random = Math.random() * totalRate;
    let rate = 0;
    let prizeIndex = -1;

    for (let i = 0; i < prizes.length; i++) {
      rate += prizes[i].rate;
      if (random < rate) {
        prizeIndex = i;
        break;
      }
    }

    if (prizeIndex === -1) {
      wx.showToast({
        title: "很遗憾，您未中奖",
        icon: "none"
      });
    } else {
      const prize = prizes[prizeIndex];
      wx.showModal({
        title: "恭喜您",
        content: `获得了${prize.name}`,
        showCancel: false
      });
    }

    this.setData({
      canLottery: false,
      prizeIndex
    });
  }
});

在这个示例中，我们使用canvas绘制了一个圆形的大转盘，奖品和抽奖率通过prizes数组控制，totalRate表示所有奖品的抽奖率之和，startLottery函数用于随机选出中奖奖品并展示。另外，通过data中的canLottery和prizeIndex变量来控制每个人只能参与一次抽奖。