JavaScript实现2种常见的抽奖效果实例代码

// 绘制转盘
function drawBg() {
    var one_angle = Math.PI * 2 / prize.length;
    var start_angle = -Math.PI / 2 - one_angle / 2
    ctx.translate(200.5, 200.5);
    ctx.arc(0, 0, 200, 0, Math.PI * 2, false);
    ctx.stroke()
	//绘制扇形
    prize.forEach((item, i) => {
        ctx.beginPath();
        ctx.moveTo(0, 0);
        ctx.arc(0, 0, 200, start_angle + one_angle * i, start_angle + one_angle * (i + 1), false);
        if (i % 2 == 0) {
            ctx.fillStyle = "rgba(230,186,127,1)"
        } else {
            ctx.fillStyle = "rgba(210,156,97,1)"
        }
        ctx.fill()
        // 绘制奖品
        var newImg = new Image()
        newImg.src = prize[i].img
        newImg.onload = function() {
            ctx.rotate(one_angle * i);
            ctx.fillStyle = "rgba(0,0,0,1)"
            //ctx.font = '16px  Microsoft YaHei';
            ctx.drawImage(newImg, 0, 0, 232, 200, -27, -190, 54, 54)
            //ctx.fillText(prize[i].name, -ctx.measureText(prize[i].name).width / 2, -112)
            ctx.rotate(-one_angle * i);
        }
        ctx.save();
    })
}

function luckyDraw(activeIndex) {
	// 旋转几圈
    var turnNum = 4
	// 当前选中下标
    var currentIndex = prize.length - (window.currentRotateAngle % 360 / (360 / prize.length))
    var turnIndex = 0
    if (activeIndex > currentIndex) {
        turnIndex = activeIndex - currentIndex
    } else {
        turnIndex = prize.length - (currentIndex - activeIndex)
    }
    var turnAngle = (turnNum + 1) * 360 - turnIndex * (360 / prize.length)
    canvasDom.style.transform = "rotate(" + (turnAngle + window.currentRotateAngle) + "deg)";
    window.currentRotateAngle = turnAngle + window.currentRotateAngle
    // 显示抽奖结果
    // setTimeout()
}

function initContent() {
	if (prize.length % 4 != 0) {
        alert("奖品个数需为4的倍数")
        return
    }
    let par_row_num = null;
    let par_col_num = null;
    for (let i = 0; i < prize.length; i++) {
        var row_num = null;
        var col_num = null;
        var x = 0;
        var y = 0;
        if (par_row_num === null) {
            row_num = 0;
            col_num = 0;
        } else if (par_row_num == 0 && par_col_num < (max_col_num - 1)) {
            row_num = par_row_num;
            col_num = par_col_num + 1;
        } else if (par_col_num == (max_col_num - 1) && par_row_num < (max_col_num - 1)) {
            row_num = par_row_num + 1;
            col_num = par_col_num;
        } else if (par_row_num == (max_col_num - 1) && par_col_num > 0) {
            row_num = par_row_num;
            col_num = par_col_num - 1;
        } else if (par_col_num == 0 && par_row_num > 0) {
            row_num = par_row_num - 1;
            col_num = par_col_num;
        }
        x = col_num * (prizeBoxWidth + prizeBoxMargin * 2) + prizeBoxMargin
        y = row_num * (prizeBoxWidth + prizeBoxMargin * 2) + prizeBoxMargin
        // 给元素设置样式 或 通过html字符串拼接容器内容
        // ......
        
        par_row_num = row_num
        par_col_num = col_num
    }
}