function rain(canvas, text, symbol, speed) {
    //获取屏幕可视区域大小
    var d = document.documentElement;
    var clinetW = d.clientWidth + 10;
    var clinetH = d.clientHeight;

    //设置画布大小
    var canvas = canvas || document.querySelector('canvas');
    canvas.width = clinetW;
    canvas.height = clinetH;

    var cxt = canvas.getContext('2d');
    var rainStr = text || 'The matrix of hackers';
    var symbol = symbol || '';
    var arr = rainStr.split(symbol);

    var fontSize = 14;
    // 计算行数
    var cols = Math.floor(clinetW / fontSize);
    // 初始化Y轴坐标
    var down = [];
    for (var i = 0; i < cols; i++) {
        down.push(Math.floor(Math.random() * -100));
    }

    function drawRain() {
        // 填充背景(ps:背景采用rgba,可尝试不同透明度的效果)
        cxt.fillStyle = 'rgba(0,0,0,.1)';
        cxt.fillRect(0, 0, clinetW, clinetH);

        // 设置字体颜色
        cxt.fillStyle = '#fff';
        for (var i = 0; i < down.length; i++) {
            var randomNum = Math.random();
            // 绘制文字
            cxt.fillText(arr[Math.floor(randomNum * arr.length)], i * fontSize, down[i] * fontSize);

            if (down[i] * fontSize > clinetH && randomNum > 0.9) {
                down[i] = 0;
            }

            down[i]++;
        }
    }
    var speed = speed || 30;
    setInterval(drawRain, speed);
}