用H5的canvas做出弹幕效果

canvas知识

绘制文字

let canvas = document.getElementById('canvas');let ctx = canvas.getContext('2d');ctx.font = '20px Microsoft YaHei';          //字体、大小ctx.fillStyle = '#000000';                  //字体颜色ctx.fillText('canvas 绘制文字', 100, 100);   //文本，字体x,y坐标

文本宽度

ctx.measureText('文本宽度').width

清除绘制内容

ctx.clearRect(0, 0, width, height);

实现步骤

1、创建canvas元素利用绝对定位覆盖在视频上
2、创建Barrage类，添加的弹幕缓存到弹幕列表中，并记录相应弹幕信息
3、绘制弹幕文本，用文本偏移量控制移动速度
4、计算当文本超出画布时移出弹幕列表

代码

//html<div style="position:relative;width:500px;height:400px;text-align:center;">
    <video controls="controls" autoplay="autoplay" style="width:100%;height:100%;">
        <source src="http://www.w3school.com.cn/i/movie.ogg" type="video/ogg" />
        <source src="http://www.w3school.com.cn/i/movie.mp4" type="video/mp4" /> 
        Your browser does not support the video tag.
    </video>
    <canvas id="canvas" width="500" height="400" style="position:absolute;top:0;left:0;">
        您的浏览器不支持canvas标签。
    </canvas>
</div>//js(function () {    class Barrage {
        constructor(canvas) {
            this.canvas = document.getElementById(canvas);
            let rect = this.canvas.getBoundingClientRect();
            this.w = rect.right - rect.left;
            this.h = rect.bottom - rect.top;
            this.ctx = this.canvas.getContext('2d');
            this.ctx.font = '20px Microsoft YaHei';
            this.barrageList = [];
        }        //添加弹幕列表
        shoot(value) {
            let top = this.getTop();
            let color = this.getColor();
            let offset = this.getOffset();
            let width = Math.ceil(this.ctx.measureText(value).width);
            let barrage = {
                value: value,
                top: top,
                left: this.w,
                color: color,
                offset: offset,
                width: width
            }
            this.barrageList.push(barrage);
        }        //开始绘制
        draw() {            if (this.barrageList.length) {
                this.ctx.clearRect(0, 0, this.w, this.h);                for (let i = 0; i < this.barrageList.length; i++) {
                    let b = this.barrageList[i];                    if (b.left + b.width <= 0) {
                        this.barrageList.splice(i, 1);
                        i--;                        continue;
                    }
                    b.left -= b.offset;
                    this.drawText(b);
                }
            }
            requestAnimationFrame(this.draw.bind(this));
        }        //绘制文字
        drawText(barrage) {
            this.ctx.fillStyle = barrage.color;
            this.ctx.fillText(barrage.value, barrage.left, barrage.top);
        }        //获取随机颜色
        getColor() {            return '#' + Math.floor(Math.random() * 0xffffff).toString(16);
        }        //获取随机top
        getTop() {            //canvas绘制文字x,y坐标是按文字左下角计算，预留30px
            return Math.floor(Math.random() * (this.h - 30)) + 30;
        }        //获取偏移量
        getOffset() {            return +(Math.random() * 4).toFixed(1) + 1;
        }
    }
    let barrage = new Barrage('canvas');
    barrage.draw();    const textList = ['弹幕', '666', '233333333', 
        'javascript', 'html', 'css', '前端框架', 'Vue', 'React',        'Angular','测试弹幕效果'
    ];
    textList.forEach((t) => {
        barrage.shoot(t);
    })
})();

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