class Roll {
  constructor(opts) {
    this.elem = opts.elem; // 图片包含滚动长度的元素的
    this.elemBox = opts.elemBox; //图片展示区域元素，为了获取展示区域的高度
    this.direction = opts.direction;
    this.time = opts.time;
    this.init();
    this.roll = this.roll.bind(this)
    this.startRoll = this.startRoll.bind(this)
    this.stopRoll = this.stopRoll.bind(this)
  }
  init(){
    this.elemHeight = this.elem.offsetHeight;
    this.elemHtml = this.elem.innerHTML;
    this.elem.innerHTML = this.elem.innerHTML + this.elemHtml+ this.elemHtml;
    this.speed;
    // 如果向上滚或者向左滚动每次减1，向下滚或者向右滚动每次加1
    if(this.direction === 'top' || this.direction === 'left'){
      this.speed = -1;
    }else{
      this.speed = 1;
    }
  }
  roll(){
    switch (this.direction) {
      case "top":
        // 如果滚动的盒子的top值超出元素的高度，则置为0
        if(Math.abs(this.elemBox.offsetTop) >= this.elemHeight){
          this.elemBox.style.top = 0;
        }else{
          this.elemBox.style.top = this.elemBox.offsetTop + this.speed + 'px';
        }
        break;
      case "bottom":
        // 如果滚动的盒子的bottom值超出元素的高度，则置为0
        if(Math.abs(this.elemBox.offsetBottom) >= this.elemHeight){
          this.elemBox.style.bottom = 0;
        }else{
          this.elemBox.style.bottom = this.elemBox.offsetBottom + this.speed + 'px';
        }
        break;
      case "left":
        // 如果滚动的盒子的left超出元素的高度，则置为0
        if(Math.abs(this.elemBox.offsetLeft) >= this.elemHeight){
          this.elemBox.style.left = 0;
        }else{
          this.elemBox.style.left = this.elemBox.offsetLeft + this.speed + 'px';
        }
        break;
      case "right":
        // 如果滚动的盒子的right超出元素的高度，则置为0
        if(Math.abs(this.elemBox.offsetRight) >= this.elemHeight){
          this.elemBox.style.right = 0;
        }else{
          this.elemBox.style.right = this.elemBox.offsetRight + this.speed + 'px';
        }
        break;
      default:
        // 默认向上滚动，如果滚动的盒子的top超出元素的高度，则置为0
        if(Math.abs(this.elemBox.offsetTop) >= this.elemHeight){
          this.elemBox.style.top = 0;
        }else{
          this.elemBox.style.top = this.elemBox.offsetTop + speed + 'px';
        }
    }
  }
  stopRoll(){
    clearInterval(this.scrollTimer)
  }
  startRoll(){
    this.scrollTimer = setInterval(this.roll,this.time)
  }
}