<template>
  <transition :name="transitionName">
    <div class="back-to-top" @click="backToTop" v-show="visible" :style="customStyle">
      <svg width="16" height="16" viewBox="0 0 17 17" xmlns="http://www.w3.org/2000/svg" class="Icon Icon--backToTopArrow" aria-hidden="true" style="height: 16px; width: 16px;">
        <title>回到顶部</title>
        <g>
          <path d="M12.036 15.59c0 .55-.453.995-.997.995H5.032c-.55 0-.997-.445-.997-.996V8.584H1.03c-1.1 0-1.36-.633-.578-1.416L7.33.29c.39-.39 1.026-.385 1.412 0l6.878 6.88c.782.78.523 1.415-.58 1.415h-3.004v7.004z" fill-rule="evenodd"></path>
        </g>
      </svg>
    </div>
  </transition>
</template>

<script>
  export default {
    name: 'BackToTop',
    props: {
      visibilityHeight: {
        type: Number,
        default: 400
      },
      backPosition: {
        type: Number,
        default: 0
      },
      customStyle: {
        type: Object,
        default: {
          right: '50px',
          bottom: '50px',
          width: '40px',
          height: '40px',
          'border-radius': '4px',
          'line-height': '45px',
          background: '#e7eaf1'
        }
      },
      transitionName: {
        type: String,
        default: 'fade'
      }
    },
    data() {
      return {
        visible: false,
        interval: null
      }
    },
    mounted() {
      window.addEventListener('scroll', this.handleScroll);
    },
    beforeDestroy() {
      window.removeEventListener('scroll', this.handleScroll);
      if (this.interval) {
        clearInterval(this.interval);
      }
    },
    methods: {
      handleScroll() {
        this.visible = window.pageYOffset > this.visibilityHeight;
      },
      backToTop() {
        const start = window.pageYOffset;
        let i = 0;
        this.interval = setInterval(() => {
          const next = Math.floor(this.easeInOutQuad(10 * i, start, -start, 500));
          if (next <= this.backPosition) {
            window.scrollTo(0, this.backPosition);
            clearInterval(this.interval)
          } else {
            window.scrollTo(0, next);
          }
          i++;
        }, 16.7)
      },
      easeInOutQuad(t, b, c, d) {
        if ((t /= d / 2) < 1) return c / 2 * t * t + b;
        return -c / 2 * (--t * (t - 2) - 1) + b;
      }
    }
  }
</script>

<style scoped>
  .back-to-top {
    position: fixed;
    display: inline-block;
    text-align: center;
    cursor: pointer;
  }

  .back-to-top:hover {
    background: #d5dbe7;
  }

  .fade-enter-active,
  .fade-leave-active {
    transition: opacity .5s;
  }

  .fade-enter,
  .fade-leave-to {
    opacity: 0
  }

  .back-to-top .Icon {
    fill: #9aaabf;
    background: none;
  }
</style>
