<template>
  <transition :name="transitionName">
    <div
      v-show="visible"
      @click="backToTop"
    >
      <slot />
    </div>
  </transition>
</template>

<script>
export default {
  name: 'BackToTop',
  props: {
    visibilityHeight: {
      type: Number,
      default: 400
    },
    backPosition: {
      type: Number,
      default: 0
    },
    transitionName: {
      type: String,
      default: 'fade'
    }
  },
  data() {
    return {
      visible: false,
      interval: null,
      isMoving: false
    }
  },
  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() {
      if (this.isMoving) return
      const start = window.pageYOffset
      let i = 0
      this.isMoving = true
      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)
          this.isMoving = false
          window.scrollTo(0, 0) // 莫名其妙的差一点没有回到顶部 暂时先用这个办法把.....
        } 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-ceiling {
  position: fixed;
  display: inline-block;
  text-align: center;
  cursor: pointer;
}
.back-to-ceiling:hover {
  background: #d5dbe7;
}
.fade-enter-active,
.fade-leave-active {
  transition: opacity .5s;
}
.fade-enter,
.fade-leave-to {
  opacity: 0
}
.back-to-ceiling .Icon {
  fill: #9aaabf;
  background: none;
}
</style>
