<template>
  <div class="waterfall wf-wrap" ref="waterfall" @scroll="onScroll">
    <ul>
      <transition-group name="list" tag="li">
        <li
          v-for="(item,index) in waterfallList"
          :key="index"
          class="wf-item"
          :style="{top:item.top+ 'px',left:item.left+'px', width:item.width+'px', height:item.height + 'px'}"
        >
          <img :src="item.src" />
        </li>
      </transition-group>
    </ul>
  </div>
</template>
<script>
import { getList } from "@/api/demo";

export default {
  name: "Waterfall",
  data() {
    return {
      waterfallList: [],

      waterfallCol: 5,
      colWidth: 236,
      marginRight: 10,
      marginBottom: 10,
      colHeights: [],

      listQuery: {
        page: 1,
        limit: 5,
        sort: "+id"
      },
      loading: false,
      show: true
    };
  },
  mounted() {
    this.init();
  },
  methods: {
    init() {
      // 初始化时，每栏高度都为0
      this.colHeights = new Array(this.waterfallCol);
      for (let i = 0; i < this.colHeights.length; i++) {
        this.colHeights[i] = 0;
      }
      this.colWidth =
        (this.$refs.waterfall.clientWidth -
          (this.waterfallCol - 1) * this.marginRight) /
        this.waterfallCol;
      this.loadImgs();
    },

    loadImgs() {
      this.loading = true;
      // 从api获取数据
      getList(this.listQuery).then(res => {
        let images = res.data.items;
        let promiseAll = [],
          imgs = [],
          total = images.length;

        for (let i = 0; i < total; i++) {
          promiseAll[i] = new Promise(resolve => {
            imgs[i] = new Image();
            imgs[i].src = images[i].image_uri;
            imgs[i].onload = () => {
              let imgData = {};
              imgData.height = (imgs[i].height * this.colWidth) / imgs[i].width;
              imgData.width = this.colWidth;
              imgData.src = images[i].image_uri;
              this.waterfallList.push(imgData);
              this.rankImgs(imgData);
              resolve(imgs[i]);
            };
          });
        }
        Promise.all(promiseAll).then(() => {
          this.loading = false;
          this.loadMore();
        });
      });
    },

    loadMore() {
      if (
        this.$refs.waterfall.clientHeight + this.$refs.waterfall.scrollTop >
          this.filterMin().minHeight &&
        this.loading == false
      ) {
        this.loading = true;
        setTimeout(() => {
          this.loadImgs();
        }, 200);
      }
    },

    rankImgs(imgData) {
      let min = this.filterMin();
      imgData.top = min.minHeight;
      imgData.left = min.minIndex * (this.colWidth + this.marginRight);

      this.colHeights[min.minIndex] += imgData.height + this.marginBottom;
    },

    filterMin() {
      let minHeight = Math.min.apply(null, this.colHeights);
      return {
        minHeight: minHeight,
        minIndex: this.colHeights.indexOf(minHeight)
      };
    },

    onScroll() {
      this.loadMore();
    }
  }
};
</script>

<style lang="scss" scoped>
ul li {
  list-style: none;
}

.wf-wrap {
  position: relative;
  width: 100%;
  height: 100%;
  overflow: scroll;
}
.wf-item {
  position: absolute;
}
.wf-item img {
  width: 100%;
  height: 100%;
}
.list-enter-active,
.list-leave-active {
  transition: all 1s;
}
.list-enter, .list-leave-to
/* .list-leave-active for below version 2.1.8 */ {
  opacity: 0;
  transform: translateY(30px);
}
</style>