Vue实现图片预览，侧边栏懒加载，不用任何插件，简单好用

实现样式

需求

实现PDF上传预览，并且不能下载

第一次实现：用vue-pdf，将上传的文件用base64传给前端展示
问题：

1. 水印第一次加载有后面又没有了。
2. 当上传大的pdf文件后，前端获取和渲染又长又慢，甚至不能用

修改实现模式

1. 前端上传PDF，后端将PDF转化成一页一页的图片
2. 前端根据page去获取一页一页的PDF图片，类似于百度文库

实现思路

配合后端实现思路

1. 获取全部页数，先把侧边栏的元素画出来占个位置
2. 获取已经看到的页数，没有默认1
3. 渲染上次看到的页数，同时侧边栏滚动到相同的index位置，通过监听元素是否进入视口去获取base64图片
4. 已经获取回来的图片不再去请求

主要重点难点是侧边栏懒加载、定位、等比例展示图片

 <div class="pdf-viewer">
   <div class="pdf-main">
     <canvas id="pdf-view"></canvas>
   </div>
   <div class="pdf-list" :class="{ collapse: collapse }">
     <div
       class="pdf-item"
       :class="{ active: currentPage === index }"
       v-for="index in pageTotalNum"
       :key="index"
       @click="changePage(index)"
       :data-index="index"
     >
       <img :src="imgList[index - 1]" alt="" />
     </div>
   </div>
 </div>

<script>
let observer = null;
export default {
  name: "PDFView",
  data() {
    return {
      currentPage: 1, //当前页数
      pageTotalNum: 1, //总页数
      imgList: [], //base64图片列表
      updateTimer: null
    };
  },
  watch: {
    /**
     * @description 监听当前页变化 滚动列表到顶部
     */
    currentPage() {
      this.$nextTick(() => {
        const activeEl = document.querySelector(".pdf-list .active");
        if (activeEl) {
          document.querySelector(".pdf-list").scrollTo({
            top: activeEl.offsetTop - 20,
            behavior: "smooth",
          });
          // 解决进来会请求当前页数 前面所有图片
          setTimeout(() => {
            if (observer) {
              observer.disconnect();
            }
            this.isEnter();
          }, 500);
        }
        // 切换页面 将查看区域滚动到最上面
        const mainEl = document.querySelector(".pdf-main");
        mainEl.scrollTo({
          top: 0,
        });
      });
    },
  },
  mounted() {
    this.getPageTotal();
  },
  beforeDestroy() {
    if (observer) {
      observer.disconnect();
    }
  },
  methods: {
    /**
     * @description 获取pdf总页数
     */
    getPageTotal() {
      const params = {
        id: this.$route.query.id,
      };
      apiGetViewPdfPageTotal(params).then((response) => {
        this.pageTotalNum = response.data;
        this.updateStudy(true);
      });
    },
    /**
     * @description 切换当前页
     */
    changePage(index) {
      this.currentPage = index;
      this.updateStudy();
      if (this.imgList[index - 1]) {
        this.drawImage(this.imgList[index - 1]);
      } else {
        this.getPdf();
      }
    },
    /**
     * @description 上一页
     */
    prePage() {
      let page = this.currentPage;
      if (page !== 1) {
        page = page > 1 ? page - 1 : this.pageTotalNum;
        this.currentPage = page;
        this.updateStudy();
        if (this.imgList[page - 1]) {
          this.drawImage(this.imgList[page - 1]);
        } else {
          this.getPdf();
        }
      }
    },
    /**
     * @description 下一页
     */
    nextPage() {
      let page = this.currentPage;
      if (page !== this.pageTotalNum) {
        page = page < this.pageTotalNum ? page + 1 : 1;
        this.currentPage = page;
        this.updateStudy();
        if (this.imgList[page - 1]) {
          this.drawImage(this.imgList[page - 1]);
        } else {
          this.getPdf();
        }
      }
    },
    /**
     * @description 更新学习 flag=true第一次进入
     */
    updateStudy(flag = false) {
      const params = {
        courseId: this.$route.query.id,
        pageRate: this.currentPage,
        flag,
        totalPageRate: this.pageTotalNum,
      };
      apiUpdateStudy(params)
        .then((response) => {
          this.currentPage = response.data.pageRate;
          if (flag) {
            this.updateTimer = setInterval(() => {
              this.updateStudy();
            }, 1000 * 10);
          }
          if (flag) {
            this.getPdf();
            // 解决第一页进来不请求的问题，一页大概能展示4-5张
            if (this.currentPage < 5) {
              this.isEnter();
            }
          }
        })
    },
    /**
     * @description 查看资料
     */
    getPdf() {
      const params = {
        id: this.$route.query.id,
        page: this.currentPage,
      };
      apiGetPdf(params).then((response) => {
        let base64 = "data:image/png;base64," + response.data;
        this.drawImage(base64);
      });
    },
    /**
     * @description 将base64图片 画到canvas上
     */
    drawImage(base64) {
      const canvas = document.getElementById("pdf-view");
      const context = canvas.getContext("2d");
      const image = new Image();
      image.src = base64;
      image.onload = () => {
        const proportion = image.width / image.height;
        // 获取style设置width:100% 的canvas宽度
        const canvasWidth = canvas.offsetWidth;
        // 图片宽度与canvas宽度比例
        const canvasWidthProportion = image.width / canvasWidth;
        // canvas宽度设置为宽度
        canvas.width = image.width;
        // 根据图片比例和宽度比例计算出canvas高度
        canvas.height = (canvasWidth / proportion) * canvasWidthProportion;
        context.drawImage(image, 0, 0);
      };
    },
    /**
     * @description 监听元素进入视口
     */
    isEnter() {
      observer = new IntersectionObserver((entries) => {
        entries.forEach((entry) => {
          const target = entry.target;
          const index = target.dataset.index;
          if (entry.isIntersecting) {
            if (!this.imgList[index - 1]) {
              this.getImgList(index);
            }
          } else {
            // console.log("元素离开视口", index);
          }
        });
      });
      this.$nextTick(() => {
      //将所有侧边栏的元素进行监听
        const els = document.querySelectorAll(".pdf-item");
        Array.from(els).forEach((el) => {
          observer.observe(el);
        });
      });
    },
    /**
     * @description 滚动获取图片
     */
    getImgList(index) {
      const params = {
        id: this.$route.query.id,
        page: index,
      };
      apiGetPdf(params).then((response) => {
        let base64 = "data:image/png;base64," + response.data;
        this.imgList[index - 1] = base64;
        // 解决请求回来页面没更新的问题
        this.$forceUpdate();
      });
    },
  },
};
</script>

<style lang="scss" scoped>
.pdf-container {
  width: 100%;
  height: 100%;
  color: #999;
}
.pdf-viewer {
  width: 100%;
  height: calc(100vh - 50px - 30px - 60px - 6px);
  position: relative;
  display: flex;
}
.pdf-list {
  width: 240px;
  overflow-y: auto;
  display: flex;
  flex-direction: column;
  padding: 20px;
  background: #000;
  box-sizing: border-box;
  // transition: all 0.3s ease-in-out;
  border-left: 1px solid #999;
  &::-webkit-scrollbar {
    width: 0px;
  }
  .pdf-item {
    height: 183px;
    min-height: 183px;
    display: inline-flex;
    justify-content: center;
    align-items: center;
    cursor: pointer;
    overflow: hidden;
    &:hover {
      ::v-deep img {
        transition: all 0.5s ease-in-out;
        transform: scale(1.1);
      }
    }
    &.active {
      box-shadow: 0px 0px 0px 4px #e6a23c;
    }
    &:not(:last-child) {
      margin-bottom: 10px;
    }
    img {
      pointer-events: none;
      width: 100%;
      // height: 100%;
    }
  }
  &.collapse {
    width: 0;
    padding: 0;
  }
}
.pdf-main {
  flex: 1;
  // width: 100%;
  // height: 100%;
  overflow-y: auto;
  background: #000;
  position: relative;
  padding: 10px 0;
  &::-webkit-scrollbar {
    width: 0px;
  }
}
.handle-btn {
  background: #000;
  display: flex;
  font-size: 12px;
  position: relative;
  height: 60px;
  padding: 0 6px;
  border-bottom: 1px solid #999;
  .right {
    width: 240px;
    display: flex;
    align-items: center;
    justify-content: flex-end;
    font-size: 32px;
  }
  .main {
    flex: 1;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 32px;
    margin-left: 250px;
    .pagination {
      display: flex;
      align-items: center;
      margin: 0 10px;
      .pagination-info {
        font-size: 14px;
        margin: 0 8px;
      }
    }
    .zoom {
      display: flex;
      align-items: center;
      margin: 0 10px;
      .scale {
        font-size: 14px;
        margin: 0 8px;
      }
    }
  }
  .tips {
    color: #e6a23c;
    font-size: 12px;
  }
  .start-test {
    display: flex;
    align-items: center;
  }
  .time {
    position: absolute;
    left: 6px;
    top: 50%;
    transform: translateY(-50%);
    > span {
      display: inline-block;
      margin-left: 10px;
    }
  }
}
i {
  cursor: pointer;
  &:hover {
    color: #fff;
  }
}
#pdf-view {
  width: 100%;
  // height: 100%;
  padding: 10px;
}
</style>

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