方案

1，先禁止原组件的预览大图功能，自己添加预览方法；
2，获取dom，添加长按手势，用定时器来模拟长按手势；
3，交互方法保存图片； 代码如下

//@click-preview 
<Uploader
  v-model="fileList3"
  :before-read="onUploaderBefore"
  :after-read="(file) => onAfterRead(file, 3)"
  @delete="(file) => onUploaderBeforeDelete(file, 3)"
  @click-preview="doorImagePreview"
  :deletable="editor"
  :max-count="editor ? 3 : fileList3.length"
  image-fit="cover"
  accept="image/*"
  capture="camera"
/>

//长按定时器
const longPressTimer = ref(null);
const doorImagePreview = (file, detail) => {
  nextTick(() => {
    let aaa = document.getElementsByClassName("van-image-preview__swipe-item");
    for (let i = 0; i < aaa.length; i++) {
      let ddd = aaa[i].getElementsByClassName("van-image-preview__image");
      if (ddd.length) {
        let eee = ddd[0].childNodes;
        if (eee.length) {
          let imagepreview = eee[0];
          imagepreview.ontouchstart = (event) => {
            longPressTimer.value = setTimeout(function () {
              console.log("进入长摁方法");
              onLongPress(file.url);
            }, 500); //这里设置定时器，定义长按500毫秒触发长按事件
            return false;
          };
          imagepreview.ontouchmove = () => {
            // clearTimeout(longPressTimer.value); //清除定时器
            // if (longPressTimer.value != 0) {
            //   //这里写要执行的内容（如onclick事件）
            //   console.log("点击但未长按");
            // }
            return false;
          };
          imagepreview.ontouchend = () => {
            clearTimeout(longPressTimer.value); //清除定时器
            longPressTimer.value = null;
          };
        }
      }
    }
  });
};


const onLongPress = (url) => {
  console.log("长按方法", url);
  //调用app保存图片方法
  channels.saveImage(url, false);
  setTimeout(() => {
    Toast("图片保存到相册");
  }, 1000);
};