unpack(_package, resolver, canonical) {
    this.items = _package.spine;
    this.manifest = _package.manifest;
    this.spineNodeIndex = _package.spineNodeIndex;
    this.baseUrl = _package.baseUrl || _package.basePath || "";
    this.length = this.items.length;
    this.items.forEach((item, index) => {
      var manifestItem = this.manifest[item.idref];
      var spineItem;
      item.index = index;
      item.cfiBase = this.epubcfi.generateChapterComponent(this.spineNodeIndex, item.index, item.idref);

      if (item.href) {
        item.url = resolver(item.href, true);
        item.canonical = canonical(item.href);
      }

      if (manifestItem) {
        item.href = manifestItem.href;
        item.url = resolver(item.href, true);
        item.canonical = canonical(item.href);

        if (manifestItem.properties.length) {
          item.properties.push.apply(item.properties, manifestItem.properties);
        }
      }

      if (item.linear === "yes") {
        item.prev = function () {
          let prevIndex = item.index;

          while (prevIndex > 0) {
            let prev = this.get(prevIndex - 1);

            if (prev && prev.linear) {
              return prev;
            }

            prevIndex -= 1;
          }

          return;
        }.bind(this);

        item.next = function () {
          let nextIndex = item.index;

          while (nextIndex < this.spineItems.length - 1) {
            let next = this.get(nextIndex + 1);

            if (next && next.linear) {
              return next;
            }

            nextIndex += 1;
          }

          return;
        }.bind(this);
      } else {
        item.prev = function () {
          return;
        };

        item.next = function () {
          return;
        };
      }

      spineItem = new src_section(item, this.hooks);
      this.append(spineItem);
    });
    this.loaded = true;
  }
Unpack items from a opf into spine items
@param  {Packaging} _package
@param  {method} resolver URL resolver
@param  {method} canonical Resolve canonical url