vant-weapp/packages/mixins/page-scroll.ts

type IPageScrollOption = WechatMiniprogram.Page.IPageScrollOption;
type Scroller = (event: IPageScrollOption) => void;
type TrivialInstance = WechatMiniprogram.Page.TrivialInstance & {
  vanPageScroller?: Scroller[];
};

function getCurrentPage(): TrivialInstance {
  const pages = getCurrentPages();
  return pages[pages.length - 1] || ({} as TrivialInstance);
}

function onPageScroll(event: IPageScrollOption) {
  const { vanPageScroller = [] } = getCurrentPage();

  vanPageScroller.forEach((scroller: Scroller) => {
    if (typeof scroller === 'function') {
      scroller(event);
    }
  });
}

export const pageScrollMixin = (scroller: Scroller) =>
  Behavior({
    attached() {
      const page = getCurrentPage();

      if (Array.isArray(page.vanPageScroller)) {
        page.vanPageScroller.push(scroller.bind(this));
      } else {
        page.vanPageScroller = [
          page.onPageScroll.bind(page),
          scroller.bind(this)
        ];
      }

      page.onPageScroll = onPageScroll;
    },

    detached() {
      const page = getCurrentPage();
      page.vanPageScroller = (page.vanPageScroller || []).filter(
        item => item !== scroller
      );
    }
  });