vant-weapp/packages/sticky/index.ts

import { VantComponent } from '../common/component';
import { pageScrollMixin } from '../mixins/page-scroll';

const ROOT_ELEMENT = '.van-sticky';

type BoundingClientRect = WechatMiniprogram.BoundingClientRectCallbackResult;

VantComponent({
  props: {
    zIndex: {
      type: Number,
      value: 99
    },
    offsetTop: {
      type: Number,
      value: 0,
      observer: 'onScroll'
    },
    disabled: {
      type: Boolean,
      observer: 'onScroll'
    },
    container: {
      type: null,
      observer: 'onScroll'
    }
  },

  mixins: [
    pageScrollMixin(function(event) {
      this.onScroll(event);
    })
  ],

  data: {
    height: 0,
    fixed: false,
    transform: 0
  },

  mounted() {
    this.onScroll();
  },

  methods: {
    onScroll({ scrollTop } = {}) {
      const { container, offsetTop, disabled } = this.data;

      if (disabled) {
        this.setDataAfterDiff({
          fixed: false,
          transform: 0
        });
        return;
      }

      this.scrollTop = scrollTop || this.scrollTop;

      if (typeof container === 'function') {
        Promise.all([this.getRect(ROOT_ELEMENT), this.getContainerRect()]).then(
          ([root, container]: BoundingClientRect[]) => {
            if (offsetTop + root.height > container.height + container.top) {
              this.setDataAfterDiff({
                fixed: false,
                transform: container.height - root.height
              });
            } else if (offsetTop >= root.top) {
              this.setDataAfterDiff({
                fixed: true,
                height: root.height,
                transform: 0
              });
            } else {
              this.setDataAfterDiff({ fixed: false, transform: 0 });
            }
          }
        );

        return;
      }

      this.getRect(ROOT_ELEMENT).then((root: BoundingClientRect) => {
        if (offsetTop >= root.top) {
          this.setDataAfterDiff({ fixed: true, height: root.height });
          this.transform = 0;
        } else {
          this.setDataAfterDiff({ fixed: false });
        }
      });
    },

    setDataAfterDiff(data) {
      wx.nextTick(() => {
        const diff = Object.keys(data).reduce((prev, key) => {
          if (data[key] !== this.data[key]) {
            prev[key] = data[key];
          }

          return prev;
        }, {});

        this.setData(diff);

        this.$emit('scroll', {
          scrollTop: this.scrollTop,
          isFixed: data.fixed || this.data.fixed
        });
      });
    },

    getContainerRect() {
      const nodesRef: WechatMiniprogram.NodesRef = this.data.container();

      return new Promise(resolve =>
        nodesRef.boundingClientRect(resolve).exec()
      );
    }
  }
});