vant-weapp/dist/mixins/observer/index.js

import { behavior } from './behavior';
import { observeProps } from './props';

export function observe(sfc, options) {
  if (sfc.computed) {
    options.behaviors.push(behavior);
    options.methods = options.methods || {};
    options.methods.$options = () => sfc;

    if (options.properties) {
      observeProps(options.properties);
    }
  }
}