Vuex hooks from Vue 2.7

Component.vue

<script>
  import { defineComponent } from "vue";
  import { useVuex } from "./useVuex";

  export default defineComponent({
    setup() {
      const { getState, getAction } = useVuex();

      const user = getState("user");
      const fetchApiExample = getAction("fetchApiExample", "MyModule");

      return {
        user,
        fetchApiExample
      };
    }
  })
</script>

useVuex.js

import { getCurrentInstance, computed } from "vue";

export const useVuex = () => {
  const $root = getCurrentInstance().proxy.$root;
  const $store = $root.$store;

  function getAction(action, namespace) {
    return function(...args) {
      if (!namespace) {
        $store.dispatch(action, ...args);
      } else {
        $store.dispatch(`${namespace}/${action}`, ...args);
      }
    };
  }

  function getState(state, namespace) {
    if (!namespace) {
      return computed(() => $store.state[state]);
    } else {
      return computed(() => $store.state[namespace][state]);
    }
  }

  function getGetter(getter, namespace) {
    if (!namespace) {
      return computed(() => $store.getters[getter]);
    } else {
      return computed(() => $store.getters[namespace][getter]);
    }
  }

  function registerModule(...args) {
    $store.registerModule(...args);
  }

  function unregisterModule(...args) {
    $store.unregisterModule(...args);
  }

  return {
    getAction,
    getState,
    getGetter,
    registerModule,
    unregisterModule,
  };
}

getCurrentInstance is an internal API and not intended as client usage. There is no guarantee that it will not break in the future. I think Vue core team should provide clear and detailed guide in terms of how to use vuex@4 and vue-router@4 with vue@2.7.x, if composition API is supported in Vue@2.7 then other official packages should support this as well