fix: avoid resetting store state when registering a dynamic module

Fixes vuejs#2197

At the moment, when registering a dynamic module, we call
`resetStoreState()` just to register the getters for the new module.

It seems unnecessary to reset the entire store state in this case, and
this actually also leads to [other issues][1].

This change is based on the test case added in
vuejs#2201

The approach taken in this change is to refactor the getter registration
into its own function, and call that new method when registering a
dynamic module instead of resetting the store state.

[1]: vuejs#2197

src/store-util.js

@@ -1,5 +1,5 @@
 import { reactive, computed, watch, effectScope } from 'vue'
-import { forEachValue, isObject, isPromise, assert, partial } from './util'
+import { isObject, isPromise, assert, partial } from './util'
 
 export function genericSubscribe (fn, subs, options) {
   if (subs.indexOf(fn) < 0) {
@@ -35,36 +35,19 @@ export function resetStoreState (store, state, hot) {
   store.getters = {}
   // reset local getters cache
   store._makeLocalGettersCache = Object.create(null)
-  const wrappedGetters = store._wrappedGetters
-  const computedObj = {}
-  const computedCache = {}
 
   // create a new effect scope and create computed object inside it to avoid
   // getters (computed) getting destroyed on component unmount.
   const scope = effectScope(true)
-
-  scope.run(() => {
-    forEachValue(wrappedGetters, (fn, key) => {
-      // use computed to leverage its lazy-caching mechanism
-      // direct inline function use will lead to closure preserving oldState.
-      // using partial to return function with only arguments preserved in closure environment.
-      computedObj[key] = partial(fn, store)
-      computedCache[key] = computed(() => computedObj[key]())
-      Object.defineProperty(store.getters, key, {
-        get: () => computedCache[key].value,
-        enumerable: true // for local getters
-      })
-    })
-  })
+  // register the newly created effect scope to the store so that we can
+  // dispose the effects when this method runs again in the future.
+  store._scope = scope
+  registerGetters(store, Object.keys(store._wrappedGetters))
 
   store._state = reactive({
     data: state
   })
 
-  // register the newly created effect scope to the store so that we can
-  // dispose the effects when this method runs again in the future.
-  store._scope = scope
-
   // enable strict mode for new state
   if (store.strict) {
     enableStrictMode(store)
@@ -86,6 +69,26 @@ export function resetStoreState (store, state, hot) {
   }
 }
 
+export function registerGetters (store, getterKeys) {
+  const computedObj = {}
+  const computedCache = {}
+
+  store._scope.run(() => {
+    getterKeys.forEach((key) => {
+      const fn = store._wrappedGetters[key]
+      // use computed to leverage its lazy-caching mechanism
+      // direct inline function use will lead to closure preserving oldState.
+      // using partial to return function with only arguments preserved in closure environment.
+      computedObj[key] = partial(fn, store)
+      computedCache[key] = computed(() => computedObj[key]())
+      Object.defineProperty(store.getters, key, {
+        get: () => computedCache[key].value,
+        enumerable: true // for local getters
+      })
+    })
+  })
+}
+
 export function installModule (store, rootState, path, module, hot) {
   const isRoot = !path.length
   const namespace = store._modules.getNamespace(path)

src/store.js

@@ -9,7 +9,8 @@ import {
   installModule,
   resetStore,
   resetStoreState,
-  unifyObjectStyle
+  unifyObjectStyle,
+  registerGetters
 } from './store-util'
 
 export function createStore (options) {
@@ -227,9 +228,15 @@ export class Store {
     }
 
     this._modules.register(path, rawModule)
-    installModule(this, this.state, path, this._modules.get(path), options.preserveState)
-    // reset store to update getters...
-    resetStoreState(this, this.state)
+    const module = this._modules.get(path)
+    const namespace = this._modules.getNamespace(path)
+    installModule(this, this.state, path, module, options.preserveState)
+
+    const getterKeys = []
+    module.forEachGetter((getter, key) => {
+      getterKeys.push(namespace + key)
+    })
+    registerGetters(this, getterKeys)
   }
 
   unregisterModule (path) {