From e794c1edde6d77cf089ac3a986cd23a0434cfc49 Mon Sep 17 00:00:00 2001
From: Guillaume Chau <guillaume.b.chau@gmail.com>
Date: Mon, 12 Jun 2023 12:26:48 +0200
Subject: [PATCH] feat: keepPreviousResult

---
 packages/docs/src/api/use-query.md            |   2 +
 .../src/components/ChannelView.vue            |   1 -
 .../src/components/KeepPreviousResult.vue     | 115 ++++++++++++++++++
 .../test-e2e-composable-vue3/src/router.ts    |   4 +
 .../tests/e2e/specs/keepPreviousResult.cy.js  |  33 +++++
 .../vue-apollo-composable/src/useQuery.ts     |   3 +-
 6 files changed, 156 insertions(+), 2 deletions(-)
 create mode 100644 packages/test-e2e-composable-vue3/src/components/KeepPreviousResult.vue
 create mode 100644 packages/test-e2e-composable-vue3/tests/e2e/specs/keepPreviousResult.cy.js

diff --git a/packages/docs/src/api/use-query.md b/packages/docs/src/api/use-query.md
index 7602746..b088a6a 100644
--- a/packages/docs/src/api/use-query.md
+++ b/packages/docs/src/api/use-query.md
@@ -42,6 +42,8 @@
 
   - `throttle`: Throttle interval in ms.
 
+  - `keepPreviousResult`: (default: `false`) Whether or not to keep previous result when the query is fetch again (for example when a variable changes). This can be useful to prevent a flash of empty content.
+
 ## Return
 
 - `result`: result data object.
diff --git a/packages/test-e2e-composable-vue3/src/components/ChannelView.vue b/packages/test-e2e-composable-vue3/src/components/ChannelView.vue
index afbf437..b79e2a6 100644
--- a/packages/test-e2e-composable-vue3/src/components/ChannelView.vue
+++ b/packages/test-e2e-composable-vue3/src/components/ChannelView.vue
@@ -34,7 +34,6 @@ export default defineComponent({
       id: props.id,
     }), {
       notifyOnNetworkStatusChange: true,
-      keepPreviousResult: true,
     })
     const channel = computed(() => result.value?.channel)
 
diff --git a/packages/test-e2e-composable-vue3/src/components/KeepPreviousResult.vue b/packages/test-e2e-composable-vue3/src/components/KeepPreviousResult.vue
new file mode 100644
index 0000000..1b7a6a5
--- /dev/null
+++ b/packages/test-e2e-composable-vue3/src/components/KeepPreviousResult.vue
@@ -0,0 +1,115 @@
+<script lang="ts" setup>
+import { useApolloClient, useQuery } from '@vue/apollo-composable'
+import gql from 'graphql-tag'
+import { computed, ref } from 'vue'
+import MessageItem from './MessageItem.vue'
+
+interface Channel {
+  id: string
+  label: string
+}
+
+const keepPreviousResult = ref(false)
+
+const channelsQuery = useQuery<{ channels: Channel[] }>(gql`
+  query channels {
+    channels {
+      id
+      label
+    }
+  }
+`)
+
+const channels = computed(() => channelsQuery.result.value?.channels ?? [])
+
+const selectedChannelId = ref<string | null>(null)
+
+const selectedChannelQuery = useQuery(gql`
+  query channel ($id: ID!) {
+    channel (id: $id) {
+      id
+      label
+      messages {
+        id
+        text
+      }
+    }
+  }
+`, () => ({
+  id: selectedChannelId.value,
+}), () => ({
+  enabled: !!selectedChannelId.value,
+  fetchPolicy: 'cache-and-network',
+  keepPreviousResult: keepPreviousResult.value,
+}))
+
+const selectedChannel = computed(() => selectedChannelQuery.result.value?.channel)
+
+const { client: apolloClient } = useApolloClient()
+
+function clearCache () {
+  apolloClient.clearStore()
+}
+</script>
+
+<template>
+  <div class="h-full flex flex-col">
+    <div class="p-4">
+      <label>
+        <input
+          v-model="keepPreviousResult"
+          type="checkbox"
+        >
+        keepPreviousResult
+      </label>
+
+      <button
+        class="ml-4 underline"
+        @click="clearCache"
+      >
+        Clear cache
+      </button>
+    </div>
+
+    <div class="flex h-full">
+      <div class="flex flex-col">
+        <button
+          v-for="channel of channels"
+          :key="channel.id"
+          class="channel-btn p-4"
+          :class="{
+            'bg-green-200': selectedChannelId === channel.id,
+          }"
+          @click="selectedChannelId = channel.id"
+        >
+          {{ channel.label }}
+        </button>
+      </div>
+
+      <div
+        v-if="selectedChannel"
+        class="the-channel flex flex-col w-full h-full overflow-auto"
+      >
+        <div class="flex-none p-6 border-b border-gray-200 bg-white">
+          # {{ selectedChannel.label }}
+        </div>
+
+        <div class="flex-1 overflow-y-auto">
+          <MessageItem
+            v-for="message of selectedChannel.messages"
+            :key="message.id"
+            :message="message"
+            class="m-2"
+          />
+        </div>
+      </div>
+
+      <div
+        v-else
+        class="no-data"
+      >
+        No data
+      </div>
+    </div>
+  </div>
+</template>
diff --git a/packages/test-e2e-composable-vue3/src/router.ts b/packages/test-e2e-composable-vue3/src/router.ts
index ef19cb0..9bf3f14 100644
--- a/packages/test-e2e-composable-vue3/src/router.ts
+++ b/packages/test-e2e-composable-vue3/src/router.ts
@@ -42,5 +42,9 @@ export const router = createRouter({
       path: '/on-result',
       component: () => import('./components/OnResult.vue'),
     },
+    {
+      path: '/keep-previous-result',
+      component: () => import('./components/KeepPreviousResult.vue'),
+    },
   ],
 })
diff --git a/packages/test-e2e-composable-vue3/tests/e2e/specs/keepPreviousResult.cy.js b/packages/test-e2e-composable-vue3/tests/e2e/specs/keepPreviousResult.cy.js
new file mode 100644
index 0000000..080651e
--- /dev/null
+++ b/packages/test-e2e-composable-vue3/tests/e2e/specs/keepPreviousResult.cy.js
@@ -0,0 +1,33 @@
+/// <reference types="Cypress" />
+
+describe('keepPreviousResult', () => {
+  beforeEach(() => {
+    cy.task('db:reset')
+    cy.visit('/keep-previous-result')
+  })
+
+  it('keepPreviousResult disabled: should clear previous data', () => {
+    cy.get('.no-data').should('be.visible')
+    cy.get('.channel-btn').eq(0).click()
+    cy.get('.no-data').should('be.visible')
+    cy.get('.the-channel').should('contain.text', '# General')
+    cy.get('.no-data').should('not.exist')
+    cy.get('.channel-btn').eq(1).click()
+    cy.get('.no-data').should('be.visible')
+    cy.get('.the-channel').should('contain.text', '# Random')
+    cy.get('.no-data').should('not.exist')
+  })
+
+  it('keepPreviousResult enabled: should display previous channel', () => {
+    cy.get('label').contains('keepPreviousResult').get('input[type="checkbox"]').check()
+    cy.get('.no-data').should('be.visible')
+    cy.get('.channel-btn').eq(0).click()
+    cy.get('.no-data').should('be.visible')
+    cy.get('.the-channel').should('contain.text', '# General')
+    cy.get('.no-data').should('not.exist')
+    cy.get('.channel-btn').eq(1).click()
+    cy.get('.no-data').should('not.exist')
+    cy.get('.the-channel').should('contain.text', '# General')
+    cy.get('.the-channel').should('contain.text', '# Random')
+  })
+})
diff --git a/packages/vue-apollo-composable/src/useQuery.ts b/packages/vue-apollo-composable/src/useQuery.ts
index 850cc44..d472cc0 100644
--- a/packages/vue-apollo-composable/src/useQuery.ts
+++ b/packages/vue-apollo-composable/src/useQuery.ts
@@ -44,6 +44,7 @@ export interface UseQueryOptions<
   throttle?: number
   debounce?: number
   prefetch?: boolean
+  keepPreviousResult?: boolean
 }
 
 interface SubscribeToMoreItem {
@@ -256,7 +257,7 @@ export function useQueryImpl<
 
     // Make the cache data available to the component immediately
     // This prevents SSR hydration mismatches
-    if (!isServer && (currentOptions.value?.fetchPolicy !== 'no-cache' || currentOptions.value.notifyOnNetworkStatusChange)) {
+    if (!isServer && !currentOptions.value?.keepPreviousResult && (currentOptions.value?.fetchPolicy !== 'no-cache' || currentOptions.value.notifyOnNetworkStatusChange)) {
       const currentResult = query.value.getCurrentResult(false)
 
       if (!currentResult.loading || currentResult.partial || currentOptions.value?.notifyOnNetworkStatusChange) {