From 5a0bab0bd262639d3a5a208764005f6fd5e8e522 Mon Sep 17 00:00:00 2001
From: Evan You <yyx990803@gmail.com>
Date: Thu, 6 May 2021 15:45:42 -0400
Subject: [PATCH] wip: defineReactive on instance with keys starting with $

---
 packages/runtime-core/src/compat/global.ts   |  8 ++------
 packages/vue-compat/__tests__/global.spec.ts | 14 ++++++++++++++
 2 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/packages/runtime-core/src/compat/global.ts b/packages/runtime-core/src/compat/global.ts
index 9f353d414d..2466706650 100644
--- a/packages/runtime-core/src/compat/global.ts
+++ b/packages/runtime-core/src/compat/global.ts
@@ -11,7 +11,6 @@ import {
   isFunction,
   extend,
   NOOP,
-  EMPTY_OBJ,
   isArray,
   isObject,
   isString,
@@ -557,11 +556,8 @@ function defineReactive(obj: any, key: string, val: any) {
 
   const i = obj.$
   if (i && obj === i.proxy) {
-    // Vue instance, add it to data
-    if (i.data === EMPTY_OBJ) {
-      i.data = reactive({})
-    }
-    i.data[key] = val
+    // target is a Vue instance - define on instance.ctx
+    defineReactiveSimple(i.ctx, key, val)
     i.accessCache = Object.create(null)
   } else if (isReactive(obj)) {
     obj[key] = val
diff --git a/packages/vue-compat/__tests__/global.spec.ts b/packages/vue-compat/__tests__/global.spec.ts
index 3cd2c9a18d..02d578772b 100644
--- a/packages/vue-compat/__tests__/global.spec.ts
+++ b/packages/vue-compat/__tests__/global.spec.ts
@@ -360,6 +360,20 @@ describe('GLOBAL_PRIVATE_UTIL', () => {
     expect(vm.$el.textContent).toBe('2')
   })
 
+  test('defineReactive on instance with key that starts with $', async () => {
+    const vm = new Vue({
+      beforeCreate() {
+        // @ts-ignore
+        Vue.util.defineReactive(this, '$foo', 1)
+      },
+      template: `<div>{{ $foo }}</div>`
+    }).$mount() as any
+    expect(vm.$el.textContent).toBe('1')
+    vm.$foo = 2
+    await nextTick()
+    expect(vm.$el.textContent).toBe('2')
+  })
+
   test('defineReactive with object value', () => {
     const obj: any = {}
     const val = { a: 1 }
-- 
2.47.3