feat(reactive): allow usage of reactive before Vue.use (#515)

src/reactivity/reactive.ts

@@ -1,5 +1,5 @@
 import { AnyObject } from '../types/basic'
-import { getVueConstructor } from '../runtimeContext'
+import { getRegisteredVueOrDefault } from '../runtimeContext'
 import { isPlainObject, def, warn } from '../utils'
 import { isComponentInstance, defineComponentInstance } from '../utils/helper'
 import { RefKey } from '../utils/symbols'
@@ -94,7 +94,7 @@ export function defineAccessControl(target: AnyObject, key: any, val?: any) {
 }
 
 function observe<T>(obj: T): T {
-  const Vue = getVueConstructor()
+  const Vue = getRegisteredVueOrDefault()
   let observed: T
   if (Vue.observable) {
     observed = Vue.observable(obj)

src/runtimeContext.ts

@@ -2,6 +2,14 @@ import type { VueConstructor } from 'vue'
 import { ComponentInstance } from './component'
 import { assert, hasOwn, warn } from './utils'
 
+let vueDependency: VueConstructor | undefined = undefined
+
+try {
+  vueDependency = require('vue')
+} catch {
+  // not available
+}
+
 let vueConstructor: VueConstructor | null = null
 let currentInstance: ComponentInstance | null = null
 
@@ -26,6 +34,17 @@ export function getVueConstructor(): VueConstructor {
   return vueConstructor!
 }
 
+// returns registered vue or `vue` dependency
+export function getRegisteredVueOrDefault(): VueConstructor {
+  let constructor = vueConstructor || vueDependency
+
+  if (__DEV__) {
+    assert(vueConstructor, `No vue dependency found.`)
+  }
+
+  return constructor!
+}
+
 export function setVueConstructor(Vue: VueConstructor) {
   if (__DEV__ && vueConstructor) {
     warn('Another instance of vue installed')