fix(watch): watch will trigger when added new keys using set (#468)

* fix(watch): watch will trigger when added new keys using `set`

* chore: add docs

* Update README.md

Co-authored-by: Anthony Fu <anthonyfu117@hotmail.com>

* Update README.md

Co-authored-by: Anthony Fu <anthonyfu117@hotmail.com>

README.md

@@ -174,6 +174,27 @@ a.list.push(
 a.list[1].count === 1 // true
 ```
 
+</details>
+
+<details>
+<summary>
+⚠️ `set` workaround for adding new reactive properties
+</summary>
+
+> ⚠️ Warning: `set` does NOT exist in Vue 3. We provide it as a workaround here, due to the limitation of [Vue 2.x reactivity system](https://vuejs.org/v2/guide/reactivity.html#For-Objects). In Vue 2, you will need to call `set` to track new keys on an `object`(similar to `Vue.set` but for `reactive objects` created by the Composition API). In Vue 3, you can just assign them like normal objects.
+
+```ts
+import { reactive, set } from '@vue/composition-api'
+
+const a = reactive({
+  foo: 1
+})
+
+// add new reactive key
+set(a, 'bar', 1)
+```
+
+
 </details>
 
 ### Template Refs

src/apis/watch.ts

@@ -50,6 +50,9 @@ export interface VueWatcher {
   lazy: boolean
   get(): any
   teardown(): void
+  run(): void
+
+  value: any
 }
 
 export type WatchStopHandle = () => void
@@ -315,6 +318,19 @@ function createWatcher(
 
   // Once again, we have to hack the watcher for proper teardown
   const watcher = vm._watchers[vm._watchers.length - 1]
+
+  // if the return value is reactive and deep:true
+  // watch for changes, this might happen when new key is added
+  if (isReactive(watcher.value) && deep) {
+    watcher.value.__ob__.dep.addSub({
+      update() {
+        // this will force the source to be revaluated and the callback
+        // executed if needed
+        watcher.run()
+      },
+    })
+  }
+
   patchWatcherTeardown(watcher, runCleanup)
 
   return () => {

test/apis/watch.spec.js

@@ -1,5 +1,5 @@
 const Vue = require('vue/dist/vue.common.js')
-const { ref, reactive, watch, watchEffect } = require('../../src')
+const { ref, reactive, watch, watchEffect, set } = require('../../src')
 
 describe('api/watch', () => {
   const anyFn = expect.any(Function)
@@ -745,4 +745,16 @@ describe('api/watch', () => {
         .then(done)
     })
   })
+
+  it('should execute watch when new key is added', () => {
+    const r = reactive({})
+
+    const cb = jest.fn()
+
+    watch(r, cb, { deep: true })
+
+    set(r, 'a', 1)
+
+    expect(cb).toHaveBeenCalled()
+  })
 })