fix(#8728): only execute the dependArray function once

src/core/observer/index.js

@@ -158,11 +158,13 @@ export function defineReactive (
     configurable: true,
     get: function reactiveGetter () {
       const value = getter ? getter.call(obj) : val
-      if (Dep.target) {
+      const targetWatcher = Dep.target
+      if (targetWatcher) {
         dep.depend()
         if (childOb) {
+          const needDepend = Array.isArray(value) && !targetWatcher.checkRelated(childOb.dep)
           childOb.dep.depend()
-          if (Array.isArray(value)) {
+          if (needDepend) {
             dependArray(value)
           }
         }

src/core/observer/watcher.js

@@ -139,6 +139,13 @@ export default class Watcher {
     }
   }
 
+  /**
+   * Check if a dependency has been associated with the current watcher.
+   */
+  checkRelated (dep: Dep) {
+    return this.newDepIds.has(dep.id)
+  }
+
   /**
    * Clean up for dependency collection.
    */

test/unit/modules/observer/observer.spec.js

@@ -6,7 +6,7 @@ import {
   del as delProp
 } from 'core/observer/index'
 import Dep from 'core/observer/dep'
-import { hasOwn } from 'core/util/index'
+import { hasOwn, _Set as Set } from 'core/util/index'
 
 describe('Observer', () => {
   it('create on non-observables', () => {
@@ -356,6 +356,33 @@ describe('Observer', () => {
     })
   })
 
+  it('Array\'s getter is triggered multiple times, the Watcher should only associate with Dep once.', () => {
+    const data = {
+      arr: [{}]
+    }
+    observe(data)
+    // mock a watcher!
+    const watcher = {
+      newDepIds: new Set(),
+      addDep (dep) {
+        this.newDepIds.add(dep.id)
+        dep.addSub(this)
+      },
+      checkRelated: function (dep) {
+        return this.newDepIds.has(dep.id)
+      }
+    }
+    spyOn(watcher, 'checkRelated').and.callThrough()
+    Dep.target = watcher
+    data.arr
+    data.arr
+    Dep.target = null
+    expect(watcher.checkRelated.calls.count()).toBe(2)
+    const allCalls = watcher.checkRelated.calls.all()
+    expect(allCalls[0].returnValue).toBe(false)
+    expect(allCalls[1].returnValue).toBe(true)
+  })
+
   it('warn set/delete on non valid values', () => {
     try {
       setProp(null, 'foo', 1)