prevent out-in transition to enter early when parent re-renders (fix #3440)

Author: yyx990803

Diff for: src/platforms/web/runtime/components/transition.js

@@ -48,6 +48,12 @@ export function extractTransitionData (comp: Component): Object {
   return data
 }
 
+function placeholder (h, rawChild) {
+  return /\d-keep-alive$/.test(rawChild.tag)
+    ? h('keep-alive')
+    : null
+}
+
 export default {
   name: 'transition',
   props: transitionProps,
@@ -101,6 +107,10 @@ export default {
       return rawChild
     }
 
+    if (this._leaving) {
+      return placeholder(h, rawChild)
+    }
+
     child.key = child.key == null
       ? `__v${child.tag + this._uid}__`
       : child.key
@@ -115,13 +125,13 @@ export default {
 
       // handle transition mode
       if (mode === 'out-in') {
-        // return empty node and queue update when leave finishes
+        // return placeholder node and queue update when leave finishes
+        this._leaving = true
         mergeVNodeHook(oldData, 'afterLeave', () => {
+          this._leaving = false
           this.$forceUpdate()
         })
-        return /\d-keep-alive$/.test(rawChild.tag)
-          ? h('keep-alive')
-          : null
+        return placeholder(h, rawChild)
       } else if (mode === 'in-out') {
         let delayedLeave
         const performLeave = () => { delayedLeave() }

Diff for: test/unit/features/transition/transition-mode.spec.js

@@ -91,6 +91,54 @@ if (!isIE9) {
       }).then(done)
     })
 
+    // #3440
+    it('dynamic components, out-in (with extra re-render)', done => {
+      let next
+      const vm = new Vue({
+        template: `<div>
+          <transition name="test" mode="out-in" @after-leave="afterLeave">
+            <component :is="view" class="test">
+            </component>
+          </transition>
+        </div>`,
+        data: { view: 'one' },
+        components,
+        methods: {
+          afterLeave () {
+            next()
+          }
+        }
+      }).$mount(el)
+      expect(vm.$el.textContent).toBe('one')
+      vm.view = 'two'
+      waitForUpdate(() => {
+        expect(vm.$el.innerHTML).toBe(
+          '<div class="test test-leave test-leave-active">one</div><!---->'
+        )
+      }).thenWaitFor(nextFrame).then(() => {
+        expect(vm.$el.innerHTML).toBe(
+          '<div class="test test-leave-active">one</div><!---->'
+        )
+        // Force re-render before the element finishes leaving
+        // this should not cause the incoming element to enter early
+        vm.$forceUpdate()
+      }).thenWaitFor(_next => { next = _next }).then(() => {
+        expect(vm.$el.innerHTML).toBe('<!---->')
+      }).thenWaitFor(nextFrame).then(() => {
+        expect(vm.$el.innerHTML).toBe(
+          '<div class="test test-enter test-enter-active">two</div>'
+        )
+      }).thenWaitFor(nextFrame).then(() => {
+        expect(vm.$el.innerHTML).toBe(
+          '<div class="test test-enter-active">two</div>'
+        )
+      }).thenWaitFor(duration + 10).then(() => {
+        expect(vm.$el.innerHTML).toBe(
+          '<div class="test">two</div>'
+        )
+      }).then(done)
+    })
+
     it('dynamic components, in-out', done => {
       let next
       const vm = new Vue({