adjust scheduler: call user watchers before component updates

Author: yyx990803

src/core/observer/scheduler.js

@@ -39,13 +39,8 @@ function resetSchedulerState () {
  */
 function flushSchedulerQueue () {
   flushing = true
-  runSchedulerQueue(queue.sort(queueSorter))
   runSchedulerQueue(userQueue)
-  // user watchers triggered more watchers,
-  // keep flushing until it depletes
-  if (queue.length) {
-    return flushSchedulerQueue()
-  }
+  runSchedulerQueue(queue.sort(queueSorter))
   // devtool hook
   /* istanbul ignore if */
   if (devtools && config.devtools) {
@@ -103,11 +98,16 @@ function runSchedulerQueue (queue: Array<Watcher>) {
 export function queueWatcher (watcher: Watcher) {
   const id = watcher.id
   if (has[id] == null) {
+    // if already flushing, and all user watchers have already been run,
+    // run the new user watcher immediately.
+    if (flushing && watcher.user && !userQueue.length) {
+      return watcher.run()
+    }
     // push watcher into appropriate queue
+    has[id] = true
     const q = watcher.user
       ? userQueue
       : queue
-    has[id] = true
     if (!flushing) {
       q.push(watcher)
     } else {

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

@@ -45,7 +45,7 @@ describe('Scheduler', () => {
     }).then(done)
   })
 
-  it('calls user watchers after directive updates', done => {
+  it('call user watchers before component re-render', done => {
     const vals = []
     function run () {
       vals.push(this.id)
@@ -67,9 +67,34 @@ describe('Scheduler', () => {
       run: run
     })
     waitForUpdate(() => {
-      expect(vals[0]).toBe(1)
-      expect(vals[1]).toBe(2)
-      expect(vals[2]).toBe(3)
+      expect(vals).toEqual([2, 1, 3])
+    }).then(done)
+  })
+
+  it('call user watcher triggered by component re-render immediately', done => {
+    // this happens when a component re-render updates the props of a child
+    const vals = []
+    queueWatcher({
+      id: 1,
+      run () {
+        vals.push(1)
+        queueWatcher({
+          id: 3,
+          user: true,
+          run () {
+            vals.push(3)
+          }
+        })
+      }
+    })
+    queueWatcher({
+      id: 2,
+      run () {
+        vals.push(2)
+      }
+    })
+    waitForUpdate(() => {
+      expect(vals).toEqual([1, 3, 2])
     }).then(done)
   })
 
@@ -106,7 +131,7 @@ describe('Scheduler', () => {
       }
     })
     waitForUpdate(() => {
-      expect(callOrder.join()).toBe('1,2,3')
+      expect(callOrder).toEqual([1, 2, 3])
     }).then(done)
   })
 })