wip: fix watch simple effect usage

packages/runtime-core/src/apiWatch.ts

@@ -77,9 +77,6 @@ function doWatch(
     | null,
   { lazy, deep, flush, onTrack, onTrigger }: WatchOptions = EMPTY_OBJ
 ): StopHandle {
-  const scheduler =
-    flush === 'sync' ? invoke : flush === 'pre' ? queueJob : queuePostFlushCb
-
   const baseGetter = isArray(source)
     ? () => source.map(s => (isRef(s) ? s.value : s()))
     : isRef(source)
@@ -111,17 +108,24 @@ function doWatch(
       }
     : void 0
 
+  const scheduler =
+    flush === 'sync' ? invoke : flush === 'pre' ? queueJob : queuePostFlushCb
+
   const runner = effect(getter, {
     lazy: true,
     // so it runs before component update effects in pre flush mode
     computed: true,
     onTrack,
     onTrigger,
-    scheduler: applyCb ? () => scheduler(applyCb) : void 0
+    scheduler: applyCb ? () => scheduler(applyCb) : scheduler
   })
 
   if (!lazy) {
-    applyCb && scheduler(applyCb)
+    if (applyCb) {
+      scheduler(applyCb)
+    } else {
+      scheduler(runner)
+    }
   } else {
     oldValue = runner()
   }

packages/runtime-core/src/scheduler.ts

@@ -8,22 +8,32 @@ export function nextTick(fn?: () => void): Promise<void> {
   return fn ? p.then(fn) : p
 }
 
-export function queueJob(job: () => void, onError?: (err: Error) => void) {
+type ErrorHandler = (err: Error) => void
+
+export function queueJob(job: () => void, onError?: ErrorHandler) {
   if (queue.indexOf(job) === -1) {
     queue.push(job)
-    if (!isFlushing) {
-      const p = nextTick(flushJobs)
-      if (onError) p.catch(onError)
-    }
+    queueFlush(onError)
   }
 }
 
-export function queuePostFlushCb(cb: Function | Function[]) {
+export function queuePostFlushCb(
+  cb: Function | Function[],
+  onError?: ErrorHandler
+) {
   if (Array.isArray(cb)) {
     postFlushCbs.push.apply(postFlushCbs, cb)
   } else {
     postFlushCbs.push(cb)
   }
+  queueFlush(onError)
+}
+
+function queueFlush(onError?: ErrorHandler) {
+  if (!isFlushing) {
+    const p = nextTick(flushJobs)
+    if (onError) p.catch(onError)
+  }
 }
 
 const dedupe = (cbs: Function[]): Function[] => Array.from(new Set(cbs))