diff --git a/packages/runtime-core/__tests__/apiWatch.spec.ts b/packages/runtime-core/__tests__/apiWatch.spec.ts
index b71d17e..63e269b 100644
--- a/packages/runtime-core/__tests__/apiWatch.spec.ts
+++ b/packages/runtime-core/__tests__/apiWatch.spec.ts
@@ -1,6 +1,9 @@
+import { reactive } from "@mini-vue/reactivity";
+import { watchEffect } from "../src/apiWatch";
+import { nextTick } from "../src/scheduler";
+
 describe("api: watch", () => {
-  it.todo("effect", async () => {
-    // 先实现个 watchEffect 玩一玩
+  it("effect", async () => {
     const state = reactive({ count: 0 });
     let dummy;
     watchEffect(() => {
diff --git a/packages/runtime-core/src/apiWatch.ts b/packages/runtime-core/src/apiWatch.ts
new file mode 100644
index 0000000..8903a27
--- /dev/null
+++ b/packages/runtime-core/src/apiWatch.ts
@@ -0,0 +1,29 @@
+import { ReactiveEffect } from "@mini-vue/reactivity";
+import { queuePreFlushCb } from "./scheduler";
+
+// Simple effect.
+export function watchEffect(effect) {
+  doWatch(effect);
+}
+
+function doWatch(source) {
+  // 把 job 添加到 pre flush 里面
+  // 也就是在视图更新完成之前进行渲染（待确认？）
+  // 当逻辑执行到这里的时候 就已经触发了 watchEffect
+  const job = () => {
+    effect.run();
+  };
+
+  // 这里用 scheduler 的目的就是在更新的时候
+  // 让回调可以在 render 前执行 变成一个异步的行为（这里也可以通过 flush 来改变）
+  const scheduler = () => queuePreFlushCb(job);
+
+  const getter = () => {
+    source();
+  };
+
+  const effect = new ReactiveEffect(getter, scheduler);
+
+  // 这里执行的就是 getter
+  effect.run();
+}
diff --git a/packages/runtime-core/src/scheduler.ts b/packages/runtime-core/src/scheduler.ts
index 1abaa10..b8e197d 100644
--- a/packages/runtime-core/src/scheduler.ts
+++ b/packages/runtime-core/src/scheduler.ts
@@ -1,10 +1,10 @@
 const queue: any[] = [];
-const activePreFlushCbs:any = [];
+const activePreFlushCbs: any = [];
 
 const p = Promise.resolve();
 let isFlushPending = false;
 
-export function nextTick(fn) {
+export function nextTick(fn?) {
   return fn ? p.then(fn) : p;
 }
 
@@ -37,6 +37,9 @@ function queueCb(cb, activeQueue) {
   // todo 这里没有考虑 activeQueue 是否已经存在 cb 的情况
   // 然后在执行 flushJobs 的时候就可以调用 activeQueue 了
   activeQueue.push(cb);
+
+  // 然后执行队列里面所有的 job
+  queueFlush()
 }
 
 function flushJobs() {
@@ -60,6 +63,6 @@ function flushJobs() {
 function flushPreFlushCbs() {
   // 执行所有的 pre 类型的 job
   for (let i = 0; i < activePreFlushCbs.length; i++) {
-    activePreFlushCbs[i]()
+    activePreFlushCbs[i]();
   }
 }