diff --git a/Cargo.toml b/Cargo.toml
index 9098dcd..f394d0e 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -9,9 +9,14 @@ edition = "2021"
 # path = "src/examples/test_sync.rs"
 
 
+# [[bin]]
+# name = "test_async"
+# path = "src/examples/test_async.rs"
+
+
 [[bin]]
 name = "test_async"
-path = "src/examples/test_async.rs"
+path = "src/examples/test_cb.rs"
 
 
 [[test]]
diff --git a/src/examples/test_cb.rs b/src/examples/test_cb.rs
new file mode 100644
index 0000000..3639d80
--- /dev/null
+++ b/src/examples/test_cb.rs
@@ -0,0 +1,34 @@
+use tokio::time::sleep;
+use tokio::time::Duration;
+
+use futures::future::BoxFuture;
+
+fn test(val: u8) -> BoxFuture<'static, Result<bool, ()>> {
+    Box::pin(async move {
+        println!("cbbbbb!!! {}", val);
+        Ok(true)
+    })
+}
+
+#[tokio::main]
+async fn main() {
+    println!("test cb");
+
+    let mut cbb: Option<fn(u8) -> BoxFuture<'static, Result<bool, ()>>> = None;
+
+    cbb = Some(test);
+
+    tokio::spawn(cbb.unwrap()(1));
+
+    tokio::spawn(test(1));
+
+    tokio::spawn(test(2));
+
+    tokio::spawn(test(3));
+
+    tokio::spawn(test(4));
+
+    tokio::spawn(test(5));
+
+    sleep(Duration::from_secs(2)).await
+}