diff --git a/Examples/coobjcBaseExample/coobjcBaseExampleTests/coobjcSequenceTests.m b/Examples/coobjcBaseExample/coobjcBaseExampleTests/coobjcSequenceTests.m
index 0861e27..ef02a3c 100644
--- a/Examples/coobjcBaseExample/coobjcBaseExampleTests/coobjcSequenceTests.m
+++ b/Examples/coobjcBaseExample/coobjcBaseExampleTests/coobjcSequenceTests.m
@@ -24,6 +24,30 @@
 #import <coobjc/coobjc.h>
 #import "coobjcCommon.h"
 
+static COPromise<NSData *> *co_downloadWithURL(NSString *url) {
+    
+    return [COPromise promise:^(COPromiseFullfill  _Nonnull fullfill, COPromiseReject  _Nonnull reject) {
+        
+        [NSURLSession sharedSession].configuration.requestCachePolicy = NSURLRequestReloadIgnoringCacheData;
+        NSURLSessionDownloadTask *task = [[NSURLSession sharedSession] downloadTaskWithURL:[NSURL URLWithString:url] completionHandler:
+                                          ^(NSURL *location, NSURLResponse *response, NSError *error) {
+                                              if (error) {
+                                                  reject(error);
+                                                  return;
+                                              }
+                                              else{
+                                                  NSData *data = [[NSData alloc] initWithContentsOfURL:location];
+                                                  
+                                                  fullfill(data);
+                                                  return;
+                                              }
+                                          }];
+        
+        [task resume];
+        
+    }];
+}
+
 SpecBegin(coSequence)
 
 describe(@"test sequence on same queue", ^{
@@ -90,7 +114,7 @@
         COCoroutine *co1 = co_sequence(^{
             int index = 0;
             while(co_isActive()){
-                yield([NSData co_downloadWithURL:@"http://pytstore.oss-cn-shanghai.aliyuncs.com/GalileoShellApp.ipa"]);
+                yield(co_downloadWithURL(@"http://pytstore.oss-cn-shanghai.aliyuncs.com/GalileoShellApp.ipa"));
                 index++;
             }
         });
@@ -117,7 +141,7 @@
         COCoroutine *co2 = co_sequence(^{
             int index = 0;
             while(co_isActive()){
-                yield([NSData co_downloadWithURL:@"http://pytstore.oss-cn-shanghai.aliyuncs.com/GalileoShellApp.ipa"]);
+                yield(co_downloadWithURL(@"http://pytstore.oss-cn-shanghai.aliyuncs.com/GalileoShellApp.ipa"));
                 index++;
             }
         });
@@ -217,7 +241,7 @@
         COCoroutine *co1 = co_sequence_onqueue(q1, ^{
             int index = 0;
             while(co_isActive()){
-                yield([NSData co_downloadWithURL:@"http://pytstore.oss-cn-shanghai.aliyuncs.com/GalileoShellApp.ipa"]);
+                yield(co_downloadWithURL(@"http://pytstore.oss-cn-shanghai.aliyuncs.com/GalileoShellApp.ipa"));
                 index++;
             }
         });
@@ -246,7 +270,7 @@
         COCoroutine *co2 = co_sequence_onqueue(q1, ^{
             int index = 0;
             while(co_isActive()){
-                yield([NSData co_downloadWithURL:@"http://pytstore.oss-cn-shanghai.aliyuncs.com/GalileoShellApp.ipa"]);
+                yield(co_downloadWithURL(@"http://pytstore.oss-cn-shanghai.aliyuncs.com/GalileoShellApp.ipa"));
                 index++;
             }
         });
diff --git a/coobjc/api/coobjc.h b/coobjc/api/coobjc.h
index 44080d9..5d5b916 100644
--- a/coobjc/api/coobjc.h
+++ b/coobjc/api/coobjc.h
@@ -160,12 +160,18 @@ NS_INLINE NSArray<id> *_Nullable batch_await(NSArray<id> * _Nonnull _promiseOrCh
 }
 
 /**
- yield with a COPromise or COChan
+ yield with a COPromise
  
- @param _promiseOrChan the COPromise object or COChan object.
+ @discussion `yield` means pause the expression execution,
+ until Generator(coroutine) call `next`.
+ 
+ So, you should passing a `COPromise` object with constructor,
+ To make the expression lazy loading.
+ 
+ @param _promise the COPromise object.
  */
-NS_INLINE void yield(id _Nonnull _promiseOrChan) {
-    co_generator_yield(_promiseOrChan);
+NS_INLINE void yield(id _Nonnull _promise) {
+    co_generator_yield(_promise);
 }
 
 /**