Xcode 升級到 Xcode 9後，整合時若提示下述錯誤：

1. MainThreadChecker: UI API called on a background thread

請檢查工程中，是否在後臺執行緒（非主執行緒）呼叫 AppKit、UIKit相關的API，比如iOS 10+ 請求通知許可權時，[application registerForRemoteNotifications];在回撥非主執行緒中執行，則Xcode 9會報上述錯誤。

1. [_notificationCenter requestAuthorizationWithOptions:UNAuthorizationOptionAlert
   |UNAuthorizationOptionBadge|UNAuthorizationOptionSound completionHandler:^(BOOL granted,NSError*_Nullable error){
2. if(granted){
3. // granted
4. NSLog(@"User authored notification.");
5. // 向APNs註冊，獲取deviceToken
6. [application registerForRemoteNotifications];
7. }else{
8. // not granted
9. NSLog(@"User denied notification.");
10. }
11. }];

應修改為：

1. [_notificationCenter requestAuthorizationWithOptions:UNAuthorizationOptionAlert|UNAuthorizationOptionBadge|UNAuthorizationOptionSound completionHandler:^(BOOL granted,NSError*_Nullable error){
2. if(granted){
3. // granted
4. NSLog(@"User authored notification.");
5. // 向APNs註冊，獲取deviceToken
6. dispatch_async(dispatch_get_main_queue
   (),^{
7. [application registerForRemoteNotifications];
8. };
9. }else{
10. // not granted
11. NSLog(@"User denied notification.");
12. }
13. }];