iOS开发 --- 推送 SDK: Main Thread Checker: UI API called on a background thread

Xcode 升级到 Xcode 9后，集成时若提示下述错误：

1. Main Thread Checker: UI API called on a background thread

请检查工程中，是否在后台线程（非主线程）调用 AppKit、UIKit相关的API，比如iOS 10+ 请求通知权限时，[application registerForRemoteNotifications];在回调非主线程中执行，则Xcode 9会报上述错误。

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注册，获取deviceToken
6. [application registerForRemoteNotifications];
7. } else {
8. // not granted
9. NSLog(@"User denied notification.");
10. }
11. }];

应修改为：

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注册，获取deviceToken
6. dispatch_async(dispatch_get_main_queue(), ^{
7. [application registerForRemoteNotifications];
8. };
9. } else {
10. // not granted
11. NSLog(@"User denied notification.");
12. }
13. }];