iOS异步变同步

        __block BOOL result = NO;
        //异步线程中操作是否完成
        __block BOOL inThreadOperationComplete = NO;
        [[UNUserNotificationCenter currentNotificationCenter] getNotificationSettingsWithCompletionHandler:^(UNNotificationSettings * _Nonnull settings) {
            if (settings.authorizationStatus == UNAuthorizationStatusDenied) {
                result = NO;
            }else if (settings.authorizationStatus == UNAuthorizationStatusNotDetermined) {
                result = NO;
            }else if (settings.authorizationStatus == UNAuthorizationStatusAuthorized) {
                result = YES;
            }else {
                result = NO;
            }
            inThreadOperationComplete = YES;
        }];
        
        while (!inThreadOperationComplete) {
            [NSThread sleepForTimeInterval:0];
        }
        return result;

另外一种写法：使用信号量

- (void)testDispatchSemaphore {
    NSLog(@"--------------------------begin-----------------");
    dispatch_semaphore_t sema = dispatch_semaphore_create(0);
    dispatch_queue_t que = dispatch_get_global_queue(0, 0);
    
    dispatch_async(que, ^{
        NSLog(@"--------------------------async begin-----------------");
        [NSThread sleepForTimeInterval:3];
        dispatch_semaphore_signal(sema);
        NSLog(@"--------------------------async complete-----------------");
    });
    
    
    dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
    NSLog(@"--------------------------complete-----------------");
    
}

--------------------------begin-----------------
--------------------------async begin-----------------
--------------------------async complete-----------------
--------------------------complete-----------------