有时候调用presentViewController之后，视图并不会立刻出现，这是因为main runloop being asleep，解决办法如下

[self presentViewController:vc animated:YES completion:nil];
CFRunLoopWakeUp(CFRunLoopGetCurrent());
//Or you can enqueue an empty block to the main queue:
[self presentViewController:vc animated:YES completion:nil];
dispatch_async(dispatch_get_main_queue(), ^{});