iOS7 之后 presentviewcontroller 延迟的解决方案

http://stackoverflow.com/questions/20087408/lag-between-viewwillappear-and-viewdidappear

两种解决方案 ：

1. 
dispatch_async(dispatch_get_main_queue(), ^{
        [presentingViewController presentViewController:VC
                                               animated:YES
                                             completion:^{
        }];
    });

2.

[self performSelectorOnMainThread:@selector(methodName) withObject:nil waitUntilDone:NO];

// methodName方法只是一个空方法就行