一个 UIViewController 子类，提供 PIN 视图展示。
典型用法是在 UINavigationController 中展示 ViewController
#import "MGPinViewController.h"- (void)showPinViewController {
MGPinViewController *pinViewController = [[MGPinViewController alloc] initWithValidPin:@"1234"];
UINavigationController *pinNavigationController = [[UINavigationController alloc] initWithRootViewController:pinViewController];
[self presentModalViewController:pinNavigationController animated:NO];
}