UIWindow *window = [UIApplication sharedApplication].keyWindow;
window 为nil的原因：在指定rootViewController之前，没有让window 先可视化
1.示例
为nil：
self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
self.window.backgroundColor = [UIColor whiteColor];
self.window.rootViewController = [[ViewController alloc] init];
[self.window makeKeyAndVisible];
不为nil：
self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
[self.window makeKeyAndVisible];
self.window.backgroundColor = [UIColor whiteColor];
self.window.rootViewController = [[ViewController alloc] init];
2.为nil的解决方法二
用 delegate方法即可
UIWindow *window = [[UIApplication sharedApplication].delegate window];