计算视图相对坐标时convertPoint:toView: ，UIApplication sharedApplication - keyWindow is nil?

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];