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

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   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];

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

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原文地址：http://www.cnblogs.com/MrXHong/p/5415295.html