計算視圖相對座標時convertPoint:toView: ，UIApplication sharedApplication - keyWindow is nil?

標籤：

   UIWindow *window = [UIApplication sharedApplication].keyWindow;

 

 window 為nil的原因：在指定rootViewController之前，沒有讓window 先可視化

 

1.樣本

 為nil：

　self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];

   self.window.backgroundColor = [UIColor whiteColor];

    self.window.rootViewController = [[ViewController alloc] init];

 

   [self.window makeKeyAndVisible]; 

 

 不為nil：

   self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];

 

   [self.window makeKeyAndVisible];

 

   self.window.backgroundColor = [UIColor whiteColor];

   self.window.rootViewController = [[ViewController alloc] init];

 

2.為nil的解決方案二

 用 delegate方法即可

 UIWindow *window = [[UIApplication sharedApplication].delegate window];

 

計算視圖相對座標時convertPoint:toView: ，UIApplication sharedApplication - keyWindow is nil?