写API的目的在于作为一个字典的作用。简单，明了，温习，复习。
//1.定义一个CGFloat类型的UIWindowLevel，UIWindowLevel分为三种不同选择，定义了UIWindow不同层级的展示方式，UIWindow在现实的时候会根据三种不同选择进行不同的排序，即level高的将排在level比他低的层级前面。
typedef
CGFloat UIWindowLevel;
//2.添加UIScreen屏幕属性
@property(nonatomic,retain)
UIScreen *screen
NS_AVAILABLE_IOS(3_2);
// default is [UIScreen mainScreen]. changing the screen may be an expensive operation and should not be done in performance-sensitive code
//3. UIWindowLevel属性
@property(nonatomic)
UIWindowLevel windowLevel;
// default = 0.0
//4.keyWindow是指定的用来接收键盘以及非触摸类的消息，而且程序中每一个时候只能有一个window是keyWindow
@property(nonatomic,readonly,getter=isKeyWindow)
BOOL keyWindow;
//5.设置当前window变成主window
- (void)becomeKeyWindow;
// override point for subclass. Do not call directly
//6.放弃当前主window
- (void)resignKeyWindow;
// override point for subclass. Do not call directly
//7.让当前window变成keyWindow
- (void)makeKeyWindow;
//8.让当前window变成keyWindow，并显示出来
- (void)makeKeyAndVisible;
// convenience. most apps call this to show the main window and also make it key. otherwise use view hidden property
//9.设置UIViewController为根控制器
@property(nonatomic,retain)
UIViewController *rootViewController
NS_AVAILABLE_IOS(4_0);
// default is nil
//10.当产生一个事件时，先传递给UIApplication，而UIAppliaction将这个事件传递UIWindow进行处理，然后由UIWindow将这个时间传递给特定的对象，即first responder，而UIWindow就是通过sendEvent寻找first responder，经过sendEvent分发到合适的对象，sendEvent就相当于时间的中转站。
- (void)sendEvent:(UIEvent *)event;
// called by UIApplication to dispatch events to views inside the window
//11.将当前window一个坐标转化为相对于另外一个窗口的坐标
- (CGPoint)convertPoint:(CGPoint)point toWindow:(UIWindow *)window;
// can be used to convert to another window
//12.将另一个窗口的一个坐标转化为当前窗口的坐标
- (CGPoint)convertPoint:(CGPoint)point fromWindow:(UIWindow *)window;
// pass in nil to mean screen
//13.转化当前窗口一个矩形坐标相对于当前窗口的坐标.
- (CGRect)convertRect:(CGRect)rect toWindow:(UIWindow *)window;
//14.转化里面那个外窗口一个矩形坐标相对于当前窗口的坐标
- (CGRect)convertRect:(CGRect)rect fromWindow:(UIWindow *)window;
@end
//15.定义UIWindow展示优先级不同层次的选择 ，根据window显示级别优先的原则，级别高的灰显示在上面，级别低的在下面，我们程序正常显示的位于最底层
UIKIT_EXTERN
const
UIWindowLevel UIWindowLevelNormal;
//默认的window就是Normal级别，优先级值这个层次的值为0，通常我们程序的界面都是处于这个Normal级别的
UIKIT_EXTERN
const UIWindowLevel UIWindowLevelAlert;
//屏幕的statusBar处于中等水平，优先级值为1000
UIKIT_EXTERN
const
UIWindowLevel UIWindowLevelStatusBar;// alert级别的，通常是UIAlertView和UIAlertSheet这些用来中断正常流程，提醒用户等操作的。优先级值为2000.按照这样说来，alertView弹框出来下面也是一层UIWindow，只不过是优先级最高的alert级别，所以是keyWindow，当前只能交互alert，这就是为什么弹框出来其他界面都不可交互的原因
//16.接下来是window变化的四个通知
UIKIT_EXTERN
NSString *const UIWindowDidBecomeVisibleNotification; //当window显示就会调用
UIKIT_EXTERN
NSString *const UIWindowDidBecomeHiddenNotification; //window被隐藏之后调用，需要注意的是，当程序进入后台时，这两个通知不会被调用，即使应用转到后台，窗口不会显示，窗口应用下的上下文中仍然被认为是可见的
UIKIT_EXTERN
NSString *const UIWindowDidBecomeKeyNotification; //window成为主窗口调用
UIKIT_EXTERN
NSString *const UIWindowDidResignKeyNotification; //window撤销主窗口调用
//17. 监听键盘变化的通知
UIKIT_EXTERN
NSString *const UIKeyboardWillShowNotification; //当键盘将要显示的时候调用
UIKIT_EXTERN
NSString *const UIKeyboardDidShowNotification; //当键盘已经显示的时候调用
UIKIT_EXTERN
NSString *const UIKeyboardWillHideNotification; //当键盘将要隐藏的时候调用
UIKIT_EXTERN
NSString *const UIKeyboardDidHideNotification;
//当键盘隐藏完毕的时候调用
//18.关于监听键盘通知的一些userInfo消息
UIKIT_EXTERN
NSString *const UIKeyboardFrameBeginUserInfoKey
NS_AVAILABLE_IOS(3_2); //动画前键盘的位置，包含CGRect的NSValue
可以在通知方法中写得到位置
CGRect rect = [[notif.userInfo objectForKey:UIKeyboardFrameEndUserInfoKey] CGRectValue];
UIKIT_EXTERN
NSString *const UIKeyboardFrameEndUserInfoKey
NS_AVAILABLE_IOS(3_2);
//动画结束后键盘的位置 包含CGRect的NSValue
UIKIT_EXTERN
NSString *const UIKeyboardAnimationDurationUserInfoKey
NS_AVAILABLE_IOS(3_0);
//动画的持续时间，数值是NSNumber
UIKIT_EXTERN
NSString *const UIKeyboardAnimationCurveUserInfoKey
NS_AVAILABLE_IOS(3_0); //动画的曲线类型（UIViewAnimationCurve），数值是NSNumber
//19.监听键盘frame大小变化的通知
UIKIT_EXTERN
NSString *const UIKeyboardWillChangeFrameNotification
NS_AVAILABLE_IOS(5_0); //当键盘frame值将要改变的时候调用
UIKIT_EXTERN
NSString *const UIKeyboardDidChangeFrameNotification
NS_AVAILABLE_IOS(5_0); //当键盘frame值已经改变的时候调用
//下面为一些废弃很久的API，不多做解释
UIKIT_EXTERN
NSString *const UIKeyboardCenterBeginUserInfoKey
NS_DEPRECATED_IOS(2_0,
3_2);
UIKIT_EXTERN
NSString *const UIKeyboardCenterEndUserInfoKey
NS_DEPRECATED_IOS(2_0,
3_2);
UIKIT_EXTERN
NSString *const UIKeyboardBoundsUserInfoKey
NS_DEPRECATED_IOS(2_0,
3_2);