获取全局对象分类

NSObject+GetGlobalObject.h

#import <Foundation/Foundation.h>

@interface NSObject (GetGlobalObject)

@property (nonatomic , weak) UIScreen *mainScreen ;

@property (nonatomic , weak) UIApplication *app ;

@property (nonatomic , weak) UIWindow *keyWindow ;

@property (nonatomic , weak) UIWindow *contentWindow ;

@property (nonatomic , weak) UIViewController *rootVC ;

@property (nonatomic, assign) id<UIApplicationDelegate> appDelegate ;

@property (nonatomic,weak) NSNotificationCenter *msgCenter ;

@end

NSObject+GetGlobalObject.m

#import "NSObject+GetGlobalObject.h"

@implementation NSObject (GetGlobalObject)
@dynamic mainScreen , app , keyWindow , contentWindow , rootVC , appDelegate , msgCenter;

- (UIScreen*)mainScreen
{
    return [UIScreen mainScreen];
}

- (UIApplication*)app
{
    return [UIApplication sharedApplication] ;
}

- (UIWindow *)keyWindow
{
    return self.app.keyWindow ;
}

- (UIWindow *)contentWindow
{
    return self.app.delegate.window ;
}
- (UIViewController*)rootVC
{
    return self.contentWindow.rootViewController ;
}

- (id<UIApplicationDelegate>)appDelegate
{
    return self.app.delegate ;
}

- (NSNotificationCenter *)msgCenter
{
    return [NSNotificationCenter defaultCenter];
}
@end

添加contentWindow属性，修改keyWindow的bug。在弹出UIAlertView后，keyWindow已经不在是delegate的window，所以rootVC应该在contentWindow属性中获取