ios category 添加属性

@interface AppDelegate (adv) 

@property (nonatomic, strong) UIView *backgroundView;

@end

@implementation AppDelegate (adv)
-(UIView *)backgroundView
{
    return objc_getAssociatedObject(self, @selector(backgroundView));
}

-(void)setBackgroundView:(UIView *)backgroundView
{
    return objc_setAssociatedObject(self, @selector(backgroundView), backgroundView, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
@end