亮度调事，不久在QR界面切换的时候还要考虑进入前台后台时的操作

1.QR界面功能实现代码。

QR界面- (void)viewWillAppear:(BOOL)animated
{[super viewWillAppear:animated];[[NSUserDefaults standardUserDefaults] setValue:@([UIScreen mainScreen].brightness) forKey:@"Brightness"];[UIScreen mainScreen].brightness = 1;
}- (void) viewWillDisappear:(BOOL)animated
{[super viewWillDisappear:animated];NSNumber* num = [[NSUserDefaults standardUserDefaults] objectForKey:@"Brightness"];if(num.floatValue > 0.1){[UIScreen mainScreen].brightness = num.floatValue;}
}

2.进入前后台时的处理。这个地方要意思，必须要在Appdelegate 

中的两个回调函数中实现，在QR()中添加进入前后台通知实现的话，会有问题。具体原历不清楚

- (void)applicationDidBecomeActive:(UIApplication *)application

- (void)applicationWillResignActive:(UIApplication *)application

- (void)applicationDidBecomeActive:(UIApplication *)application {application.applicationIconBadgeNumber = 0;if(当前界面是QR界面）{[UIScreen mainScreen].brightness = 1; }
}- (void)applicationWillResignActive:(UIApplication *)application
{NSNumber* num = [[NSUserDefaults standardUserDefaults] objectForKey:@"Brightness"];if(num.floatValue > 0.1){[UIScreen mainScreen].brightness = num.floatValue;}NSLog(@"num = %@", num);
}