双击home后强制退出保存数据

在双击home后强制退出，导致保存失败。

其实苹果有一个UIApplicationWillResignActiveNotification通知：

看代码吧：

    
       //首先声明app
       UIApplication * app= [UIApplication sharedApplication];
      //接下来四行是新的功能，能检查设备是否支持后台，老系统和一代的backgroundSupported = NO;支持的自然就是YES啦
    UIDevice* device = [UIDevice currentDevice];
    BOOL backgroundSupported = NO;
    if ([device respondsToSelector:@selector(isMultitaskingSupported)])
    backgroundSupported = device.multitaskingSupported;
    
      //如果设备不支持后台，那么就继续使用applicationWillTerminate:
    if(backgroundSupported==NO)
    {
        [[NSNotificationCenter defaultCenter ] addObserver:self
                                                  selector:@selector(applicationWillTerminate:)
                                                      name:UIApplicationWillTerminateNotification
                                                    object:app];
    }
       //如果支持
    else{
        
        //这个新的UIApplicationWillResignActiveNotification就是在用户按下home键时的通知，建议用它的selector去保存数据，能防止用户直接在任务管理器里强制退出
        [[NSNotificationCenter defaultCenter ] addObserver:self
                                                  selector:@selector(applicationDidEnterBackground:)
                                                      name:UIApplicationWillResignActiveNotification
                                                    object:app];
        
                //这个UIApplicationWillEnterForegroundNotification是app在切回前台时的通知
        [[NSNotificationCenter defaultCenter ] addObserver:self
                                                  selector:@selector(applicationDidBecomeActive:)
                                                      name:UIApplicationWillEnterForegroundNotification
                                                    object:app];
        
    }