iPhone上的距离传感器通过UIDevice开启，开启后系统以Notification的方式通知。

①开启距离传感器。监听通知：

- (void)viewDidLoad {
    [super viewDidLoad];

    // 开启传感器在iOS3之后通过UIDevice的currentDevice的proximityMonitoringEnabled设置
    [UIDevice currentDevice].proximityMonitoringEnabled = YES;
    
    // 监听到物体靠近时，系统会发出通知
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(proximityStateChange:) name:UIDeviceProximityStateDidChangeNotification object:nil];
    
}

- (void)proximityStateChange:(NSNotification *)nof{
    
    NSLog(@"%d",[UIDevice currentDevice].proximityState);
    if ([UIDevice currentDevice].proximityState) {
        NSLog(@"靠近");
    }else{
        NSLog(@"离开");
    }
    
}

- (void)dealloc{
    
    [[NSNotificationCenter defaultCenter] removeObserver:self];
    
}