结束后台任务并等待应用程序再次激活-BLE处理数据

我的应用程序通过BLE从外围设备下载大量数据。如果我锁定屏幕,我的应用程序会移动到后台,并启动后台任务。下载完成得很好,但如果处理开始(因为数据量很大,需要很长时间),应用程序就会崩溃,因为它无法连接到数据库。

我想在那一刻停止执行,等待应用程序再次激活,但不知为什么我无法实现这一点。我想我需要某种信号来等待应用程序激活。

这里是我迄今为止的代码:

- (void)viewDidLoad
{
    //Some other code

    //initialize flag        
    isInBackgroud = NO;

    // check if app is in the background
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(appDidEnterBackground) name:UIApplicationDidEnterBackgroundNotification object:nil];

    // check if app is in the foreground
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(appDidEnterForeground) name:UIApplicationDidBecomeActiveNotification object:nil];
}

- (void)appDidEnterBackground {
    NSLog(@"appDidEnterBackground");
    isInBackground = YES;
    UIApplication *app = [UIApplication sharedApplication];
    NSLog(@"remaining Time: %f", [app backgroundTimeRemaining]);
    bgTask = [app beginBackgroundTaskWithExpirationHandler:^{
        NSLog(@"expirationHandler");
        [app endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    }];
}

- (void)appDidEnterForeground {
    NSLog(@"appDidEnterForeground");
    isInBackground = NO;
    if (bgTask != UIBackgroundTaskInvalid) {
        UIApplication *app = [UIApplication sharedApplication];
        [app endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    }
}

//BLE connection and reading data via notification
//when finished [self processData] is called.

- (void)processData {
    if (isInBackground) {
        //set reminder
        UILocalNotification *localNotification = [[UILocalNotification alloc] init];
        localNotification.fireDate = [NSDate date];
        localNotification.alertBody = [NSString stringWithFormat:@"Data was downloaded, return to the application to proceed processing your data."];
        localNotification.timeZone = [NSTimeZone defaultTimeZone];
        [[UIApplication sharedApplication] scheduleLocalNotification:localNotification];
        UIApplication *app = [UIApplication sharedApplication];

        //end background task
        [app endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;

        //wait for application to become active again
        while (isInBackground) {
            NSLog(@"isInBackground");
            NSLog(@"remaining Time: %f", [app backgroundTimeRemaining]);
            sleep(1);
        }

        //process data
    }

所以我注意到如果我打电话 [app endBackgroundTask:bgTask]; 应用程序只是继续运行,但当我想连接到我的数据库时就会崩溃。这就是为什么我添加了 while(isInBackground) 环我知道这不是一个好的做法,因为它在做笔记时会主动浪费CPU时间。我当时应该使用信号灯,但我想不出怎么做。

因为我正在积极地放弃这个循环, appDidEnterForegronund 从不调用,循环将永远运行。