后台任务占用100％CPU并耗尽电池电量

Question

我正在开发一个企业应用程序，它必须定时报告信息，特别是当应用程序在后台时。

- (void)applicationWillResignActive:(UIApplication *)application {
     //stuff
     UIApplication *app = [UIApplication sharedApplication];

     //create new uiBackgroundTask
     bgTask = [app beginBackgroundTaskWithExpirationHandler:^{
     [app endBackgroundTask:bgTask];
     bgTask = UIBackgroundTaskInvalid;
     }];
     //and create new timer with async call:
     dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
     //run function methodRunAfterBackground
     timerOne = [NSTimer scheduledTimerWithTimeInterval:100 target:self selector:@selector(refreshInactiveTimer) userInfo:nil repeats:YES];
     timerTwo = [NSTimer scheduledTimerWithTimeInterval:300 target:self selector:@selector(postActivity) userInfo:nil repeats:YES];
     [[NSRunLoop currentRunLoop] addTimer:timerOne forMode:NSDefaultRunLoopMode];
     [[NSRunLoop currentRunLoop] addTimer:timerTwo forMode:NSDefaultRunLoopMode];
     [[NSRunLoop currentRunLoop] run];
     });
}

- (void)applicationWillEnterForeground:(UIApplication *)applicationq {
    [[UIApplication sharedApplication] endBackgroundTask:bgTask];
    [timerOne invalidate];
    [timerTwo invalidate];
}

-(void)refreshInactiveTimer {  //this function plays a short inaudible sound which refreshes the background timer
    AVSpeechSynthesizer *synth = [[AVSpeechSynthesizer alloc] init];
    AVSpeechUtterance *utter = [[AVSpeechUtterance alloc] initWithString:@"up"];
    [utter setVolume:0.0];
    [synth speakUtterance:utter];
}

-(void)postActivity {
    //this function makes a NSURLRequest that posts small data back to server side
}

代码按预期工作，但问题是它占用了100％的CPU（根据分析器）。我注意到，当我在拔下电源插头时进行测试时，电池似乎也很快耗尽，所以CPU肯定会消失。也许我在这里错过了一些小事。

注意：是的我知道这是不好的做法，Apple会拒绝使用这种方法的应用程序。但是，正如我所提到的，这是一个永远不会在App Store中存在且永远不会分发给普通公众的企业应用程序。