如何解决iOS后台应用程序取消无法解决？-HowtotroubleshootiOSbackgroundappfetchnotworking?

20  

Your problem is that you are returning from performFetchWithCompletionHandler before you call the completion handler, since the network fetch operation is occurring the in the background and you call the completion handler in your delegate method. Since iOS thinks you aren't playing by the rules it will deny your ability to use background fetch.

您的问题是您在调用完成处理程序之前从performFetchWithCompletionHandler返回，因为网络提取操作正在后台进行，您在委托方法中调用完成处理程序。由于iOS认为您不遵守规则，因此它将拒绝您使用后台获取的能力。

To fix the problem you need to call beginBackgroundTaskWithExpirationHandler and then end that task after you have called the completion handler.

要解决此问题，您需要调用beginBackgroundTaskWithExpirationHandler，然后在调用完成处理程序后结束该任务。

Something like:

就像是：

UIBackgroundTaskIdentifier backgroundTask

-(void)application:(UIApplication *)application performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler
{
    fetchCompletiOnHandler= completionHandler;

    fetchStart = [NSDate date];

    self.backgroundTask = [application beginBackgroundTaskWithExpirationHandler:^{
        [application endBackgroundTask:self.backgroundUpdateTask];
        self.backgroundTask = UIBackgroundTaskInvalid;
    }];

    [[NSUserDefaults standardUserDefaults] setObject:fetchStart forKey:kLastCheckedContentDate];

    [FeedParser parseFeedAtUrl:url withDelegate:self];
}

-(void)onParserFinished
{
    DDLogVerbose(@"AppDelegate/onParserFinished");

    UIBackgroundFetchResult result = UIBackgroundFetchResultNoData;

    NSDate *fetchEnd = [NSDate date];
    NSTimeInterval timeElapsed = [fetchEnd timeIntervalSinceDate:fetchStart];
    DDLogVerbose(@"Background Fetch Duration: %f seconds", timeElapsed);
    if ([self.mostRecentContentDate compare:item.date] <0) {
        DDLogVerbose(@"got new content: %@", item.date);
        self.mostRecentCOntentDate= item.date;
        [self scheduleNotificationWithItem:item];
        result = UIBackgroundFetchResultNewData;
    }
    else {
        DDLogVerbose(@"no new content.");
        UILocalNotification* localNotification = [[UILocalNotification alloc] init];
        localNotification.alertBody = [NSString stringWithFormat:@"Checked for new posts in %f seconds", timeElapsed];
        [[UIApplication sharedApplication] scheduleLocalNotification:localNotification];
    }
    fetchCompletionHandler(result);
    [[UIApplication sharedApplication] application endBackgroundTask:self.backgroundUpdateTask];
    self.backgroundTask = UIBackgroundTaskInvalid;
}

My test app using this approach has been executing a fetch every 15 minutes initially, but it becomes less frequent over time. Without the background task it exhibited the same issue you are seeing.

我使用这种方法的测试应用程序最初每15分钟执行一次获取，但随着时间的推移它会变得不那么频繁。如果没有后台任务，它会显示您遇到的相同问题。

I found that setting the background fetch interval to something other than UIApplicationBackgroundFetchIntervalMinimum also helps. My test app is running with a background fetch interval of 3600 (one hour) and has been reliably triggering for several days now; even after a phone restart and not running the app again. The actual trigger interval is 2-3 hours however.

我发现将背景提取间隔设置为UIApplicationBackgroundFetchIntervalMinimum以外的其他内容也有帮助。我的测试应用程序正在以3600（一小时）的后台获取间隔运行，并且已经可靠地触发了几天;即使手机重启后又没有再运行应用程序。但实际的触发间隔是2-3小时。

My sample app is here

我的示例应用就在这里

10  

In order to implement background fetch there are three things you must do:

为了实现后台提取，您必须做三件事：

• Check the box Background fetch in the Background Modes of your app’s Capabilities.
• 选中应用功能的后台模式中的背景提取框。
• Use setMinimumBackgroundFetchInterval(_:) to set a time interval appropriate for your app.
• 使用setMinimumBackgroundFetchInterval（_ :)设置适合您的应用程序的时间间隔。
• Implement application(_:performFetchWithCompletionHandler:) in your app delegate to handle the background fetch.
• 在您的app委托中实现应用程序（_：performFetchWithCompletionHandler :)以处理后台提取。

Background Fetch Frequency

背景提取频率

How frequent our application can perform Background Fetch is determined by the system and it depends on:

我们的应用程序执行背景提取的频率由系统决定，它取决于：

• Whether network connectivity is available at that particular time
• 网络连接是否在该特定时间可用
• Whether the device is awake i.e. running
• 设备是否清醒即运行
• How much time and data your application has consumed in the previous Background Fetch
• 您的应用程序在之前的Background Fetch中消耗了多少时间和数据

In other words, your application is entirely at the mercy of the system to schedule background fetch for you. In addition, each time your application uses background fetch, it has at most 30 seconds to complete the process.

换句话说，您的应用程序完全受制于为您安排后台获取的系统。此外，每次应用程序使用后台提取时，最多只需30秒即可完成此过程。

Include in your AppDelegate (change the below code to your fetching needs)

包含在AppDelegate中（将以下代码更改为您的提取需求）

-(void) application:(UIApplication *)application performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler {

    NSLog(@"Background fetch started...");

    //---do background fetch here---
    // You have up to 30 seconds to perform the fetch

    BOOL downloadSuccessful = YES;

    if (downloadSuccessful) {
        //---set the flag that data is successfully downloaded---
        completionHandler(UIBackgroundFetchResultNewData);
    } else {
        //---set the flag that download is not successful---
        completionHandler(UIBackgroundFetchResultFailed);
    }

    NSLog(@"Background fetch completed...");
}

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    [[UIApplication sharedApplication] setMinimumBackgroundFetchInterval:UIApplicationBackgroundFetchIntervalMinimum];
    return YES;
}

To check if Background Refresh is enabled for your app use the below code:

要检查您的应用是否启用了后台刷新，请使用以下代码：

UIBackgroundRefreshStatus status = [[UIApplication sharedApplication] backgroundRefreshStatus];
switch (status) {
    case UIBackgroundRefreshStatusAvailable:
    // We can do background fetch! Let's do this!
    break;
    case UIBackgroundRefreshStatusDenied:
    // The user has background fetch turned off. Too bad.
    break;
    case UIBackgroundRefreshStatusRestricted:
    // Parental Controls, Enterprise Restrictions, Old Phones, Oh my!
    break;
}