iOS 应用进入后台时下载数据

Question

我有一个应用程序在第一次安装时会下载大量数据（主要是图像和文档文件）。目前，我只能在 HUD 中显示进度。但我希望我能以某种方式允许在应用程序进入后台（或设备被锁定）时下载数据。由于该应用程序的目标设备是运行 iOS 7.0 及更高版本，因此我正在使用 NSURLSession 下载数据。我已经浏览了 Whosebug 上的各种主题以及教程 here。即使按照教程对我的应用程序进行了更改，我的应用程序也不会继续下载。我在 iPad 上测试了它。当应用程序被发送到后台（或锁定）时，下载暂停并在应用程序进入前台时恢复。

我不确定我的方法是错误的还是我的实现有缺陷。欢迎任何 help/advice。

The flow of the app is as follows: 'LoginViewController' calls an internal method downloadData which has an object of the SaveProjectData class that performs the task of downloading.

LoginViewController.m

@implementation LoginViewController
- (void)viewDidLoad {
}
- (IBAction)sumitButtonDidClick:(id)sender {

    if ([self checkNetworkConnection] ==NotReachable) {

        UIAlertView * alertView=[[UIAlertView alloc] initWithTitle:@"Network Error" message:@"No internet Connection." delegate:nil cancelButtonTitle:@"OK" otherButtonTitles: nil];
        [alertView show];

    } else {

       MBProgressHUD *hud = [MBProgressHUD showHUDAddedTo:self.view animated:YES];
        hud.mode = MBProgressHUDModeDeterminateHorizontalBar;
        hud.label.text = NSLocalizedString(@"Downloading...", @"HUD loading title");

         dispatch_async(dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0), ^{

        double delayInSeconds = 1.0;
        dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));

            if ([self loginStatus]) {

                NSString * userName = self.user_nameTxtField.text;
                 CallProjectAPI * callProjectAPIinst = [[CallProjectAPI alloc] init];

                NSString * appStatus = [[NSUserDefaults standardUserDefaults] objectForKey:@"appStatus"];
                if ([appStatus isEqualToString:@"N"]) {
                       [callProjectAPIinst dataFromJSONFile];
                      [callProjectAPIinst saveImageName];

                } else {

                     [self doSomeWorkWithProgress];
                    [callProjectAPIinst callAPI];
                     [self doSomeWorkWithProgress];
                    [self downloadData]; 
                }


                hud.label.text = NSLocalizedString(@"Complete!", @"HUD completed title");


                porjectVC = [self.storyboard instantiateViewControllerWithIdentifier:@"ProjectViewController"];
                dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
                [self presentViewController:porjectVC animated:YES completion:nil];
                     [hud hideAnimated:YES];
                  });
                [[NSUserDefaults standardUserDefaults] setObject:userName forKey:@"userName"];

            } else {

                dispatch_async(dispatch_get_main_queue(), ^{
                     [hud hideAnimated:YES];
                    UIAlertView * alert=[[UIAlertView alloc]  initWithTitle:@"Alert" message:@"User Name or Password is wrong." delegate:nil cancelButtonTitle:@"ok" otherButtonTitles:nil];
                    [alert show];

                });

            }


         });

    }
}
-(void)downloadData{


    if ([self checkNetworkConnection] ==NotReachable) {

        UIAlertView * alertView=[[UIAlertView alloc] initWithTitle:@"Network Error" message:@"No internet Connection." delegate:nil cancelButtonTitle:@"OK" otherButtonTitles: nil];
        [alertView show];

    } else {
        NSString * status =[[NSUserDefaults standardUserDefaults] objectForKey:@"DownloadData"];
        if (status == nil) {




                SaveProjectData * saveProjectDataInst = [[SaveProjectData alloc] init];

                 [self doSomeWorkWithProgress];
                [saveProjectDataInst saveAboutImage];
                [self doSomeWorkWithProgress];
                [saveProjectDataInst saveConstructionUpdateImages];
                 [self doSomeWorkWithProgress];
                [saveProjectDataInst saveLogImages];
                [saveProjectDataInst saveSmallImages];
                 [self doSomeWorkWithProgress];
                [saveProjectDataInst saveFloorPlanImages];
                 [self doSomeWorkWithProgress];
                [saveProjectDataInst saveUnitPlanImages];
                 [self doSomeWorkWithProgress];
                [saveProjectDataInst saveMasterPlanImages];
                 [self doSomeWorkWithProgress];
                [saveProjectDataInst saveBrochurs];
                 [self doSomeWorkWithProgress];
                [saveProjectDataInst saveGalleryImages];
                 [self doSomeWorkWithProgress];
                [saveProjectDataInst saveAmenitiesImages];
                 [self doSomeWorkWithProgress];
                [saveProjectDataInst saveVideos];
                 [self doSomeWorkWithProgress];
                [saveProjectDataInst saveMapImage];
                 [self doSomeWorkWithProgress];
                [saveProjectDataInst saveBannerImage];
                 [self doSomeWorkWithProgress];
                [saveProjectDataInst savewalkthrough];
                 [self doSomeWorkWithProgress];
                [saveProjectDataInst saveFlatImages];
                 [self doSomeWorkWithProgress];
                [saveProjectDataInst saveEventImages];
                 [self doSomeWorkWithProgress];

                [[NSUserDefaults standardUserDefaults] setObject:@"YES" forKey:@"DownloadData"];
               [saveProjectDataInst getUpdatedListForEachProject];
        }
    }
}
- (void)doSomeWorkWithProgress {

    progress += 0.049f;
    dispatch_async(dispatch_get_main_queue(), ^{
        // Instead we could have also passed a reference to the HUD
        // to the HUD to myProgressTask as a method parameter.
        [MBProgressHUD HUDForView:self.view].progress = progress;
    });


}
@end

Another approach could be to provide a button that the user clicks and data is downloaded while the user can still continue to use the device for other perposes. Any pointers as to how I can implement it?

Answer 1

iirc 你不应该依赖于当你的应用程序在后台下载东西，因为你不知道它什么时候会由于内存压力或其他原因而被杀死

这是有关它的 Apple 文档

https://developer.apple.com/library/content/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/TheAppLifeCycle/TheAppLifeCycle.html#//apple_ref/doc/uid/TP40007072-CH2-SW7

Answer 2

@HimanshuMahajan here 给出了这个问题（类似的）的答案我正在添加该答案并发布一个对我有用的解决方案，并在 iPad 上成功测试了它。

1) 在 ViewController

的头文件中使用以下行

@property (nonatomic) UIBackgroundTaskIdentifier backgroundTask;

2) 在 ViewDidLoad 中分配 UIBackgroundTaskIdentifier，如：

self.backgroundTask = UIBackgroundTaskInvalid;

3) 使用下面的代码行，这里我只是在 beginBackgroundTaskWithExpirationHandler: 块

中保持 getDataFromServer 方法

self.backgroundTask = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{

        [[UIApplication sharedApplication] endBackgroundTask:self.backgroundTask];
        self.backgroundTask = UIBackgroundTaskInvalid;
    }];

    /* Here your downloading Code, let say getDataFromServer method */

    [self getDataFromServer]; // Its dummy method

    /* Your downloading Code End Here */

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

        [[UIApplication sharedApplication] endBackgroundTask:self.backgroundTask];
        self.backgroundTask = UIBackgroundTaskInvalid;
    });

4) 如果你想检查后台下载数据的剩余时间，在applicationDidEnterBackground中包含以下行：(UIApplication *)AppDelegate的应用委托方法：

NSLog(@"Background time remaining = %.1f seconds", [UIApplication sharedApplication].backgroundTimeRemaining);

Adding to the answer:

5) 在applicationDidEnterBackground:(UIApplication *)application方法中添加如下代码，允许后台执行无时间限制

UIApplication *app = [UIApplication sharedApplication];
    UIBackgroundTaskIdentifier bgTask;
    bgTask = [app beginBackgroundTaskWithExpirationHandler:^{
        [app endBackgroundTask:bgTask];
    }];

希望有人认为答案有用！

iOS 应用进入后台时下载数据

Downloading data when iOS app goes in background

objective-c

background-process

ios

nsurlsession