URLSessionDidFinishEventsForBackgroundURLSession 未调用 - Objective-C

Question

NSURLSession 委托方法
URLSessionDidFinishEventsForBackgroundURLSession 不是在通话吗？

我已经在项目功能设置中启用了后台模式。

这是代码

AppDelegate.h方法

@interface AppDelegate : UIResponder <UIApplicationDelegate>

@property (strong, nonatomic) UIWindow *window;

@property (nonatomic, copy) void(^backgroundTransferCompletionHandler)();

@end

AppDelegate.m方法

-(void)application:(UIApplication *)application handleEventsForBackgroundURLSession:(NSString *)identifier completionHandler:(void (^)())completionHandler{

    self.backgroundTransferCompletionHandler = completionHandler;

}

ViewController.m方法

- (void)viewDidLoad
{
    [super viewDidLoad];
    //Urls
    [self initializeFileDownloadDataArray];

    NSArray *URLs = [[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask];
    self.docDirectoryURL = [URLs objectAtIndex:0];

    NSURLSessionConfiguration *sessionConfiguration = [NSURLSessionConfiguration backgroundSessionConfiguration:@"com.GACDemo"];
    sessionConfiguration.HTTPMaximumConnectionsPerHost = 5;


    self.session = [NSURLSession sessionWithConfiguration:sessionConfiguration
                                                 delegate:self
                                            delegateQueue:nil];
}

NSUrlSession 方法

-(void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session{
    AppDelegate *appDelegate = [UIApplication sharedApplication].delegate;

    // Check if all download tasks have been finished.
    [self.session getTasksWithCompletionHandler:^(NSArray *dataTasks, NSArray *uploadTasks, NSArray *downloadTasks) {

        if ([downloadTasks count] == 0) {
            if (appDelegate.backgroundTransferCompletionHandler != nil) {
                // Copy locally the completion handler.
                void(^completionHandler)() = appDelegate.backgroundTransferCompletionHandler;

                // Make nil the backgroundTransferCompletionHandler.
                appDelegate.backgroundTransferCompletionHandler = nil;

                [[NSOperationQueue mainQueue] addOperationWithBlock:^{
                    // Call the completion handler to tell the system that there are no other background transfers.
                    completionHandler();

                    // Show a local notification when all downloads are over.
                    UILocalNotification *localNotification = [[UILocalNotification alloc] init];
                    localNotification.alertBody = @"All files have been downloaded!";
                    [[UIApplication sharedApplication] presentLocalNotificationNow:localNotification];
                }];
            }
        }
    }];
}

我可以一个一个地下载所有文件，但是下载完所有文件后，URLSessionDidFinishEventsForBackgroundURLSession 方法没有调用。

我必须在仅下载所有文件后执行一些操作方法。

Answer 1

如果出现以下情况，将不会调用这些委托方法：

当任务完成时，应用程序已经运行；
该应用程序已通过 double-tapping 在设备的主页按钮上终止并手动将其终止；或
如果启动后台失败NSURLSession同一个标识符

所以，显而易见的问题是：

应用程序是如何终止的？如果没有终止，或者如果终止不正确（例如，您通过主页按钮上的 double-tapping 手动终止它），这将阻止调用这些委托方法。
你看到 handleEventsForBackgroundURLSession 被调用了吗？
您是在物理设备上执行此操作吗？这在模拟器上的表现不同。

最重要的是，此处不足以诊断出确切的问题，但这些是可能无法调用该委托方法的常见原因。

你后来说：

Actually this is the first time I'm using NSURLSession class. My actual requirement is once the download (all the images) is completed then only I can retrieve the images from document directory and I can show in UICollectionView.

I'm following this tutorial from APPCODA. Here is the link http://appcoda.com/background-transfer-service-ios7

如果那是您的要求，那么背景 NSURLSession 可能有点矫枉过正。它比标准 NSURLSession 慢，而且更复杂。如果您确实需要大量下载以便在应用 suspended/terminated.

后在后台继续，请仅使用后台 sessions

您引用的那个教程似乎是对一个相当复杂的主题的通俗介绍（尽管我不同意 URLSessionDidFinish... 实施，如评论中所讨论的那样）。我会做类似的事情：

- (void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session {
    // log message so we can see completion in device log; remove this once you're done testing the app

    NSLog(@"%s", __FUNCTION__);

    // Since you may be testing whether the terminated app is awaken when the
    // downloads are done, you might want to post local notification. 
    // (Otherwise, since you cannot use the debugger, you're just staring
    // at the device console hoping you see your log messages.) Local notifications
    // are very useful in testing this, so you can see when this method is 
    // called, even if the app wasn't running. Obviously, you have to register
    // for local notifications for this to work.
    //
    // UILocalNotification *notification = [[UILocalNotification alloc] init];
    // notification.fireDate = [NSDate date];
    // notification.alertBody = [NSString stringWithFormat:NSLocalizedString(@"Downloads done", nil. nil)];
    //
    // [[UIApplication sharedApplication] scheduleLocalNotification:notification];

    // finally, in `handleEventsForBackgroundURLSession` you presumably
    // captured the `completionHandler` (but did not call it). So this 
    // is where you'd call it on the main queue. I just have a property 
    // of this class in which I saved the completion handler.

    dispatch_async(dispatch_get_main_queue(), ^{
        if (self.savedCompletionHandler) {
            self.savedCompletionHandler();
            self.savedCompletionHandler = nil;
        }
    });
}

我的问题是，如果您只是为 collection 视图下载图像，您是否真的需要背景 session。只有当图像太多（或者它们太大）以至于在应用程序仍处于运行.

时无法合理下载它们时，我才会这样做

为了完整起见，我将在下面分享后台下载的完整演示：

//  AppDelegate.m

#import "AppDelegate.h"
#import "SessionManager.h"

@interface AppDelegate ()

@end

@implementation AppDelegate

// other app delegate methods implemented here

// handle background task, starting session and saving 
// completion handler

- (void)application:(UIApplication *)application handleEventsForBackgroundURLSession:(NSString *)identifier completionHandler:(void (^)())completionHandler {
    [SessionManager sharedSession].savedCompletionHandler = completionHandler;
}

@end

和

//  SessionManager.h

@import UIKit;

@interface SessionManager : NSObject

@property (nonatomic, copy) void (^savedCompletionHandler)();

+ (instancetype)sharedSession;
- (void)startDownload:(NSURL *)url;

@end

和

//  SessionManager.m

#import "SessionManager.h"

@interface SessionManager () <NSURLSessionDownloadDelegate, NSURLSessionDelegate>
@property (nonatomic, strong) NSURLSession *session;
@end

@implementation SessionManager

+ (instancetype)sharedSession {
    static id sharedMyManager = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedMyManager = [[self alloc] init];
    });
    return sharedMyManager;
}

- (instancetype)init {
    self = [super init];
    if (self) {
        NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:@"foo"];
        self.session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:nil];
    }
    return self;
}

- (void)startDownload:(NSURL *)url {
    [self.session downloadTaskWithURL:url];
}

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location {
    NSLog(@"%s: %@", __FUNCTION__, downloadTask.originalRequest.URL.lastPathComponent);

    NSError *error;
    NSURL *documents = [[NSFileManager defaultManager] URLForDirectory:NSDocumentDirectory inDomain:NSUserDomainMask appropriateForURL:nil create:false error:&error];
    NSAssert(!error, @"Docs failed %@", error);

    NSURL *localPath = [documents URLByAppendingPathComponent:downloadTask.originalRequest.URL.lastPathComponent];
    if (![[NSFileManager defaultManager] moveItemAtURL:location toURL:localPath error:&error]) {
        NSLog(@"move failed: %@", error);
    }
}

- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
    NSLog(@"%s: %@ %@", __FUNCTION__, error, task.originalRequest.URL.lastPathComponent);
}

- (void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session {
    NSLog(@"%s", __FUNCTION__);

    // UILocalNotification *notification = [[UILocalNotification alloc] init];
    // notification.fireDate = [NSDate date];
    // notification.alertBody = [NSString stringWithFormat:NSLocalizedString(@"Downloads done", nil. nil)];
    //
    // [[UIApplication sharedApplication] scheduleLocalNotification:notification];

    if (self.savedCompletionHandler) {
        self.savedCompletionHandler();
        self.savedCompletionHandler = nil;
    }
}

@end

最后，发起请求的视图控制器代码：

//  ViewController.m

#import "ViewController.h"
#import "SessionManager.h"

@implementation ViewController

- (IBAction)didTapButton:(id)sender {

    NSArray *urlStrings = @[@"http://spaceflight.nasa.gov/gallery/images/apollo/apollo17/hires/s72-55482.jpg",
                            @"http://spaceflight.nasa.gov/gallery/images/apollo/apollo10/hires/as10-34-5162.jpg",
                            @"http://spaceflight.nasa.gov/gallery/images/apollo-soyuz/apollo-soyuz/hires/s75-33375.jpg",
                            @"http://spaceflight.nasa.gov/gallery/images/apollo/apollo17/hires/as17-134-20380.jpg",
                            @"http://spaceflight.nasa.gov/gallery/images/apollo/apollo17/hires/as17-140-21497.jpg",
                            @"http://spaceflight.nasa.gov/gallery/images/apollo/apollo17/hires/as17-148-22727.jpg"];

    for (NSString *urlString in urlStrings) {
        NSURL *url = [NSURL URLWithString:urlString];
        [[SessionManager sharedSession] startDownload:url];
    }

    // explicitly kill app if you want to test background operation
    //
    // exit(0);
}

- (void)viewDidLoad {
    [super viewDidLoad];

    // if you're going to use local notifications, you must request permission

    UIUserNotificationSettings *settings = [UIUserNotificationSettings settingsForTypes:UIUserNotificationTypeAlert categories:nil];
    [[UIApplication sharedApplication] registerUserNotificationSettings:settings];
}

@end

Answer 2

如 Apple 所述：

如果 iOS 应用程序被系统终止并重新启动，该应用程序可以使用相同的标识符来创建新的配置对象和会话，并检索在终止。此行为仅适用于系统正常终止应用程序。如果用户从多任务屏幕终止应用程序，系统将取消所有会话的后台传输。此外，系统不会自动重新启动用户强制退出的应用程序。用户必须明确重新启动应用程序才能再次开始传输。

URLSessionDidFinishEventsForBackgroundURLSession 未调用 - Objective-C

URLSessionDidFinishEventsForBackgroundURLSession Not Calling- Objective-C

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