问iPad Pro第三代无缘无故杀死前台应用

EN

//Assume the following instance variables are set up:

self.operationQueue = NSOperationQueue to download the images.

self.urlSession = NSURLSession with ephemeralSessionConfiguration, 60 second timeoutIntervalForRequest

self.conditions = NSMutableArray to house the NSConditions used below.

self.countRemaining = NSUInteger which keeps track of how many images are left to be downloaded.

//Starts the downloading process by setting up the variables needed for downloading.
-(void)startDownloading
{
    //If the operation queue doesn't exist, re-create it here.
    if(!self.operationQueue)
    {
        self.operationQueue = [[NSOperationQueue alloc] init];
        [self.operationQueue addObserver:self forKeyPath:KEY_PATH options:0 context:nil];
        [self.operationQueue setName:QUEUE_NAME];
        [self.operationQueue setMaxConcurrentOperationCount:2];
    }

    //If the session is nil, re-create it here.
    if(!self.urlSession)
    {
        self.urlSession = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration ephemeralSessionConfiguration]
                                                        delegate:self
                                                   delegateQueue:nil];
    }

    if([self.countRemaining count] == 0)
    {
        [self performSelectorInBackground:@selector(startDownloadForNextBatch:) withObject:nil];

        self.countRemaining = 1;
    }
}

//Starts each batch. Called again on observance of the operation queue's task count being 0.
-(void)startDownloadForNextBatch:
{
    [NSThread sleepForTimeInterval:20.0]; // 20 second gap between batches

    self.countRemaining = //Go get the count remaining from the database.

    if (countRemaining > 0)
    {
        NSArray *imageRecordsToDownload = //Go get the next batch of URLs for the images to download from the database.

        [imageRecordsToDownload enumerateObjectsUsingBlock:^(NSDictionary *imageRecord,
                                                                          NSUInteger index,
                                                                          BOOL *stop)
         {
            NSInvocationOperation *invokeOp = [[NSInvocationOperation alloc] initWithTarget:self
                                                                                   selector:@selector(downloadImageForRecord:)
                                                                                     object:imageRecord];
            [self.operationQueue addOperation:invokeOp];
         }];
    }
}

//Performs one image download.
-(void)downloadImageForRecord:(NSDictionary *)imageRecord
{
    NSCondition downloadCondition = [[NSCondition alloc] init];
    [self.conditions addObject:downloadCondition];

    [[self.urlSession downloadTaskWithURL:imageURL
                        completionHandler:^(NSURL *location,
                                            NSURLResponse *response,
                                            NSError *error)
              {
                  if(error)
                  {
                    //Record error below.
                  }
                  else
                  {
                    //Move the downloaded image to the correct directory.
                    NSError *moveError;
                    [[NSFileManager defaultManager] moveItemAtURL:location toURL:finalURL error:&moveError];

                    //Create a thumbnail version of the image for use in a search grid.
                  }

                  //Record the final outcome for this record by updating the database with either an error code, or the file path to where the image was saved.

                  //Sleep for some time to allow the CPU to rest.
                  [NSThread sleepForTimeInterval:0.05]; // 0.05 second gap between images.

                  //Finally, signal our condition.
                  [downloadCondition signal];
              }]
        resume];

    [downloadCondition lock];
    [downloadCondition wait];
    [downloadCondition unlock];
}

//If the downloads need to be stopped, for whatever reason (i.e. the user logs out), this function is called to stop the process entirely:
-(void)stopDownloading
{
    //Immediately suspend the queue.
    [self.operationQueue setSuspended:YES];

    //If any conditions remain, signal them, then remove them. This was added to avoid deadlock issues with the user logging out and then logging back in in rapid succession.
    [self.conditions enumerateObjectsUsingBlock:^(NSCondition *condition,
                                                  NSUInteger idx,
                                                  BOOL * _Nonnull stop)
     {

         [condition signal];
     }];

    [self setConditions:nil];
    [self setConditions:[NSMutableArray array]];
    [self.urlSession invalidateAndCancel];
    [self setImagesRemaining:0];
    [self.operationQueue cancelAllOperations];
    [self setOperationQueue:nil];
}