Implementing Asynchronous Downloads in a Queue Using NSURLConnection

Asynchronous Download in Queue using NSURLConnection

Asynchronous downloading has become a crucial aspect of modern software development. With the increasing demand for high-speed internet and mobile devices, developers need to ensure that their applications can handle multiple downloads simultaneously without compromising performance. In this article, we’ll explore how to implement asynchronous downloads in a queue using NSURLConnection.

Introduction

NSURLConnection is a built-in iOS framework that allows you to download data from remote sources asynchronously. While it’s suitable for single-download scenarios, it might not be the best fit for multiple downloads in sequence. In this section, we’ll discuss why queuing NSOperations can be an effective solution and how to implement it.

Understanding NSOperation Queues

NSOperation is a framework that allows you to execute asynchronous tasks on a background thread. By creating a queue of these operations, you can manage the execution order and dependencies between them. This approach provides several benefits, including:

Efficient resource utilization
Improved performance
Better error handling

In our scenario, we’ll use NSOperation Queues to manage the download process.

Storing Download Items in an Array

Instead of queuing up individual NSOperations, it’s more efficient to store a list of download items (e.g., URLs) and execute them sequentially. When one download finishes, you can check the array for any pending downloads and kick off a new request if necessary.

Here’s some sample code to illustrate this concept:

{
 {< highlight objective-c >}
#import <Foundation/Foundation.h>

@interface DownloadManager : NSObject

@property (nonatomic) NSArray *downloadItems;

- (void)startDownload;
- (void)downloadItem:(NSURL *)url;

@end

@implementation DownloadManager

@synthesize downloadItems = _downloadItems;

- (void)startDownload {
    // Start the first download
    [self downloadItem:_downloadItems[0]];
}

- (void)downloadItem:(NSURL *)url {
    // Asynchronous download using NSURLConnection
    NSURLSession *session = [[NSURLSession alloc] initWithSessionConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]];
    URLSessionTask *task = [session dataTaskWithURL:url completionHandler:^(NSData * _Nullable data, urlSessionTask * _Nonnull task, NSError * _Nullable error) {
        if (error != nil) {
            // Handle errors
            return;
        }

        // Process the downloaded data
        NSLog(@"Downloaded: %@", url);
    }];

    [task resume];
}

@end }
</ highlight >

Storing Download Items on Disk

To persist the download items array between app restarts, you can store it on disk using a serialization technique. This approach ensures that your app picks up where it left off when it resumes.

Here’s an example of how to serialize and deserialize an NSArray using Property List Archives (PLIST):

{
 {< highlight objective-c >}
#import <Foundation/Foundation.h>

@interface DownloadManager : NSObject

@property (nonatomic) NSArray *downloadItems;

- (void)startDownload;
- (void)downloadItem:(NSURL *)url;

+ (NSArray *)loadDownloadItemsFromPlistFile:(NSString *)filePath error:(NSError **)error;

@end

@implementation DownloadManager

@synthesize downloadItems = _downloadItems;

+ (NSArray *)loadDownloadItemsFromPlistFile:(NSString *)filePath error:(NSError **)error {
    NSArray *data;
    NSError *error;
    
    NSPropertyListFormat format = NSPropertyListFormatBinary;
    NSData *plistData = [[NSFileManager defaultManager] contentsOfURL:[NSURL fileURLWithPath:filePath] type:NSFileTypePropertyList options:NSStoreCreateWithSecurity error:&error];
    
    if (![plistData containString:@"downloadItems"]) {
        // Handle empty or invalid PLIST
        return nil;
    }
    
    data = [NSPropertyListSerialization propertyListWithData:plistData options:format format:NULL error:&error];
    
    if (error != NULL) {
        // Handle serialization errors
        return nil;
    }
    
    // Deserialize the array from plist
    id object = [data copy];
    NSArray *downloadItems = nil;
    
    if ([object isKindOfClass:[NSArray class]]) {
        downloadItems = (NSArray *)object;
    } else if ([object isKindOfClass:[NSValue class]] && [(NSValue *)object valueIsArray]) {
        downloadItems = [((NSArray *)[(NSValue *)object value]) copy];
    }
    
    return downloadItems;
}

- (void)saveDownloadItemsToPlistFile:(NSString *)filePath {
    NSPropertyListFormat format = NSPropertyListFormatBinary;
    NSArray *downloadItems = self.downloadItems;
    NSData *plistData = [NSKeyedArchiver archivedDataWithRootObject:downloadItems requiringSecureCoding:NO];
    
    [[NSFileManager defaultManager] createFileAtPath:filePath contents:nil attributes:nil];
    
    [plistData writeToFile:filePath atomically:YES encoding:NSUTF8StringEncoding error:nil];
}

@end }
</ highlight >

Implementing Asynchronous Downloads in a Queue

Now that we have the necessary components in place, let’s put everything together. Here’s an example implementation:

{
 {< highlight objective-c >}
#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>

@interface DownloadManager : NSObject

@property (nonatomic) NSArray *downloadItems;
@property (nonatomic, strong) NSOperationQueue *queue;

- (void)startDownload;
- (void)downloadItem:(NSURL *)url;

@end

@implementation DownloadManager

@synthesize downloadItems = _downloadItems;
@synthesize queue = _queue;

- (instancetype)init {
    self = [super init];
    
    if (self) {
        // Initialize the queue
        _queue = [[NSOperationQueue alloc] init];
    }
    
    return self;
}

- (void)startDownload {
    // Start the first download
    [_queue addOperation:[NSBlockOperation blockOperationWithExecutor:_queue]];
}

- (void)downloadItem:(NSURL *)url {
    // Asynchronous download using NSURLConnection
    NSURLSession *session = [[NSURLSession alloc] initWithSessionConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]];
    URLSessionTask *task = [session dataTaskWithURL:url completionHandler:^(NSData * _Nullable data, urlSessionTask * _Nonnull task, NSError * _Nullable error) {
        if (error != nil) {
            // Handle errors
            return;
        }

        // Process the downloaded data
        NSLog(@"Downloaded: %@", url);
    }];

    [task resume];
}

@end }
</ highlight >

Error Handling and Dependencies

In our implementation, we’re assuming that each download item is independent of the others. However, in real-world scenarios, you might need to consider dependencies between downloads. For example:

If a download requires additional resources or data from other sources.
If a download is dependent on the completion of another download.

To handle these cases, you can use NSOperation’s dependency features:

{
 {< highlight objective-c >}
#import <Foundation/Foundation.h>

@interface DownloadManager : NSObject

@property (nonatomic) NSArray *downloadItems;
@property (nonatomic, strong) NSOperationQueue *queue;

- (void)startDownload;
- (void)downloadItem:(NSURL *)url;

@end

@implementation DownloadManager

@synthesize downloadItems = _downloadItems;
@synthesize queue = _queue;

- (void)downloadItem:(NSURL *)url {
    // Asynchronous download using NSURLConnection
    NSURLSession *session = [[NSURLSession alloc] initWithSessionConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]];
    URLSessionTask *task = [session dataTaskWithURL:url completionHandler:^(NSData * _Nullable data, urlSessionTask * _Nonnull task, NSError * _Nullable error) {
        if (error != nil) {
            // Handle errors
            return;
        }

        // Process the downloaded data
        NSLog(@"Downloaded: %@", url);
    }];

    [task resume];

    // Set up dependencies for other downloads
    NSOperation *operation = [[NSBlockOperation alloc] init];
    [operation addExecutionBlock:^{
        // Perform additional tasks or download resources
    }];
    [_queue addOperation:operation dependentOn:self];
}

@end }
</ highlight >

Conclusion

In this article, we’ve explored the basics of building a download manager that implements asynchronous downloads in a queue. We’ve covered topics such as:

Storing and deserializing data on disk using Property List Archives (PLIST)
Implementing asynchronous downloads using NSURLConnection
Handling errors and dependencies between downloads

By following these best practices, you can build a robust and scalable download manager that meets your application’s requirements.

Last modified on 2023-11-15