NSURLConnection and grand central dispatch

Question

Is it advisable to wrap up NSUrlConnection in a gcd style blocks and run it on a low_priority queue?

I need to ensure that my connections are not happening on the main thread and the connections need to be asynchronous. I also need several simultaneous requests to go at once.

If I go the gcd route, I'm not sure which thread the NSUrlConnectionDelegate methods get invoked on.

NSURLConnection relies on delegates so once the connection is complete, whatever wrapper class that handles it will need to invoke its caller. I'm not certain how to deal with all of the various callbacks that are invoked when the connection work starts up/finishes:

- (void)connection:(NSURLConnection *)aConnection didReceiveResponse:(NSURLResponse *)response;
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)incrementalData;
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error;
- (void)connectionDidFinishLoading:(NSURLConnection *)connection;

Should I just call the synchronous versions but wrapped in gcd blocks? And if I want to cancel a call, use 'dispatch_suspend'?

dispatch_async(queue,^{
      NSString* result = [self mySynchronousHttp:someURLToInvoke];
      });

// If I need to cancel
dispatch_suspend(queue);

Answer 1

I recommend you to see WWDC Sessions about network application in iPhone OS.

• WWDC 2010 Session 207 - Network Apps for iPhone OS, Part 1
• WWDC 2010 Session 208 - Network Apps for iPhone OS, Part 2

The lecturer said

"Threads Are Evil™"

for network programming and recommended to use asynchronous network programming with RunLoop. Use background thread (Grand Central Dispatch Concurrent Queue) for thread-safe data processing, not for network programming.

By the way, Blocks and Grand Central Dispatch sessions are also worth to see.

• WWDC 2010 Session 206 - Introducing Blocks and Grand Central Dispatch on iPhone
• WWDC 2010 Session 211 - Simplifying iPhone App Development with Grand Central Dispatch

I wrote a wrapper class for asynchronous NSURLConnection.

---

AsyncURLConnection.h

#import <Foundation/Foundation.h>

typedef void (^completeBlock_t)(NSData *data);
typedef void (^errorBlock_t)(NSError *error);

@interface AsyncURLConnection : NSObject
{
    NSMutableData *data_;
    completeBlock_t completeBlock_;
    errorBlock_t errorBlock_;
}

+ (id)request:(NSString *)requestUrl completeBlock:(completeBlock_t)completeBlock errorBlock:(errorBlock_t)errorBlock;
- (id)initWithRequest:(NSString *)requestUrl completeBlock:(completeBlock_t)completeBlock errorBlock:(errorBlock_t)errorBlock;
@end

---

AsyncURLConnection.m

#import "AsyncURLConnection.h"

@implementation AsyncURLConnection

+ (id)request:(NSString *)requestUrl completeBlock:(completeBlock_t)completeBlock errorBlock:(errorBlock_t)errorBlock
{
    return [[[self alloc] initWithRequest:requestUrl
        completeBlock:completeBlock errorBlock:errorBlock] autorelease];
}

- (id)initWithRequest:(NSString *)requestUrl completeBlock:(completeBlock_t)completeBlock errorBlock:(errorBlock_t)errorBlock
{

    if ((self=[super init])) {
        data_ = [[NSMutableData alloc] init];

        completeBlock_ = [completeBlock copy];
        errorBlock_ = [errorBlock copy];

        NSURL *url = [NSURL URLWithString:requestUrl];
        NSURLRequest *request = [NSURLRequest requestWithURL:url];
        [NSURLConnection connectionWithRequest:request delegate:self];
    }

    return self;
}

- (void)dealloc
{
    [data_ release];

    [completeBlock_ release];
    [errorBlock_ release];
    [super dealloc];
}

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
{
    [data_ setLength:0];
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
    [data_ appendData:data];
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
    completeBlock_(data_);
}

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
    errorBlock_(error);
}

@end

---

How to use AsyncURLConnection class.

/*
 * AsyncURLConnection -request:completeBlock:errorBlock: have to be called
 * from Main Thread because it is required to use asynchronous API with RunLoop.
 */

[AsyncURLConnection request:url completeBlock:^(NSData *data) {

    /* success! */

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

        /* process downloaded data in Concurrent Queue */

        dispatch_async(dispatch_get_main_queue(), ^{

            /* update UI on Main Thread */

        });
    });

} errorBlock:^(NSError *error) {

    /* error! */

}];