Multiple async NSURLConnections example

Yannick Compernol — Sat, 14 Mar 2009 23:23:17 +0000

After posting the custom NSURLConnection subclass blog post, I thought I might post an example of how it would be used, since quite a lot of visitors came to the blog searching for the keywords: “multiple asynchronous NSURLConnection”. The code itself is pretty straightforward, but might prove handy for those developers setting their first steps in Cocoa Touch.

The basic idea is to use a dictionary which stores the received data per tag, remember that the actual connection cannot be used as the key of a dictionary. Each NSURLConnection methods retrieve the NSMutableData object from the dictionary based on the tag and does it magic with it.

If you don’t know how to use the NSURLConnection class, take a look at the documentation. That should get you going in no time.

First of all add a mutable dictionary to the header file:

NSMutableDictionary *receivedData;

Here’s the actual code. The load method starts the actual asynchronous calls. The startAsyncLoad:tag and dataForConnection methods are convenience methods to keep the code tidy & readable.

- (void)startAsyncLoad:(NSURL*)url tag:(NSString*)tag {
   NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];
   CustomURLConnection *connection = [[CustomURLConnection alloc] initWithRequest:request delegate:self startImmediately:YES tag:tag];
 
   if (connection) {
      [receivedData setObject:[[NSMutableData data] retain] forKey:connection.tag];
   }
}
 
- (NSMutableData*)dataForConnection:(CustomURLConnection*)connection {
   NSMutableData *data = [receivedData objectForKey:connection.tag];
   return data;
}
 
- (void)load {
   receivedData = [[NSMutableDictionary alloc] init];
 
   NSURL *url1 = [NSURL URLWithString:@"http://blog.emmerinc.be"];
   NSURL *url2 = [NSURL URLWithString:@"http://www.emmerinc.be"];
   NSURL *url3 = [NSURL URLWithString:@"http://twitter.com/emmerinc"]; 
 
   [self startAsyncLoad:url1 tag:@"tag1"];
   [self startAsyncLoad:url2 tag:@"tag2"];
   [self startAsyncLoad:url3 tag:@"tag3"];
}
 
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
   NSMutableData *dataForConnection = [self dataForConnection:(CustomURLConnection*)connection];
   [dataForConnection setLength:0];
}
 
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
   NSMutableData *dataForConnection = [self dataForConnection:(CustomURLConnection*)connection];
   [dataForConnection appendData:data];
}
 
- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
   NSMutableData *dataForConnection = [self dataForConnection:(CustomURLConnection*)connection];
   [connection release];
 
   // Do something with the dataForConnection.
}

In the connectionDidFinishLoading: method the tag of the connection should be used to determine what to do with the actual received data. And that is about it!

Don’t forget to release the dictionary in the deconstructor or after all the asynchronous calls have been completed though.

Custom NSURLConnection class with tag

Yannick Compernol — Mon, 02 Mar 2009 21:41:15 +0000

A caveat one quickly encounters when using multiple asynchronous NSURLConnections with the same target delegate is that there is no easy way to distinguish one connection from the other, in for instance the connection:didReceiveData: method.

Since using multiple asynchronous connections implies handling the received data chunks yourself, one is likely to create a dictionary which contains a NSMutableData object for each NSURLConnection key. Unfortunately does a NSDictionary copy its keys and this is not something a NSURLConnection supports. A quick solution to avoid this is to make a subclass with a tag to distinguish each connection. This tag can be used as key in the dictionary.

Interface:

@interface CustomURLConnection : NSURLConnection {
   NSString *tag;
}
 
@property (nonatomic, retain) NSString *tag;
 
- (id)initWithRequest:(NSURLRequest *)request delegate:(id)delegate startImmediately:(BOOL)startImmediately tag:(NSString*)tag;
 
@end

Implementation:

@implementation CustomURLConnection
 
@synthesize tag;
 
- (id)initWithRequest:(NSURLRequest *)request delegate:(id)delegate startImmediately:(BOOL)startImmediately tag:(NSString*)tag {
   self = [super initWithRequest:request delegate:delegate startImmediately:startImmediately];
 
   if (self) {
      self.tag = tag;
   }
   return self;
}
 
- (void)dealloc {
   [tag release];
   [super dealloc];
}
 
@end

As you can see, creating a subclass with a tag property is rather easy and solves the dictionary issue.

Emmer Inc » NSURLConnection

Multiple async NSURLConnections example

Custom NSURLConnection class with tag