How can I avoid data corruption with multiple instances of NSUrlConnection

Question

I have written an iOS app that calls NSUrlConnection multiple times to download image data from the web. Sometimes, one NSUrlConnection has not finished before the other starts. I am seeing corrupt jpeg data and I think it is because my didReceiveData delegate is saving data from two separate NSUrlConnections and munging the two jpeg data streams together into one data variable, hence causing the corruption.

My question is: what is the best way to avoid this? There doesn't seem to be a way to make each NSUrlConnection instance save to a separate data variable, or make each instance wait until the previous instance is done before saving.

My code basically follows Apple's example here except I call a loadData function multiple times which creates the NSURLRequest and NSURLConnection. http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/URLLoadingSystem/Tasks/UsingNSURLConnection.html

Thanks in advance for any help.

Answer 1

When your delegate's connection:didReceiveData: method is called, you'll have the connection instance as the first parameter. So you'll need to use that to keep track of which connection just received data.

Apple's sample maintains one instance of NSMutableData. Your code will require several instances, one for each active connection.

Or, of course, you could have a separate delegate object (an individual instance) for each connection. That may be easier.