In the little analytics I’ve had to this point, I’ve discovered that people have come here while searching for an example NSURLConnection delegate. So we’re going to walk through an simple example I’ve coded up.
This delegate will fetch a requested URL and make the data available to the class. We’ll allow for simultaneous connections, and callbacks using blocks.
What’s Going On
Let’s look at the full gist and walk through the code.
The Public Interface
There are only two methods and two properties that are public for our delegate. One method allows for the canceling of all running NSURLConnections. The other
is used to start a request.
The Private Interface
We have two additional properties defined. One is a mutable array where we will store active connections. The other is a
NSOperationQueue which is where the delegate callbacks will run.
A Private Class
NSURLConnection doesn’t have much data with it. If we only allowed one
call at a time, we could simply have a single property for the data we
receive. But since we’ll have multiple connections running at once, it is
impractical to manage them1 in our delegate class. Instead we create a
child class of
NSURLConnection that will store our two blocks and the
received data and response. Once a connection is started, it continues to exist
until it is complete, so this is an easy place to store such information.
This method is simple. We create an
NSURLRequest and then create a
connection with that. If we can’t create a connection, we call the failure
block with a custom error2. Then we assign values to our custom
properties and start the connection. Once the connection has started we track
it in our array.
The Delegate Callbacks (NSURLConnectionDelegate)
You have to implement two delegate protocols to use
NSURLConnectionDelegate only has one required method as of iOS 5.0.
This is an easy one to handle. We simply call our failure block with the error and remove the connection from our tracking array.
The Delegate Callbacks (NSURLConnectionDataDelegate)
This delegate has the meat of the callbacks3. They handle the responses we receive from the server.
Apple’s documentation informs us that every time this method is called we need to zero out the received data. Besides that anything we do is up to us. We store the response for use for later, in case of error.
connection:didReceiveData:] provides us with the data the server has sent. We go ahead and append it our connection’s data.
When this callback is called, we know the connection is done. We don’t know if it was successful, but no more callbacks will be called after this. So here we investigate the response object to see if the call was a success.
Since an HTTP response might indicate failure we check to see if the response
NSHTTPURLResponse and see if it has an error status code. If so, we
call the failure block with an error holding the status code’s message.
If the response wasn’t an HTTP response, or didn’t have an error, we go ahead and call the completion block so the user has the received data.
You might consider managing connections an their associated data with an
NSDictionaryusing the connection as a key. This doesn’t work since
NSURLConnection’s don’t implement
NSCopyingand can’t be used as keys in
CFDictionaryworks, but is more work with memory management. And don’t even consider using similarly ordered arrays, because then you can’t delete items without weird issues. ↩
This NSError code is ugly and bad. So bad. You shouldn’t use it.↩
So in pre-iOS 5 these were part of
NSURLConnectionDelegateprotocol but have since moved into their own. Why two protocols? I don’t know, but that’s how it is.↩