CODEWITHSUNDEEP
iosobjective-casynchronousnsurlconnectiongrand-central-dispatch
puntofisso
puntofisso

Reputation: 393

Async NSURLConnection triggering other Async NSURLConnection: what is the best way of doing this?

this is an open question aiming at understanding what is the best practice or the most common solution for a problem that I think might be common.

Let's say I have a list of URLs to download; the list is itself hosted on a server, so I start a NSURLConnection that downloads it. The code in connectionDidFinishLoading will use the list of URLs to instantiate one new NSURLConnection, asynchronously, per each URL; this in turn will trigger even more NSURLConnections, and so on - until there are no more URLs. See it as a tree of connections.

What is the best way to detect when all connections have finished?

I'm aiming the question to iOS7, but comments about other versions are welcome.

Upvotes: 1

Views: 80

Answers (3)

Duncan C
Duncan C

Reputation: 131418

As Rob says in his answer you might want to consider NSURLSession rather than doing this yourself. It has a number of advantages.

Other options are building your own download manager class, or using a ready-made third party framework like AFNetworking. I've only worked with AFNetworking a little bit but from what I've seen its elegant, powerful, and easy to use.

Our company wrote an async download manager class based on NSURLConnection for a project that predates both AFNetworking and NSURLSession. It's not that hard, but it isn't as flexible as either NSURLSession or AFNetworking.

Upvotes: 1

Rob
Rob

Reputation: 437552

A couple of thoughts:

  1. In terms of triggering the subsequent downloads after you retrieve the list from the server, just put the logic to perform those subsequent downloads inside the completion handler block (or completion delegate method) of the first request.

  2. In terms of downloading a bunch of files, if targeting iOS 7 and later, you might consider using NSURLSession instead of NSURLConnection.

    • First, the downloading of files with a nice modest memory footprint is enabled by initiating "download" tasks (rather than "data" tasks).

    • Second, you can do the downloads using a background NSURLSessionConfiguration, which will let the downloads continue even if the user leaves the app. See the Downloading Content in the Background section of the App Programming Guide for iOS. There are a lot of i's that need dotting and t's that need crossing if you do this, but it's a great feature to consider implementing.

    See WWDC 2013 What's New in Foundation Networking for an introduction to NSURLSession. Or see the relevent chapter of the URL Loading System Programming Guide.

  3. In terms of keeping track of whether you're done, as Wain suggests, you can just keep track of the number of requests issued and the number of requests completed/failed, and in your "task completion" logic, just compare these two numbers, and initiate the "all done" logic if the number of completions matches the number of requests. There are a bunch of ways of doing this, somewhat dependent upon the details of your implementation, but hopefully this illustrates the basic idea.

Upvotes: 4

Wain
Wain

Reputation: 119031

Instead of using GCD you should consider using NSOperationQueue. You should also limit the number of concurrent operations, certainly on mobile devices, to perhaps 4 so you don't flood the network with requests.

Now, the number of operations on the queue is the remaining count. You can add a block to the end of each operation to check the queue count and execute any completion logic.

Upvotes: 2

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