如何在NSOperation中实现异步网络调用

Question

I have an NSOperation derived class which performs asynchronous download.
Because the download operation has to be initiated from the main thread, the response is also completed on the main thread.

Once the download has completed, I want to perform more heavy operations on the downloaded data within the NSOperation .

How do I get the operations after the download has completed to be performed in a background thread again within the existing NSOperation ?

Answer 1

If you instead of moving to main thread in the NSOperation, rather start the connection in the current thread being run by your NSOperation.

Then, keep the NSURLConnection alive by adding a NSPort to the NSRunLoop .
Now all the callbacks will be received in the same thread as the one you started the connection in - and you may continue to perform your additional work here, which is already in the background.

Answer 2

NSOperation is simply a managed task. You should use it for doing network calls outside of the main thread.

The subtlety in dealing with asynchronous code inside NSOperations lies in the NSOperation will complete and exit (and be disposed of from memory) before the callback returns. The NSOperationQueue doesn't know or care that your code is asynchronous.

Using the existing runloop for the operation and explicitly setting complete on the NSOperation will keep the operation alive to recieve the callback and to mark itself complete.

When the async callback returns (before setting the operation to complete) the code inside the NSOperation should make a call to the main thread possibly by sending a block to update the UI.

Dont expect to keep the NSOperation around after it completes to do anything with it. Its a disposable task object which should not live after it completes.

As suggested by the existing answer, you should do as much of the processing in the NSOperation that does the network call itself.