Question

I would like to start a server request, you can cancel.

My idea is to start the request in a thread so that the user interface does not freeze. So you can kill the whole thread including the request with a click on a “Cancel”-button.

With Android it works: the server request gets started in a “AsyncTask” and in the “onReturn()”-method I can react as soon as the server request finish.

How can I implement this using Objective-C on iOS?
My first attempt was a “NSInvocationOperation”. You can cancel the operation, but it’s difficult to handle when a request is completed and results are available. I think NSInvocationOperation is not the solution for my issue.

The would you recommend to me? Is NSThread the right choice for me?

Thank you very much!

Answer 1

Note!

This extremely old answer is now here only for historic purposes.

The wonderful ASIHttpRequest library no longer exists; technology is totally different now.

---

It is unbelievably simple to do this with ASIHttpRequest.

(Asynchronous is so simple, there is no reason you would ever do it not-asynchronously.)

Here are some rough extracts that might get you started.

...
ASIFormDataRequest *request;
...
NSURL *url = [NSURL URLWithString:@"https://blah.blah/blah.cgi?blah"];
request = [ASIFormDataRequest requestWithURL:url];

[request setPostValue:@"fred" forKey:@"username"];
[request setPostValue:@"flint" forKey:@"passie"];
[request setPostValue:@"stone" forKey:@"town"];

// send up data...
[request setData:[NSData dataWithBytes:blah length:blah] forKey:@"thefile"];

// or perhaps something like...
[request setData:imageData withFileName:@"blah.png"
        andContentType:@"image/jpeg" forKey:@"photoimage"];

[request setDelegate:self];
[request setDidFinishSelector:@selector(postingDone:)];
[request setDidFailSelector:@selector(postingDoneProblem:)];
[request startAsynchronous];
...

-(void) postingDone:(ASIHTTPRequest *)request
    {
    // it worked
    }
-(void) postingDoneProblem:(ASIHTTPRequest *)request
    {
    // failed
    }

Couldn’t really be any easier. You’re basically just typing out the fields and values.

Per your question, here is how you cancel an “in-flight” request… just set the delegate to nil and then “cancel” it.

  [myRequest setDelegate:nil];
  [myRequest cancel];
  [myRequest release];

ASIHttpRequest is the “miracle library”. If you are new to iOS, ASIHttpRequest is simply THE most used 3rd party library. Essentially, every single iPhone app of the 300,000 iPhone apps uses it.

If at all possible BE SURE to donate a few bucks to the guy — if he stops supporting that library, 100,000 iPhone programmers are buggered!

the documentation is trivial, a child can follow it:
http://allseeing-i.com/ASIHTTPRequest/How-to-use
“Creating an asynchronous request”

it is probably – almost certainly – the most amazingly simple networking library on any platform. It is trivial to do what you describe, happily. Enjoy.