Question

Let’s say that if I read from http://www.example.com/number, I get a random number. In my iPhone app, I want to be able to continuously read from that address and display the new number on the screen after each request is finished. Let’s also assume that I want this process to start as soon as the view loads. Lastly, as a side-note, I’m using ASIHTTPRequest to simplify the web requests.

Approach 1: In my viewDidLoad method I could synchronously read from the URL in a loop (execution will not continue until I get a response from the HTTP request). Pros: the requests are serial and I have full control to respond to each one. Cons: the UI never gets updated because I never exit the function and give control back to the run time loop. Clearly, this is not a good solution.

Approach 2: In my viewDidLoad method I create a timer which calls a fetchURL function once per second. Pros: each request is in a separate thread, and the UI updates after each request is finished. Cons: the requests are in separate threads, and cannot be controlled well. For example, if there is a connection timeout on the first request, I want to be able to display an error popup, and not have any further requests happen until settings are changed. However, with this approach, if it takes 3 seconds to timeout, two additional requests will have already been started in that time. If I just slow down the timer, then data comes in too slowly when the connection is working well.

It seems like there should be some approach which would merge the benefits of the first two approaches I mentioned. I would like a way that I could decide whether on not to send the next request based on the result of the previous request.

Approach 3: I considered using a timer which fires more quickly (say every .25 seconds), but have the timer’s function check a flag to see what to do next. So, if the previous request has finished, it sends a new request (unless there was an error). Otherwise, if the previous request has not finished, the timer’s function returns without sending a new request. By firing this timer more quickly, you would get better response time, but the flag would let me get the synchronization I wanted.

It seems like Approach 3 would do what I want, but it also seems a little forced. Does anyone have a suggestion for a better approach to this, or is something like Approach 3 the best way to do it?

Answer 1

I believe an NSOperation is what you need. Use the number 1 solution above, but place the code in your NSOperation’s main method. Something like this:

The .h file

@interface MyRandomNumberFetcher : NSOperation {

}

@end

The .m file

@implementation MyRandomNumberFetcher

- (void) main {
      // This is where you start the web service calls.
}

@end

I’d also recommend adding a reference to the UI controller so your operation queue class can call it back when it’s appropriate.