Correct way to poll a webservice in an IPhone app

I am trying to determine the best strategy to poll a webservice once a minute, parse the xml returned and then update an object stored in a shared instance. This process needs to run in a separate thread, and will continue as long as the app is running.

It seems that I could put all the code to call the webservice and parse the xml into an NSOperation and add that NSOperation to an NSOperationQueue stored in the app delegate as soon as the app launches.

Is it a correct approach to use an NSTimer inside the main method of the NSOperation so that the operation will loop once a minute, indefinitely? In that scenario the NSOperation would never actually return - this seems what I want but I am not sure if this is the right way to think about it.

The problem I am trying to solve is开发者_JAVA技巧 of course extremely common, so I am trying to figure out the correct way to implement it. Any advice greatly appreciated.

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The real correct way to do it is to use push notifications. If any of your users have cell plans with limited data or data charged based on usage, they will thank you for it.

But if you insist on polling, you may as well use the NSTimer directly rather than messing with a timer inside an NSOperation. This will run on the main thread, but you could have the timer callback use performSelectorInBackground:withObject: to do processing in the background. Or you could just skip the timer altogether and run the whole polling sequence on a separate NSThread, and use sleepForTimeInterval: to delay between polls.

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I would highly recommend you take a look at ASIHTTPRequest. What an amazing little class, and really well documented.

Edit:

Take a look at this answer for what seems to be the optimal solution.

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one approach: create a thread and use a run loop, updating or idling as appropriate. then you can perform the request from the secondary thread and post it to the rest of the app after it's been parsed/prepped.

this way offers more control over pause/resume/delays/timing, and you can easily control the number of active requests (which should be exactly zero or one).

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I wouldn't use NSTimer for this problem/design. I would create NSThread from the AppDelegate when the application starts. I would lower the priority of this thread. Inside the NSThread main method is basically a loop.

-(void)main {

    while(true) {
        // get raw data from url
        // hash the result
        // compare the hash to the last time
        if (currentHash != lastHash) {
          // post a notification to default center with the new data
          lastHash = currentHash;
        }
        // sleep the thread sleepForTimeInterval
    }
}

Your Model object would subscribe to the notification from the thread and parse the new data and updates ivars. Your View object would listen to the Model using KVO and display any updates/changes.