Question

I am interacting with a web-controlled hardware device. You send it a request via a URL (e.g., http://device/on?port=1 or http://device/off?port=3) to turn stuff on and off, and it sends back “success” or “failure”. It is a simple device, however, so while it’s processing a request — i.e., until it returns the status of the request that it’s processing — it will ignore all subsequent requests. It does not queue them up; they just get lost.

So I need to send serial, synchronous requests. I.e., req#1, wait for response#1, req#2, wait for response#2, req#3, wait for response #3, etc.

Do I need to manage my own thread-safe queue of requests, have the UI thread push requests into one end of the queue, and have another thread pull the requests off, one at a time, as soon as the previous one either completes or times out, and send the results back to the UI thread? Or am I missing something in the API that already does this?

Thanks!

…R

Answer 1

What should work is to use an NSOperationQueue instance, and a number of NSOperation instances that perform the various URL requests.

First, set up a queue in the class that will be enqueueing the requests. Make sure to keep a strong reference to it, i.e.

@interface MyEnqueingClass ()

@property (nonatomic, strong) NSOperationQueue *operationQueue;

@end

Somewhere in the implementation, say the init method:

    _operationQueue = [[NSOperationQueue alloc] init];
    _operationQueue.maxConcurrentOperationCount = 1;

You want basically a serial queue, hence the maxConcurrentOperationCount of 1.

After setting this up, you’ll want to write some code like this:

[self.operationQueue addOperationWithBlock:^{
    NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:@"my://URLString"]];
    NSError *error;
    NSURLResponse *response;
    NSData *responseData = [NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error];
    if (!responseData)
    {
        //Maybe try this request again instead of completely restarting? Depends on your application.
        [[NSOperationQueue mainQueue] addOperationWithBlock:^{
            //Do something here to handle the error - maybe you need to cancel all the enqueued operations and start again?
            [self.operationQueue cancelAllOperations];
            [self startOver];
        }];
    }
    else
    {
        //Handle the success case;
    }
}];

[self.operationQueue addOperationWithBlock:^{
    //Make another request, according to the next instuctions?
}];

In this way you send synchronous NSURLRequests and can handle the error conditions, including by bailing out completely and starting all over (the lines with -cancelAllOperations called). These requests will be executed one after the other.

You can also of course write custom NSOperation subclasses and enqueuing instances of those rather than using blocks, if that serves you.

Hope this helps, let me know if you have any questions!