Question

Many, if not most, web services have a rate limit for clients. Delicious says a client can make one request per second; Twitter has limits per end-point; I’m sure Facebook and Flickr and Foursquare have their own idea.

You can easily limit an iOS application to a single request at a time using an NSOperationQueue.

But how do you limit an application to making, say, only one request per second?

I’ve looked at the sample code by Apple, AFNetworking, ASINetwork and a few others, and none seem to solve this problem. This seems odd to me. I’ll concede that I could be missing something very obvious…

Some parameters:

• Assume I have an NSOperationQueue for network operations and the request is an NSOperation (could also be a GCD queue I suppose, but this is what I’ve mostly been working with)
• The same rate limit is used for each request in the queue
• I’m looking for a solution in iOS, but general ideas might be useful

Possible solutions:

• sleep statement in the NSOperation (it’s a queue/thread so this wouldn’t block anything else)
• NSTimer in the NSOperation
• performSelector: in the NSOperation (I patched ASINetworking to use this approach, though I’m not using it and didn’t push the change upstream)
• Start/stop the queue (using KVO?) to make sure the rate limit is not exceeded
• Special “sleep” NSOperation. This would be a task that the next network operation would be dependent upon
• Completely ignore the rate limit and just pause a bit when you get the “exceeded rate limit” error response

These all seem quite messy. Operations that sleep would likely prevent forms of “priority” queue. Starting/stopping the queue seems fragile. Ignoring the limit is rude.

To be clear, I have solved this problem. But the solution seems “messy” and somewhat fragile. I’d like to know if there’s a better, cleaner option.

Ideas?

Answer 1

@implementation SomeNSOperationSubClass {
    BOOL complete;
    BOOL stopRunLoop;
    NSThread *myThread;
}

-(void) rateLimitMonitor:(NSTimer *)theTimer {
    [theTimer invalidate];
}

-(void) main {
    myThread = [NSThread currentThread];

    NSTimer *myTimer = [NSTimer timerWithTimeInterval:1 target:self  selector:@selector(rateLimitMonitor:) userInfo:nil repeats:NO];
    [[NSRunLoop currentRunLoop] addTimer:myTimer forMode:NSDefaultRunLoopMode];

    [self doAsyncThing];

    while ((!stopRunLoop || [myTimer isValid]) && [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]]);
    complete = YES;
}

-(void) internalComplete {
    stopRunLoop = YES;
}

-(void) setComplete {
    [self performSelector:@selector(internalComplete) onThread:myThread withObject:nil waitUntilDone:NO];
}

-(BOOL) isFinished {
    return complete;
}

@end

and in your async callback

    [myNSOperationSubClass setComplete];