#import <AVFoundation/AVFoundation.h>

// Maximum number of queued still image requests.

int const MaxStillImageRequests = 10;

@interface MyController : UIViewController

@property dispatch_source_t timer;

@property int stillImageRequests; // Number of queued still image capture requests.

@property int imagesCaptured; // Number of still images captured.

-(void)captureImages:(int)count;

@end

...

//

// Capture still images in rapid succession

//

//     count = number of images to capture

//

-(void)captureImages:(int)count

{

    self.stillImageRequests = 0;

    self.imagesCaptured = 0;

    dispatch_queue_t timerQueue =

        dispatch_queue_create("timer queue",DISPATCH_QUEUE_SERIAL);

    // Create dispatch source that submits the event handler block based on a timer.

    self.timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER,

                                        0, // unused

                                        DISPATCH_TIMER_STRICT,

                                        timerQueue);

    // Set the event handler block for the timer dispatch source.

    dispatch_source_set_event_handler(self.timer, ^{

        // This block will attempt to capture a new still image each time it is called.

        // Captured requested number of images?

        if (self.imagesCaptured >= count)

        {

            // Done capturing, kill the timer.

            dispatch_source_cancel(self.timer);

        }

        else if (self.stillImageRequests >= MaxStillImageRequests)

        {

           // Don't capture another image if the maximum

           // number of outstanding still image requests has

           // been exceeded.

        }

        else

        {

            self.stillImageRequests++;

            self.imagesCaptured++;

            // Capture a still image.

            AVCaptureStillImageOutput *stillImageOutput =

               <# a AVCaptureStillImageOutput #>;

            [stillImageOutput captureStillImageAsynchronouslyFromConnection:

                 [stillImageOutput connectionWithMediaType:AVMediaTypeVideo]

                     completionHandler:

             ^(CMSampleBufferRef imageDataSampleBuffer, NSError *error)

             {

                 self.stillImageRequests--;

                 if (error)

                 {

                     // Handle the error.

                 }

                 else if (imageDataSampleBuffer)

                 {

                     // Do something with the captured image.

                 }

             }];

        }

    });

    // Timer interval -- use your device’s activeVideoMaxFrameDuration

    uint64_t interval = 0.04;

    // Set timer start time and interval.

    dispatch_source_set_timer(self.timer,

                              dispatch_time(DISPATCH_TIME_NOW, 0), // start time

                              interval * NSEC_PER_SEC, // interval

                              interval * NSEC_PER_SEC); // leeway

    dispatch_resume(self.timer);

}