Obj. C - QR Reading application runs too slow

Question

I made an QR Code reading application with AVFoundation by tutorial on this site (tutorial of Appcoda). After reading QR code, the app shows an UIAlertView. But it takes nearly 2 minutes (sometimes more than 3mins). I'll paste the whole ViewController.m file here. I hope it is enough. (UIAlertView is in captureOutput method)

//
//  ViewController.m
//  Yuvio
//
//  Created by İhsan Batuğhan YILMAZ on 29/08/15.
//  Copyright © 2015 Farabius. All rights reserved.
//

#import "ViewController.h"
#import <AVFoundation/AVFoundation.h>


@interface ViewController ()

@property (nonatomic) BOOL isReading;

@property (nonatomic, strong) AVCaptureSession *captureSession;
@property (nonatomic, strong) AVCaptureVideoPreviewLayer *videoPreviewLayer;

-(BOOL)startReading;
-(void) stopReading;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    _isReading=NO;
    _captureSession = nil;
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

-(IBAction)startStopReading:(id)sender {
    if (!_isReading) {
        if ([self startReading]) {
            [_btnStart setTitle:@"Stop"];
        }
    }
    else{
        [self stopReading];
        [_btnStart setTitle:@"Start!"];
    }

    _isReading = !_isReading;
}
- (BOOL)startReading {
    NSError *error;

    AVCaptureDevice *captureDevice = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];

    AVCaptureDeviceInput *input = [AVCaptureDeviceInput deviceInputWithDevice:captureDevice error:&error];
    if (!input) {
        NSLog(@"%@", [error localizedDescription]);
        return NO;
    }

    _captureSession = [[AVCaptureSession alloc] init];
    [_captureSession addInput:input];

    AVCaptureMetadataOutput *captureMetadataOutput = [[AVCaptureMetadataOutput alloc] init];
    [_captureSession addOutput:captureMetadataOutput];

    dispatch_queue_t dispatchQueue;
    dispatchQueue = dispatch_queue_create("myQueue", NULL);
    [captureMetadataOutput setMetadataObjectsDelegate:self queue:dispatchQueue];
    [captureMetadataOutput setMetadataObjectTypes:[NSArray arrayWithObject:AVMetadataObjectTypeQRCode]];

    _videoPreviewLayer = [[AVCaptureVideoPreviewLayer alloc] initWithSession:_captureSession];
    [_videoPreviewLayer setVideoGravity:AVLayerVideoGravityResizeAspectFill];
    [_videoPreviewLayer setFrame:_cameraView.layer.bounds];
    [_cameraView.layer addSublayer:_videoPreviewLayer];

    [_captureSession startRunning];

    return YES;
}

-(void)captureOutput:(AVCaptureOutput *)captureOutput didOutputMetadataObjects:(NSArray *)metadataObjects fromConnection:(AVCaptureConnection *)connection{
    if (metadataObjects != nil && [metadataObjects count] > 0) {
        AVMetadataMachineReadableCodeObject *metadataObj = [metadataObjects objectAtIndex:0];
        if ([[metadataObj type] isEqualToString:AVMetadataObjectTypeQRCode]) {

            UIAlertView *alert = [[UIAlertView alloc]
                                  initWithTitle:@"QR Detected"
                                  message:[metadataObj stringValue]
                                  delegate:self
                                  cancelButtonTitle:@"OK" otherButtonTitles:nil, nil];
            [alert show];

            [self performSelectorOnMainThread:@selector(stopReading) withObject:nil waitUntilDone:NO];
            [_btnStart performSelectorOnMainThread:@selector(setTitle:) withObject:@"Start!" waitUntilDone:NO];
            _isReading = NO;
        }
    }
}
-(void)stopReading{
    [_captureSession stopRunning];
    _captureSession = nil;

    [_videoPreviewLayer removeFromSuperlayer];
    return;
}

@end

Answer 1

I think that problem is in using UI functions outside main thread. Try this code:

-(void)captureOutput:(AVCaptureOutput *)captureOutput didOutputMetadataObjects:(NSArray *)metadataObjects fromConnection:(AVCaptureConnection *)connection{
    if (metadataObjects != nil && [metadataObjects count] > 0) {
        AVMetadataMachineReadableCodeObject *metadataObj = [metadataObjects objectAtIndex:0];
        if ([[metadataObj type] isEqualToString:AVMetadataObjectTypeQRCode]) {
            __weak ViewController *weakSelf = self;
            dispatch_async(dispatch_get_main_queue(), ^{
                [weakSelf processQRCode:metadataObj];
            });
        }
    }
}

-(void)processQRCode:(AVMetadataMachineReadableCodeObject *)codeObject{
    UIAlertView *alert = [[UIAlertView alloc]
                          initWithTitle:@"QR Detected"
                          message:[codeObject stringValue]
                          delegate:self
                          cancelButtonTitle:@"OK" otherButtonTitles:nil, nil];
    [alert show];

    [self stopReading];
    [_btnStart setTitle:@"Start!" forState:UIControlStateNormal];
    _isReading = NO;
}

I checked your view controller with this fix and it works fast.