使用AVCaptureDevice读取条形码，我想在扫描后立即显示动画的ActivityIndi​​cator

Question

I have the UIActivityIndicator Place in the view, and I am try to get it to show after I scan the barcode (Because for some reason it won't immediately segue to my info page that has a loading screen on it.) I try to show the Indicator in CaptureOutput but the [loadView setHidden:NO] is just ignored. Why is this happening? And is there a way to just segue immediately after the scan?

BRISBNScanner.h

#import <UIKit/UIKit.h>
#import <AVFoundation/AVFoundation.h>

@interface BRISBNScanner : UIViewController <AVCaptureMetadataOutputObjectsDelegate>
{

    IBOutlet UIActivityIndicatorView *loadView;

}

@property (strong, nonatomic) AVCaptureDevice* device;
@property (strong, nonatomic) AVCaptureDeviceInput* input;
@property (strong, nonatomic) AVCaptureMetadataOutput* output;
@property (strong, nonatomic) AVCaptureSession* session;
@property (strong, nonatomic) AVCaptureVideoPreviewLayer* preview;
@property (strong, nonatomic) NSString *isbnText;

@end

BRISBNScanner.m

#import "BRISBNScanner.h"
#import "BRScanInfoView.h"

@interface BRISBNScanner ()

@end

@implementation BRISBNScanner

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        // Custom initialization
    }
    return self;
}

- (void)viewDidLoad
{



    [super viewDidLoad];
    // Do any additional setup after loading the view.
    // Device
    self.device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];

    // Input
    self.input = [AVCaptureDeviceInput deviceInputWithDevice:self.device error:nil];

    // Output
    self.output = [[AVCaptureMetadataOutput alloc] init];
    [self.output setMetadataObjectsDelegate:self queue:dispatch_get_main_queue()];

    // Session
    self.session = [[AVCaptureSession alloc] init];
    [self.session addInput:self.input];
    [self.session addOutput:self.output];
    self.output.metadataObjectTypes = @[AVMetadataObjectTypeEAN13Code];


    // Preview
    self.preview = [AVCaptureVideoPreviewLayer layerWithSession:self.session];
    self.preview.videoGravity = AVLayerVideoGravityResizeAspectFill;
    self.preview.frame = CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height);
    [self.view.layer insertSublayer:self.preview atIndex:0];



    // Start
    [self.session startRunning];
}

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

- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputMetadataObjects:(NSArray *)metadataObjects fromConnection:(AVCaptureConnection *)connection
{
    [loadView setHidden:NO];

    [self.session stopRunning];



    for(AVMetadataObject *metadataObject in metadataObjects)
    {

        AVMetadataMachineReadableCodeObject *readableObject = (AVMetadataMachineReadableCodeObject *)metadataObject;
        if ([metadataObject.type isEqualToString:AVMetadataObjectTypeEAN13Code])
        {
            NSLog(@"EAN 13 = %@", readableObject.stringValue);
            _isbnText = readableObject.stringValue;

            [loadView setHidden:YES];

            [self performSegueWithIdentifier:@"showInfo" sender:self];
            NSMutableArray *viewControllers = [NSMutableArray arrayWithArray: self.navigationController.viewControllers];
            [viewControllers removeObjectIdenticalTo:self];
            [self.navigationController setViewControllers: viewControllers animated: YES];

        }
    }
}
- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    if ([[segue identifier] isEqualToString:@"showInfo"]) {
        BRScanInfoView *scanView = (BRScanInfoView *)[segue destinationViewController];
        //The view that uses the ISBN to search for stuff
        scanView.isbnTextprop  = _isbnText;
    }
}


@end

Answer 1

Unless the session's metadataObjectsCallbackQueue is set to the main queue (or one that's set up to execute on it) you're not calling [loadView setHidden:NO] from the main thread. (This is probably also why [self performSegueWithIdentifier:@"showInfo" sender:self] doesn't immediately segue to your info page). Since most of UIKit isn't thread safe this leads to "undefined" behaviour, in this case not changing anything or only after a delay respectively.

My suggestion would be to wrap all your ui-updates in a

dispatch_async(dispatch_get_main_queue(), ^{
    //ui-updates
});