How to renderInContext on background queue?

Question

I'm trying to make snapshot of UIView . I doing this with 'standard' things, but, as I making high performance app, I want to run it on background queue. I know that call from background thread UIKits isn't good.

In this method I was tried to walk around and make snap of layer:

    -(void)caps{

    dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, .1 * NSEC_PER_SEC);

    dispatch_queue_t myQueue = dispatch_queue_create("com.myapp.shot", NULL);

         dispatch_after(popTime, myQueue, ^(void){
             [self capture];

         });
}



-(void)capture{
    NSLog(@"Start");
    float height;
    UIDeviceOrientation orientation = [[UIDevice currentDevice] orientation];
    if(orientation == UIDeviceOrientationLandscapeLeft || orientation == UIDeviceOrientationLandscapeRight){
        height = 600;
    }else{
        height = 500;
    }
     dispatch_async(dispatch_get_main_queue(), ^{
  /// in .h @property CALayer *liar;
         if (!self.liar) {
             self.liar = myWebView.scrollView.layer;
             NSLog(@"MAIN THREAD %@", self.liar);
         }

     });
    UIGraphicsBeginImageContext(CGSizeMake(self.view.frame.size.width, height));

    NSLog(@"NEw THREAD %@", self.liar);
    [self.liar renderInContext:UIGraphicsGetCurrentContext()];

    self.sharedImage = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

    NSLog(@"Finish %@", self.sharedImage);
}

Mostly it's working fine, but in some cases (when the myWebView 's UIViewController isn't visible - browser tabs hide it) it throws me errors:

bool _WebTryThreadLock(bool), 0x1adb1730: Tried to obtain the web lock from a thread other than the main thread or the web thread. This may be a result of calling to UIKit from a secondary thread. Crashing now...
1   0x35f3b435 WebThreadLock
2   0x305b0b31 <redacted>
3   0x305b09ed <redacted>
4   0x305b0769 <redacted>
5   0x305b02cb <redacted>
6   0x30561353 <redacted>
7   0x301e7943 <redacted>
8   0x301e3167 <redacted>
9   0x30212425 <redacted>
10  0x30234f77 <redacted>
11  0x12f38f -[Browser capture]
12  0x12f10f __20-[Browser caps]_block_invoke
13  0x3896b0c3 <redacted>
14  0x3896fa47 <redacted>
15  0x3896c72f <redacted>
16  0x3896fe3d <redacted>
17  0x3896cf93 <redacted>
18  0x38970745 <redacted>
19  0x389709c5 <redacted>
20  0x38a9adff _pthread_wqthread
21  0x38a9acc4 start_wqthread

So, what's wrong, why it's worked perfectly if UIViewController is visible, and crashes so badly if it's transited off for another UIViewController ?

Answer 1

All the UIKit features are explicitly main-thread-only except a few of exceptions. Many of UIGraphics~ series functions are the exception cases. Anyway, anything else are not exceptions. I suspect these lines may cause some issues.

UIDeviceOrientation orientation = [[UIDevice currentDevice] orientation];

self.view.frame.size.width

Then, it would be better to check other part of your code for accidental call of UI~ series features.