Question

I am using gyroscope to handle rotation. For every degree that I rotate iPad, I should redraw the image on the screen to change the height of the mask.

But redrawing stops the gyroscope.

What can I do for this situation?

Edit code added

- (UIImage *)reflectedImage:(UIImageView *)fromImage withHeight:(NSUInteger)height
{
CGImageRef gradientMaskImage = CreateGradientImage(1, height);

CGImageRef masked = CGImageCreateWithMask([fromImage.image CGImage], gradientMaskImage);
CGImageRelease(gradientMaskImage);

UIImage *theImage = [UIImage imageWithCGImage:masked];

return theImage;
}

Gyroscope will give me a value, and I calculate a height for the image. After that I call this function to redraw the mask and image. So if I roll the device, image will blind up or blind down.

Answer 1

You can redraw the image in a background thread. When the image is ready, update the UI on the main thread.

static dispatch_queue_t background_queue;
- (void)updateImage {
  if (background_queue == nil){
        background_queue = dispatch_queue_create("com.myappname.myIdentifier", 0);
  }

  dispatch_async(background_queue, ^ {  // render the image in the background thread
     UIImage * theImage = [self reflectedImage:fromImage withHeight:height]; 
     dispatch_sync(dispatch_get_main_queue(), ^{
        imageView.image = theImage;    // Update the imageview in the main thread
     });
  });
}

Edit: code fixes