Silence Main Thread checker for specific action

Question

I have a UICollectionViewCel', an embedded UIView with a shadow and within that a UIImageView.

The following code fetches an average RGB based on the image within the imageivew (Thanks Paul Hudson)

    var averageColor: UIColor? {

    guard let inputImage = CIImage(image: self) else { return nil }
    let extentVector = CIVector(x: inputImage.extent.origin.x, y: inputImage.extent.origin.y, z: inputImage.extent.size.width, w: inputImage.extent.size.height)

    guard let filter = CIFilter(name: "CIAreaAverage", parameters: [kCIInputImageKey: inputImage, kCIInputExtentKey: extentVector]) else { return nil }
    guard let outputImage = filter.outputImage else { return nil }

    var bitmap = [UInt8](repeating: 0, count: 4)
    let context = CIContext(options: [.workingColorSpace: kCFNull])
    context.render(outputImage, toBitmap: &bitmap, rowBytes: 4, bounds: CGRect(x: 0, y: 0, width: 1, height: 1), format: .RGBA8, colorSpace: nil)

    return UIColor(red: CGFloat(bitmap[0]) / 255, green: CGFloat(bitmap[1]) / 255, blue: CGFloat(bitmap[2]) / 255, alpha: CGFloat(bitmap[3]) / 255)
}

Now this is quite an expensive bit of code. Thus the idea is to pop it onto the global queue than once I have the colour to the set the shadow colour on the main queue.

To achieve this I do the following

      DispatchQueue.global().async {
        if let color = self.cellImageView.image?.averageColor?.cgColor {
            DispatchQueue.main.async {
                self.cardView.layer.shadowColor = color
            }
        }
    }

The result is a pretty smooth scrolling action but incomes XCODES thread checker. I don't want to turn it off in the scheme settings but want to get the thread checker to ignore this syntax.

Does anybody know how to achieve this?

Thanks

Answer 1

The problem is that you’re accessing a property of a UIKit control from a background thread. Instead, fetch the image from the image view on the main thread. Then, once you’ve done that, you can calculate the average color on a background thread, and then dispatch the layer update back to the main thread:

guard let image = cellImageView.image else { return }

DispatchQueue.global().async {
    guard let color = image.averageColor?.cgColor else { return }

    DispatchQueue.main.async {
        self.cardView.layer.shadowColor = color
    }
}