Why does UIGraphicsGetCurrentContext return nil after UIGraphicsBeginImageContext

Question

I am following a code example to make a blurred UILabel, https://stackoverflow.com/a/62224908/2226315.

My requirement is to make the label on blur after label initialization instead of calling the blur method at runtime. However, when I try to call blur after label gets initialized the value returned from UIGraphicsGetCurrentContext is nil hence having a "Fatal error: Unexpectedly found nil while unwrapping an Optional value"

UIGraphicsBeginImageContext(bounds.size)
print("DEBUG: bounds.size", bounds.size)
self.layer.render(in: UIGraphicsGetCurrentContext()!) // <- return nil
var image = UIGraphicsGetImageFromCurrentImageContext()
UIGraphicsEndImageContext()
print("DEBUG: image image", image)

I tried adding the code in all the following places individually, the context can be fetched now however it does not generate the blur effect as expected.

override func layoutSubviews() {
    super.layoutSubviews()
    self.blur()
}

// OR
    
override func draw(_ rect: CGRect) {
    super.draw(rect)
    self.blur()
}

Full code snippet,

class BlurredLabel: UILabel {

    func blur(_ blurRadius: Double = 2.5) {        
        let blurredImage = getBlurryImage(blurRadius)
        let blurredImageView = UIImageView(image: blurredImage)
        blurredImageView.translatesAutoresizingMaskIntoConstraints = false
        blurredImageView.tag = 100
        blurredImageView.contentMode = .center
        blurredImageView.backgroundColor = .white
        addSubview(blurredImageView)
        NSLayoutConstraint.activate([
            blurredImageView.centerXAnchor.constraint(equalTo: centerXAnchor),
            blurredImageView.centerYAnchor.constraint(equalTo: centerYAnchor)
        ])
    }

    func unblur() {
        subviews.forEach { subview in
            if subview.tag == 100 {
                subview.removeFromSuperview()
            }
        }
    }

    private func getBlurryImage(_ blurRadius: Double = 2.5) -> UIImage? {
        UIGraphicsBeginImageContext(bounds.size)
        layer.render(in: UIGraphicsGetCurrentContext()!)
        guard let image = UIGraphicsGetImageFromCurrentImageContext(),
            let blurFilter = CIFilter(name: "CIGaussianBlur") else {
            UIGraphicsEndImageContext()
            return nil
        }
        UIGraphicsEndImageContext()

        blurFilter.setDefaults()

        blurFilter.setValue(CIImage(image: image), forKey: kCIInputImageKey)
        blurFilter.setValue(blurRadius, forKey: kCIInputRadiusKey)

        var convertedImage: UIImage?
        let context = CIContext(options: nil)
        if let blurOutputImage = blurFilter.outputImage,
            let cgImage = context.createCGImage(blurOutputImage, from: blurOutputImage.extent) {
            convertedImage = UIImage(cgImage: cgImage)
        }

        return convertedImage
    }
}

REFERENCE

• Add blur view to label?
• How to blur UILabel text

UPDATE

Usage based on "Eugene Dudnyk" answer

definitionLabel = BlurredLabel()
definitionLabel.numberOfLines = 0
definitionLabel.lineBreakMode = .byWordWrapping
definitionLabel.textColor = UIColor(named: "text")
definitionLabel.text = "Lorem Ipsum is simply dummy text"
definitionLabel.clipsToBounds = false
definitionLabel.isBluring = true

Answer 1

Here is a better solution - instead of retrieving the blurred image, just let the label blur itself.

When you need it to be blurred, set label.isBlurring = true. Also, this solution is better for performance, because it reuses the same context and does not need the image view.

@IBDesignable
class BlurredLabel: UILabel {
    @IBInspectable
    public var isBlurring: Bool = false {
        didSet {
            setNeedsDisplay()
        }
    }

    @IBInspectable
    public var blurRadius: Double = 2.5 {
        didSet {
            blurFilter?.setValue(blurRadius, forKey: kCIInputRadiusKey)
        }
    }

    lazy var blurFilter: CIFilter? = {
        let blurFilter = CIFilter(name: "CIGaussianBlur")
        blurFilter?.setDefaults()
        blurFilter?.setValue(blurRadius, forKey: kCIInputRadiusKey)
        return blurFilter
    }()
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        configure()
    }
    
    required init?(coder: NSCoder) {
        super.init(coder: coder)
        configure()
    }

    func configure() {
        layer.isOpaque = false
        layer.needsDisplayOnBoundsChange = true
        layer.contentsScale = UIScreen.main.scale
        layer.contentsGravity = .center
        isOpaque = false
        isUserInteractionEnabled = false
        contentMode = .redraw
    }

    override func display(_ layer: CALayer) {
        let bounds = layer.bounds
        guard !bounds.isEmpty && bounds.size.width < CGFloat(UINT16_MAX) else {
            layer.contents = nil
            return
        }
        UIGraphicsBeginImageContextWithOptions(layer.bounds.size, layer.isOpaque, layer.contentsScale)
        if let ctx = UIGraphicsGetCurrentContext() {
            self.layer.draw(in: ctx)
        
            var image = UIGraphicsGetImageFromCurrentImageContext()?.cgImage
            if isBlurring, let cgImage = image {
                blurFilter?.setValue(CIImage(cgImage: cgImage), forKey: kCIInputImageKey)
                let ciContext = CIContext(cgContext: ctx, options: nil)
                if let blurOutputImage = blurFilter?.outputImage,
                   let cgImage = ciContext.createCGImage(blurOutputImage, from: blurOutputImage.extent) {
                    image = cgImage
                }
            }
            layer.contents = image
        }
        UIGraphicsEndImageContext()
    }
}

Answer 2

Transformed @EugeneDudnyk answer to UIView extension so it can be used also with TextView.

extension UIView {
    struct BlurableKey {
        static var blurable = "blurable"
    }
    
    func blur(radius: CGFloat) {
        guard superview != nil else { return }
        
        UIGraphicsBeginImageContextWithOptions(CGSize(width: frame.width, height: frame.height), false, 1)
        layer.render(in: UIGraphicsGetCurrentContext()!)
        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        
        guard
            let blur = CIFilter(name: "CIGaussianBlur"),
            let image = image
        else {
            return
        }
  
        blur.setValue(CIImage(image: image), forKey: kCIInputImageKey)
        blur.setValue(radius, forKey: kCIInputRadiusKey)
        
        let ciContext  = CIContext(options: nil)
        let boundingRect = CGRect(
            x:0,
            y: 0,
            width: frame.width,
            height: frame.height
        )
        
        guard
            let result = blur.value(forKey: kCIOutputImageKey) as? CIImage,
            let cgImage = ciContext.createCGImage(result, from: boundingRect)
        else {
            return
        }
                
        let blurOverlay = UIImageView()
        blurOverlay.frame = boundingRect
        blurOverlay.image = UIImage(cgImage: cgImage)
        blurOverlay.contentMode = .left
        
        addSubview(blurOverlay)
     
        objc_setAssociatedObject(
            self,
            &BlurableKey.blurable,
            blurOverlay,
            objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN
        )
    }
    
    func unBlur() {
        guard
            let blurOverlay = objc_getAssociatedObject(self, &BlurableKey.blurable) as? UIImageView
        else {
            return
        }
        
        blurOverlay.removeFromSuperview()
        
        objc_setAssociatedObject(
            self,
            &BlurableKey.blurable,
            nil,
            objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN
        )
    }
    
    var isBlurred: Bool {
        return objc_getAssociatedObject(self, &BlurableKey.blurable) is UIImageView
    } 
}