为什么UIGraphicsGetCurrentContext在UIGraphicsBeginImageContext之后返回nil

问题描述

我正在遵循一个代码示例来制作模糊的UILabel，。

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

我的要求是在标签初始化后使标签模糊，而不是在运行时调用 blur 方法。但是，当我尝试在标签初始化后调用 blur 时，从 UIGraphicsGetCurrentContext 返回的值是 nil ，因此出现致命错误：在展开可选值时意外发现nil

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)

我尝试将代码分别添加到以下所有位置，现在可以获取上下文，但是不会产生预期的模糊效果。

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()
}

完整代码段，

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
    }
}

参考

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

更新

基于 Eugene Dudnyk的用法；答案

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

推荐答案

这是一个更好的解决方案-而不是检索模糊的图像，只需让标签本身模糊即可。

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

当您需要对其模糊时，将 label.isBluring = true 。
另外，此解决方案的性能更好，因为它可以重复使用相同的上下文并且不需要图像视图。

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

class BlurredLabel: UILabel {
    
    var isBlurring = false {
        didSet {
            setNeedsDisplay()
        }
    }

    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)
        layer.isOpaque = false
        layer.needsDisplayOnBoundsChange = true
        layer.contentsScale = UIScreen.main.scale
        layer.contentsGravity = .center
        isOpaque = false
        isUserInteractionEnabled = false
        contentMode = .redraw
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    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()
    }
}

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