如何在Swift中制作IBDesignable图像下采样函数？

Question

运行应用程序时，我遇到了图像使用过多内存的问题。 905kb 的 SVG 图像在呈现时大约需要 150mb，并且系统正在缓存该图像。在不同页面上加载多个不同的 SVG 图像会导致内存压力问题。

所以我开始研究图像下采样并找到了解决方案。我设法创建一个函数来根据图像的大小对图像进行下采样（有帮助）。目前，我可以使用下采样器类对图像进行下采样，该类需要为我想要启用的每个 imageView 设置为自定义类下采样。

但是我正在尝试为 UIImageView 创建一个 IBDesignable 扩展，以便我可以使用任何自定义类，但仍然可以获得对图像进行下采样的好处。

这是下采样类的代码

import UIKit

class DownsamplingImageView: UIImageView {
    
    @IBInspectable
    var downSample: Bool = true {
        didSet {
            setNeedsLayout()
        }
    }
    
    override func layoutSubviews() {
        super.layoutSubviews()
        
        if downSample, let image = self.image {
            let targetSize = self.bounds.size
            let downsampledImage = image.downsample(to: targetSize)
            self.image = downsampledImage
        }
    }
}

extension UIImage {
    
    func downsample(to targetSize: CGSize) -> UIImage? {
        let sourceSize = self.size
        let widthRatio = targetSize.width / sourceSize.width
        let heightRatio = targetSize.height / sourceSize.height
        let scaleFactor = max(widthRatio, heightRatio)
        
        let scaledSize = CGSize(width: sourceSize.width * scaleFactor, height: sourceSize.height * scaleFactor)
        
        UIGraphicsBeginImageContextWithOptions(scaledSize, false, 0.0)
        self.draw(in: CGRect(origin: CGPoint.zero, size: scaledSize))
        let downsampledImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        
        return downsampledImage
    }
}

这就是我尝试进行扩展的方法，但不幸的是它不起作用。

import UIKit
@IBDesignable
extension UIImageView {
    
    @IBInspectable
    var downSample: Bool {
        get {
            return image != nil
        }
        set {
            setNeedsLayout()
        }
    }
    
    override open func layoutSubviews() {
        super.layoutSubviews()
        
        if downSample, let image = self.image {
            let targetSize = self.bounds.size
            let downsampledImage = image.downsample(to: targetSize)
            self.image = downsampledImage
        }
    }
}

extension UIImage {
    
    func downsample(to targetSize: CGSize) -> UIImage? {
        let sourceSize = self.size
        let widthRatio = targetSize.width / sourceSize.width
        let heightRatio = targetSize.height / sourceSize.height
        let scaleFactor = max(widthRatio, heightRatio)
        
        let scaledSize = CGSize(width: sourceSize.width * scaleFactor, height: sourceSize.height * scaleFactor)
        
        UIGraphicsBeginImageContextWithOptions(scaledSize, false, 0.0)
        self.draw(in: CGRect(origin: CGPoint.zero, size: scaledSize))
        let downsampledImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        
        return downsampledImage
    }
}

有人可以帮忙或解释一下为什么 UIImageView 上的扩展不起作用但使用自定义类可以吗？还有什么办法可以让扩展工作吗？

PS：150mb 使用量减少到仅 6mb 使用量，幸运的是，当用户离开页面时它就被释放了！ 万一有人想要启用下采样，第一个代码可以完美运行，您所要做的就是将 UIImageView 自定义类设置为

DownsamplingImageView

Answer 1

我最终使用图像下采样器来解决这个问题。

import UIKit

class DownsamplingImageView: UIImageView {
    
    @IBInspectable
    var downSample: Bool = true {
        didSet {
            setNeedsLayout()
        }
    }
    
    override func layoutSubviews() {
        super.layoutSubviews()
        
        if downSample, let image = self.image {
            let targetSize = self.bounds.size
            let downsampledImage = image.downsample(to: targetSize)
            self.image = downsampledImage
        }
    }
}

extension UIImage {
    
    func downsample(to targetSize: CGSize) -> UIImage? {
        let sourceSize = self.size
        let widthRatio = targetSize.width / sourceSize.width
        let heightRatio = targetSize.height / sourceSize.height
        let scaleFactor = max(widthRatio, heightRatio)
        
        let scaledSize = CGSize(width: sourceSize.width * scaleFactor, height: sourceSize.height * scaleFactor)
        
        UIGraphicsBeginImageContextWithOptions(scaledSize, false, 0.0)
        self.draw(in: CGRect(origin: CGPoint.zero, size: scaledSize))
        let downsampledImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        
        return downsampledImage
    }
}

致谢：我忘了我是从哪里得到这个的。我只是回答这个问题，以防有人遇到同样的问题。归功于原主人。