iOS 開發中盡管高性能、高刷新的 WKWebview 在混合開發中大放異彩表現優異，但加載網頁過程中出現異常白屏的現象卻仍然屢見不鮮，且現有的 api 協議處理捕捉不到這種異常 case，造成用戶無用等待體驗很差。
針對業務場景需求，實現加載白屏檢測。考慮采用字節跳動團隊提出的 webview 優化技術方案。在合適的加載時機對當前 webview 可視區域截圖，并對此快照進行像素點遍歷，如果非白屏顏色的像素點超過一定的閾值，認定其為非白屏，反之重新加載請求。

下面是實現代碼，對 WKWebView 的擴展

import UIKit
import WebKit

/// 檢查 Web 是否白屏
extension WKWebView {
    enum WebLoadingStatus{
        case normal
        case error
    }
    
    func judgeLoadingStatus(webView: WKWebView?, completion: ((_ status: WebLoadingStatus) -> Void)?) {
        
        if #available(iOS 11.0, *) {
            guard let webView = webView else {
                return
            }
            
            let window = UIApplication.shared.keyWindow
            guard let window = window else {
                return
            }
            
            let webViewRect = webView.convert(webView.bounds, to: window)
            
            let shotConfiguration = WKSnapshotConfiguration()
            shotConfiguration.rect = CGRect(x: webViewRect.origin.x, y: webViewRect.origin.y, width: webViewRect.size.width, height: webViewRect.size.height)
            
            webView.takeSnapshot(with: shotConfiguration) { [weak self] (image, error) in
                guard let self = self, let image = image else {
                    return
                }
                
                let scaleImage = self.scaleImage(image: image)
                let isWhiteScreen = self.searchEveryPixel(image: scaleImage)
                
                if isWhiteScreen {
                    completion?(.error)
                } else {
                    completion?(.normal)
                }
            }
        }
    }
    
    /// 遍歷像素點，判斷白色像素是否占比大于 95%
    /// - Parameter image: 圖片
    /// - Returns: 占比
    private func searchEveryPixel(image: UIImage) -> Bool {
        let cgImage = image.cgImage
        let width: size_t = cgImage?.width ?? 0
        let height: size_t = cgImage?.height ?? 0
        
        let dataProvider = cgImage?.dataProvider
        let data = dataProvider?.data
        let buffer: UnsafePointer<UInt8> = CFDataGetBytePtr(data)
        
        var whiteCount = 0
        var totalCount = 0
        
        for x in 0 ..< height {
            for y in 0 ..< width {
                
                let point = CGPoint(x: x, y: y)
                let pixelInfo: Int = (width * Int(point.y)) + Int(point.x) * 4
                
                let red = CGFloat(buffer[pixelInfo])
                let green = CGFloat(buffer[pixelInfo + 1])
                let blue = CGFloat(buffer[pixelInfo + 2])
                
                if red == 255.0 && green == 255.0 && blue == 255.0 {
                    whiteCount += 1
                }
                
                totalCount += 1
            }
        }
        
        let proportion: CGFloat = CGFloat(whiteCount / totalCount)
        
        if proportion > 0.95 {
            return true
        } else {
            return false
        }
    }
    
    /// 縮放圖片
    /// 為了提升檢測性能，考慮將快照縮放至1/5，減少像素點總數，從而加快遍歷速度
    /// - Parameter image: 原始圖片
    /// - Returns: 縮放后圖片
    private func scaleImage(image: UIImage) -> UIImage {
        let scale = 0.2
        // 縮略圖的尺寸在原圖寬高 * 縮放系數后可能不是整數，在布置畫布重繪時默認向上取整，這就造成畫布比實際縮略圖大
        // 造成不準確，所以此處選擇 向下取整
        let newSize = CGSize(width: floor(image.size.width * scale), height: floor(image.size.height * scale))
        
        if #available(iOS 13.0, *) {
            let renderer = UIGraphicsImageRenderer(size: newSize)
            
            renderer.image { rendererContext in
                return image.draw(in: CGRect(x: 0, y: 0, width: newSize.width, height: newSize.height))
            }
        }
        return image
    }
}