前言

自ios8推出wkwebview以來，極大改善了網頁加載速度及內存泄漏問題，逐漸全面取代笨重的UIWebview。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。

獲取快照

ios官方提供了簡易的獲取webview快照接口，通過異步回調拿到當前可視區域的屏幕截圖。

- (void)takeSnapshotWithConfiguration:(nullable WKSnapshotConfiguration *)snapshotConfiguration completionHandler:(void (^)(UIImage * _Nullable snapshotImage, NSError * _Nullable error))completionHandler API_AVAILABLE(ios(11.0));

其中snapshotConfiguration 參數可用于配置快照大小范圍，默認截取當前客戶端整個屏幕區域。由于可能出現導航欄成功加載而內容頁卻空白的特殊情況，導致非白屏像素點數增加對最終判定結果造成影響，考慮將其剔除。

- (void)judgeLoadingStatus:(WKWebView *)webview { if (@available(iOS 11.0, *)) { if (webView && [webView isKindOfClass:[WKWebView class]]) { CGFloat statusBarHeight = [[UIApplication sharedApplication] statusBarFrame].size.height; //狀態欄高度 CGFloat navigationHeight = webView.viewController.navigationController.navigationBar.frame.size.height; //導航欄高度 WKSnapshotConfiguration *shotConfiguration = [[WKSnapshotConfiguration alloc] init]; shotConfiguration.rect = CGRectMake(0, statusBarHeight + navigationHeight, _webView.bounds.size.width, (_webView.bounds.size.height - navigationHeight - statusBarHeight)); //僅截圖檢測導航欄以下部分內容 [_webView takeSnapshotWithConfiguration:shotConfiguration completionHandler:^(UIImage * _Nullable snapshotImage, NSError * _Nullable error) {//todo }]; } }}

縮放快照

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。

- (UIImage *)scaleImage: (UIImage *)image { CGFloat scale = 0.2; CGSize newsize; newsize.width = floor(image.size.width * scale); newsize.height = floor(image.size.height * scale); if (@available(iOS 10.0, *)) { UIGraphicsImageRenderer * renderer = [[UIGraphicsImageRenderer alloc] initWithSize:newsize]; return [renderer imageWithActions:^(UIGraphicsImageRendererContext * _Nonnull rendererContext) { [image drawInRect:CGRectMake(0, 0, newsize.width, newsize.height)]; }]; }else{ return image; }}

縮小前后性能對比（實驗環境：iPhone11同一頁面下）：

縮放前白屏檢測：

耗時20ms

縮放后白屏檢測：

耗時13ms

注意這里有個小坑。由于縮略圖的尺寸在 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（混蛋啊 摔！）。在遍歷縮略圖像素時，會將圖外畫布上的像素納入考慮范圍，導致實際白屏頁 像素占比并非100% 如圖所示。因此使用floor將其尺寸大小向下取整。

遍歷快照

遍歷快照縮略圖像素點，對白色像素(R:255 G: 255 B: 255)占比大于95%的頁面，認定其為白屏。

- (BOOL)searchEveryPixel:(UIImage *)image { CGImageRef cgImage = [image CGImage]; size_t width = CGImageGetWidth(cgImage); size_t height = CGImageGetHeight(cgImage); size_t bytesPerRow = CGImageGetBytesPerRow(cgImage); //每個像素點包含r g b a 四個字節 size_t bitsPerPixel = CGImageGetBitsPerPixel(cgImage); CGDataProviderRef dataProvider = CGImageGetDataProvider(cgImage); CFDataRef data = CGDataProviderCopyData(dataProvider); UInt8 * buffer = (UInt8*)CFDataGetBytePtr(data); int whiteCount = 0; int totalCount = 0; for (int j = 0; j < height; j ++ ) { for (int i = 0; i < width; i ++) { UInt8 * pt = buffer + j * bytesPerRow + i * (bitsPerPixel / 8); UInt8 red = * pt; UInt8 green = *(pt + 1); UInt8 blue = *(pt + 2);// UInt8 alpha = *(pt + 3); totalCount ++; if (red == 255 && green == 255 && blue == 255) {whiteCount ++; } } } float proportion = (float)whiteCount / totalCount ; NSLog(@'當前像素點數：%d,白色像素點數:%d , 占比: %f',totalCount , whiteCount , proportion ); if (proportion > 0.95) { return YES; }else{ return NO; }}

總結

typedef NS_ENUM(NSUInteger,webviewLoadingStatus) { WebViewNormalStatus = 0, //正常 WebViewErrorStatus, //白屏 WebViewPendStatus, //待決};// 判斷是否白屏- (void)judgeLoadingStatus:(WKWebview *)webview withBlock:(void (^)(webviewLoadingStatus status))completionBlock{ webviewLoadingStatus __block status = WebViewPendStatus; if (@available(iOS 11.0, *)) { if (webview && [webview isKindOfClass:[WKWebView class]]) { CGFloat statusBarHeight = [[UIApplication sharedApplication] statusBarFrame].size.height; //狀態欄高度 CGFloat navigationHeight = webview.viewController.navigationController.navigationBar.frame.size.height; //導航欄高度 WKSnapshotConfiguration *shotConfiguration = [[WKSnapshotConfiguration alloc] init]; shotConfiguration.rect = CGRectMake(0, statusBarHeight + navigationHeight, webview.bounds.size.width, (webview.bounds.size.height - navigationHeight - statusBarHeight)); //僅截圖檢測導航欄以下部分內容 [webview takeSnapshotWithConfiguration:shotConfiguration completionHandler:^(UIImage * _Nullable snapshotImage, NSError * _Nullable error) {if (snapshotImage) { CGImageRef imageRef = snapshotImage.CGImage; UIImage * scaleImage = [self scaleImage:snapshotImage]; BOOL isWhiteScreen = [self searchEveryPixel:scaleImage]; if (isWhiteScreen) { status = WebViewErrorStatus; }else{ status = WebViewNormalStatus; }}if (completionBlock) { completionBlock(status);} }]; } }}// 遍歷像素點 白色像素占比大于95%認定為白屏- (BOOL)searchEveryPixel:(UIImage *)image { CGImageRef cgImage = [image CGImage]; size_t width = CGImageGetWidth(cgImage); size_t height = CGImageGetHeight(cgImage); size_t bytesPerRow = CGImageGetBytesPerRow(cgImage); //每個像素點包含r g b a 四個字節 size_t bitsPerPixel = CGImageGetBitsPerPixel(cgImage); CGDataProviderRef dataProvider = CGImageGetDataProvider(cgImage); CFDataRef data = CGDataProviderCopyData(dataProvider); UInt8 * buffer = (UInt8*)CFDataGetBytePtr(data); int whiteCount = 0; int totalCount = 0; for (int j = 0; j < height; j ++ ) { for (int i = 0; i < width; i ++) { UInt8 * pt = buffer + j * bytesPerRow + i * (bitsPerPixel / 8); UInt8 red = * pt; UInt8 green = *(pt + 1); UInt8 blue = *(pt + 2);// UInt8 alpha = *(pt + 3); totalCount ++; if (red == 255 && green == 255 && blue == 255) {whiteCount ++; } } } float proportion = (float)whiteCount / totalCount ; NSLog(@'當前像素點數：%d,白色像素點數:%d , 占比: %f',totalCount , whiteCount , proportion ); if (proportion > 0.95) { return YES; }else{ return NO; }}//縮放圖片- (UIImage *)scaleImage: (UIImage *)image { CGFloat scale = 0.2; CGSize newsize; newsize.width = floor(image.size.width * scale); newsize.height = floor(image.size.height * scale); if (@available(iOS 10.0, *)) { UIGraphicsImageRenderer * renderer = [[UIGraphicsImageRenderer alloc] initWithSize:newsize]; return [renderer imageWithActions:^(UIGraphicsImageRendererContext * _Nonnull rendererContext) { [image drawInRect:CGRectMake(0, 0, newsize.width, newsize.height)]; }]; }else{ return image; }}

僅需在合適的view生命周期內回調使用該函數方法即可檢測出頁面狀態是否白屏，且性能損耗可忽略不計。

聲明

作者：BBTime鏈接：https://juejin.im/post/6885298718174609415

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