前言

    自ios8推出wkwebview以来，极大改善了网页加载速度及内存泄漏问题，逐渐全面取代笨重的UIWebview。尽管高性能、高刷新的WKWebview在混合开发中大放异彩表现优异，但加载网页过程中出现异常白屏的现象却仍然屡见不鲜，且现有的api协议处理捕捉不到这种异常case，造成用户无用等待体验很差。
    针对业务场景需求，实现加载白屏检测。考虑采用字节跳动团队提出的webview优化技术方案。在合适的加载时机对当前webview可视区域截图，并对此快照进行像素点遍历，如果非白屏颜色的像素点超过一定的阈值，认定其为非白屏，反之重新加载请求。

获取快照

    ios官方提供了简易的获取webview快照接口，通过异步回调拿到当前可视区域的屏幕截图。

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

其中snapshotConfiguration 参数可用于配置快照大小范围，默认截取当前客户端整个屏幕区域。由于可能出现导航栏成功加载而内容页却空白的特殊情况，导致非白屏像素点数增加对最终判定结果造成影响，考虑将其剔除。

- (void)judgeLoadingStatus:(WKWebView *)webview {
  if (@available(iOS 11.0, *)) {
    if (webView && [webView isKindOfClass:[WKWebView class]]) {

      CGFloat statusBarHeight = [[UIApplication sharedApplication] statusBarFrame].size.height; //状态栏高度
      CGFloat navigationHeight = webView.viewController.navigationController.navigationBar.frame.size.height; //导航栏高度
      WKSnapshotConfiguration *shotConfiguration = [[WKSnapshotConfiguration alloc] init];
      shotConfiguration.rect = CGRectMake(0, statusBarHeight + navigationHeight, _webView.bounds.size.width, (_webView.bounds.size.height - navigationHeight - statusBarHeight)); //仅截图检测导航栏以下部分内容
      [_webView takeSnapshotWithConfiguration:shotConfiguration completionHandler:^(UIImage * _Nullable snapshotImage, NSError * _Nullable error) {
        //todo
      }];
    }
  }
}

缩放快照

为了提升检测性能，考虑将快照缩放至1/5，减少像素点总数，从而加快遍历速度。

- (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;
  }
}

缩小前后性能对比（实验环境：iPhone11同一页面下）：

缩放前白屏检测：

耗时20ms

缩放后白屏检测：

耗时13ms

     注意这里有个小坑。由于缩略图的尺寸在 原图宽高*缩放系数后可能不是整数，在布置画布重绘时默认向上取整，这就造成画布比实际缩略图大（混蛋啊 摔！）。在遍历缩略图像素时，会将图外画布上的像素纳入考虑范围，导致实际白屏页 像素占比并非100% 如图所示。因此使用floor将其尺寸大小向下取整。

遍历快照

    遍历快照缩略图像素点，对白色像素(R:255 G: 255 B: 255)占比大于95%的页面，认定其为白屏。

- (BOOL)searchEveryPixel:(UIImage *)image {
  CGImageRef cgImage = [image CGImage];
  size_t width = CGImageGetWidth(cgImage);
  size_t height = CGImageGetHeight(cgImage);
  size_t bytesPerRow = CGImageGetBytesPerRow(cgImage); //每个像素点包含r g b a 四个字节
  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(@"当前像素点数：%d,白色像素点数:%d , 占比: %f",totalCount , whiteCount , proportion );
  if (proportion > 0.95) {
    return YES;
  }else{
    return NO;
  }
}

总结

typedef NS_ENUM(NSUInteger,webviewLoadingStatus) {

  WebViewNormalStatus = 0, //正常

  WebViewErrorStatus, //白屏

  WebViewPendStatus, //待决
};

// 判断是否白屏
- (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; //状态栏高度
      CGFloat navigationHeight = webview.viewController.navigationController.navigationBar.frame.size.height; //导航栏高度
      WKSnapshotConfiguration *shotConfiguration = [[WKSnapshotConfiguration alloc] init];
      shotConfiguration.rect = CGRectMake(0, statusBarHeight + navigationHeight, webview.bounds.size.width, (webview.bounds.size.height - navigationHeight - statusBarHeight)); //仅截图检测导航栏以下部分内容
      [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);
        }
      }];
    }
  }
}

// 遍历像素点 白色像素占比大于95%认定为白屏
- (BOOL)searchEveryPixel:(UIImage *)image {
  CGImageRef cgImage = [image CGImage];
  size_t width = CGImageGetWidth(cgImage);
  size_t height = CGImageGetHeight(cgImage);
  size_t bytesPerRow = CGImageGetBytesPerRow(cgImage); //每个像素点包含r g b a 四个字节
  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(@"当前像素点数：%d,白色像素点数:%d , 占比: %f",totalCount , whiteCount , proportion );
  if (proportion > 0.95) {
    return YES;
  }else{
    return NO;
  }
}

//缩放图片
- (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;
  }
}

仅需在合适的view生命周期内回调使用该函数方法即可检测出页面状态是否白屏，且性能损耗可忽略不计。