需求说明

1、可以将整个网页进行截屏，不止单屏幕。
2、限制的高度为6000单位。不限制的话占用内存太高。

核心思路与代码

先截当前页面图片盖住webview，做成webview没动的假象，
记录初始偏移量，截图完毕后恢复。
然后开启一个绘图上下文，截取一屏后滚动到下一屏截图。
整个web截完或者超出6000单位则停止截图，生产image。

//MARK: webSnapShot
    func webSnapShot(_ completionHandler:  @escaping (_ capturedImage: UIImage?) -> Void) {
        guard let webview = self.tabWebView?.webView else {
            return
        }
        
        let offset = webview.scrollView.contentOffset
        
        guard let snapShotView = self.snapshotView(afterScreenUpdates: true) else {
            print("截图失败")
            return
        }
        
        snapShotView.frame = self.frame
        self.superview?.addSubview(snapShotView)
        
        if webview.frame.size.height < webview.scrollView.contentSize.height {
            webview.scrollView.contentOffset = CGPoint(x: 0, y: webview.scrollView.contentSize.height - webview.frame.size.height)
        }

        DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + Double(Int64(0.3 * Double(NSEC_PER_SEC))) / Double(NSEC_PER_SEC)) { () -> Void in
            if offset.y > 6000.0 {
                webview.scrollView.contentOffset = CGPoint(x: 0, y: offset.y - 6000.0 + self.height)
            } else {
                webview.scrollView.contentOffset = CGPoint.zero
            }
            
            self.webSnapShotWithOffSet({  (capturedImage) -> Void in
                webview.scrollView.contentOffset = offset
                webview.layoutIfNeeded()
                snapShotView.removeFromSuperview()
                completionHandler(capturedImage)
            })
        }
        
    }
    
    fileprivate func webSnapShotWithOffSet(_ completionHandler: @escaping (_ capturedImage: UIImage?) -> Void) {
        guard let webview = self.tabWebView?.webView else {
            return
        }
        
        var totalSize = webview.scrollView.contentSize
        if webview.scrollView.contentSize.height > 6000.0 {
            totalSize = CGSize(width: self.width, height: 6000.0)
        }
        
        let page = floorf(Float(totalSize.height / self.bounds.height))
        
        webview.snp.removeConstraints()
        
        webview.snp.makeConstraints({ (make) in
            make.leading.right.equalToSuperview()
            make.height.equalTo(totalSize.height)
            self.webViewTopConstraint = make.top.equalTo(0).constraint
        })
        
        UIGraphicsBeginImageContextWithOptions(totalSize, false, UIScreen.main.scale)
        
        self.ContentPageDraw(0, maxIndex: Int(page), drawCallback: {  () -> Void in
            
            let capturedImage = UIGraphicsGetImageFromCurrentImageContext()
            UIGraphicsEndImageContext()
            
            webview.snp.removeConstraints()
            webview.snp.makeConstraints({ (make) in
                make.edges.equalToSuperview()
            })
            
            completionHandler(capturedImage)
        })
    }
        
    fileprivate func ContentPageDraw(_ index: Int, maxIndex: Int, drawCallback: @escaping () -> Void) {
        let splitFrame = CGRect(x: 0, y: CGFloat(index) * self.height, width: self.width, height: self.height)

        webViewTopConstraint?.update(offset:  -(CGFloat(index) * self.height))
        
        DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + Double(Int64(0.2 * Double(NSEC_PER_SEC))) / Double(NSEC_PER_SEC)) { () -> Void in
            self.drawHierarchy(in: splitFrame, afterScreenUpdates: true)
            
            if index < maxIndex {
                self.ContentPageDraw(index + 1, maxIndex: maxIndex, drawCallback: drawCallback)
            }else {
                drawCallback()
            }
        }
    }

开始的做法会先滚到webview最上面开始截图。但是当偏移过大时，在恢复偏移量时跳跃太大导致webview被回收了。
因此添加了判断，如果webview的contentsize超过6000，则不滚到最上面开始截图，直接在初始位置开始向下截图。