在Swift 3中使用LibXML2下载服务器时解析大型XML

Question

我在Swift 3中使用LibXML2中的SAX解析器存在问题。
我想要iOS中的Android的XMLPullParser之类的东西。其中从服务器下载XML并下载它时解析流。

我的XML看起来是这样的：

<?xml version="1.0" encoding="UTF-8" ?> 
<ResultList id="12345678-0" platforms="A;B;C;D;E"> 
    <Book id="1111111111" author="Author A" title="Title A" price="9.95" ... /> 
    <Book id="1111111112" author="Author B" title="Title B" price="2.00" ... /> 
    <Book id="1111111113" author="Author C" title="Title C" price="5.00" ... /> 
    <ResultInfo bookcount="3" /> 
</ResultList> 

因此，所有的数据都存储在属性，而不是子节点。

我做我自己下面的类大多是由这些例子基于：
XMLPerformance，XMLPerformance-Swift和iOS-XML-Streaming

import Foundation 

class LibXMLParser: NSObject, URLSessionDataDelegate { 

    var url: URL? 
    var delegate: LibXMLParserDelegate? 
    var done = false 
    var context: xmlParserCtxtPtr? 

    var simpleSAXHandlerStruct: xmlSAXHandler = { 
     var handler = xmlSAXHandler() 

     handler.initialized = XML_SAX2_MAGIC 
     handler.startElementNs = startElementSAX 
     handler.endElementNs = endElementSAX 
     handler.characters = charactersFoundSAX 
     //handler.error = errorEncounteredSAX 

     return handler 
    }() 

    init(url: URL) { 
     super.init() 

     self.url = url 
    } 

    func parse() { 
     self.done = false 
     let session = URLSession(configuration: .default, delegate: self, delegateQueue: OperationQueue.main) 
     let dataTask = session.dataTask(with: URLRequest(url: url!)) 
     dataTask.resume() 

     self.context = xmlCreatePushParserCtxt(&simpleSAXHandlerStruct, Unmanaged.passUnretained(self).toOpaque(), nil, 0, nil) 
     self.delegate?.parserDidStartDocument() 

     repeat { 
      RunLoop.current.run(mode: .defaultRunLoopMode, before: Date.distantFuture) 
     } while !self.done 

     xmlFreeParserCtxt(self.context) 
     self.delegate?.parserDidEndDocument() 
    } 

    func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive data: Data) { 
     print("Did receive data") 
     data.withUnsafeBytes { (bytes: UnsafePointer<CChar>) -> Void in 
      xmlParseChunk(self.context, bytes, CInt(data.count), 0) 
     } 
    } 

    func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) { 
     xmlParseChunk(self.context, nil, 0, 1) 
     self.done = true 
    } 

    func urlSession(_ session: URLSession, didBecomeInvalidWithError error: Error?) { 
     self.done = true 
     //self.delegate?.parserErrorOccurred(error) 
    } 

    func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) { 
     self.done = true 
     //self.delegate?.parserErrorOccurred(error) 
    } 
} 

private func startElementSAX(_ ctx: UnsafeMutableRawPointer?, name: UnsafePointer<xmlChar>?, prefix: UnsafePointer<xmlChar>?, URI: UnsafePointer<xmlChar>?, nb_namespaces: CInt, namespaces: UnsafeMutablePointer<UnsafePointer<xmlChar>?>?, nb_attributes: CInt, nb_defaulted: CInt, attributes: UnsafeMutablePointer<UnsafePointer<xmlChar>?>?) { 
    let parser = Unmanaged<LibXMLParser>.fromOpaque(ctx!).takeUnretainedValue() 
    parser.delegate?.parserDidStartElement(String(cString: name!), nb_attributes: nb_attributes, attributes: attributes) 
} 

private func endElementSAX(_ ctx: UnsafeMutableRawPointer?, name: UnsafePointer<xmlChar>?, 
          prefix: UnsafePointer<xmlChar>?, 
          URI: UnsafePointer<xmlChar>?) { 
    let parser = Unmanaged<LibXMLParser>.fromOpaque(ctx!).takeUnretainedValue() 
    parser.delegate?.parserDidEndElement(String(cString: name!)) 
} 

private func charactersFoundSAX(_ ctx: UnsafeMutableRawPointer?, ch: UnsafePointer<xmlChar>?, len: CInt) { 
    let parser = Unmanaged<LibXMLParser>.fromOpaque(ctx!).takeUnretainedValue() 
    parser.delegate?.parserFoundCharacters(String(cString: ch!)) 
} 

我初始化这个类有URL。当我打电话给parse()时，它会创建一个URLSession和一个URLSessionDataTask，其中一个代表自我覆盖方法didReceive data: Data。 之后，我创建xmlParserCtxtPtr并循环，直到dataTask完成。

当它接收到数据时，我用xmlParseChunk方法解析它，startElementSAX调用我从ViewController类设置的委托。 （我只需要元素名称，属性数量和属性。）
到目前为止这么好。

在我的视图控制器（的UITableViewController）我有以下代码：

func downloadBooksLibXML() { 
    print("Downloading…") 
    UIApplication.shared.isNetworkActivityIndicatorVisible = true 

    DispatchQueue.global().async { 
     print("Setting up parser") 
     let parser = LibXMLParser(url: URL(string: self.baseUrl + self.parameters!)!) 
     parser.delegate = self 
     parser.parse() 
    } 
} 

func parserDidStartDocument() { 

} 

func parserDidEndDocument() { 
    DispatchQueue.main.sync { 
     UIApplication.shared.isNetworkActivityIndicatorVisible = false 
     self.isDone = true 
     print("Finished") 
    } 
} 

func parserDidStartElement(_ elementName: String, nb_attributes: CInt, attributes: UnsafeMutablePointer<UnsafePointer<xmlChar>?>?) { 
    print(elementName) 
    switch elementName { 
    case "Book": 
     DispatchQueue.main.async { 
      let book = self.buildBook(nb_attributes: nb_attributes, attributes: attributes) 
      self.books.append(book) 

      self.tableView.beginUpdates() 
      self.tableView.insertRows(at: [IndexPath(row: self.books.count - 1, section: 0)], with: .automatic) 
      self.tableView.endUpdates() 
      self.navigationItem.title = String(format: NSLocalizedString("books_found", comment: "Books found"), "\(self.books.count)") 
     } 
    case "ResultList": 
     break 
    case "ResultInfo": 
     break 
    default: 
     break 
    } 
} 

func buildBook(nb_attributes: CInt, attributes: UnsafeMutablePointer<UnsafePointer<xmlChar>?>?) -> Book { 
    let fields = 5 /* (localname/prefix/URI/value/end) */ 
    let book = Book() 
    for i in 0..<Int(nb_attributes) { 
     if let localname = attributes?[i * fields + 0], 
      //let prefix = attributes?[i * fields + 1], 
      //let URI = attributes?[i * fields + 2], 
      let value_start = attributes?[i * fields + 3], 
      let value_end = attributes?[i * fields + 4] { 

      let localnameString = String(cString: localname) 
      let string_start = String(cString: value_start) 
      let string_end = String(cString: value_end) 
      let diff = string_start.characters.count - string_end.characters.count 
      if diff > 0 { 
       let value = string_start.substring(to: string_start.index(string_start.startIndex, offsetBy: diff)) 
       book.setValue(value, forKey: localnameString) 
      } 
     } 
    } 
    return book 
} 

func parserDidEndElement(_ elementName: String) { 

} 

func parserFoundCharacters(_ string: String) { 

} 

func parserErrorOccurred(_ parseError: Error?) { 

} 

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更新

因此获得的属性值的问题已得到修复由来自nwellnhof的回答。我已将上面的代码更新为更好的代码。它现在不再遍历所有的属性。 现在我的新问题：

我创建了方法buildBook以获得XML属性的Book对象。 我已经将这个方法从What is the right way to get attribute value in libXML sax parser (C++)?转换到了Swift，并且使用setValue(value: Any?, forKey: String)来设置我的书对象的属性。

但现在我的问题是，它不会更新tableView。 我试着在后台线程中使用DispatchQueue.global().sync同步执行buildBook方法，并在异步主线程中使用DispatchQueue.main.async更新tableView。但它然后崩溃在tableView.endUpdates()，虽然它在主线程中。

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任何帮助将不胜感激。

Answer 1

看起来像一个简单的错误的错误。遍历C中的属性阵列，我会写这样的：

for (int i = 0; i < nb_attributes; i++) 

但是，你正在使用的closed range operator其中包括上限：

for i in 0...Int(nb_attributes) 

所以，你应该使用half-open range operator代替：

for i in 0..<Int(nb_attributes) 

顺便说一句，的libxml2也有C＃的XmlTextReader建模的pull parser interface这比SAX解析器更容易使用。