在 Swift 4 中解析 XML

Question

我是 Swift 中 XML 解析的新手，我在从 Swift 中的 URL 解析 XML上找到了这段代码，但是EXC_BAD_INSTRUCTION当我尝试运行代码时出现错误。错误描述如下：fatal error: unexpectedly found nil while unwrapping an Optional value

这是我的简单 XML 文件：

<xml>
    <book>
        <title>Book Title</title>
        <author>Book Author</author>
    </book>
</xml>

以下代码创建一个XMLParser对象并解析位于我的文档中的 XML 文件。

// get xml file path from Documents and parse

let filePath = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).last?.appendingPathComponent("example.xml")

let parser = XMLParser(contentsOf: filePath!)
parser?.delegate = self

if (parser?.parse())! {
    print(self.results)
}

在这里，我实现了这些XMLParserDelegate方法并定义了我的字典：

// a few constants that identify what element names we're looking for inside the XML

let recordKey = "book"
let dictionaryKeys = ["title","author"]

// a few variables to hold the results as we parse the XML

var results: [[String: String]]!          // the whole array of dictionaries
var currentDictionary: [String: String]!  // the current dictionary
var currentValue: String?                 // the current value for one of the keys in the dictionary

// start element
//
// - If we're starting a "record" create the dictionary that will hold the results
// - If we're starting one of our dictionary keys, initialize `currentValue` (otherwise leave `nil`)


func parser(_ parser: XMLParser, didStartElement elementName: String, namespaceURI: String?, qualifiedName qName: String?, attributes attributeDict: [String : String] = [:]) {

    if elementName == recordKey {

        currentDictionary = [String : String]()

    } else if dictionaryKeys.contains(elementName) {

        currentValue = String()

    }
}

// found characters
//
// - If this is an element we care about, append those characters.
// - If `currentValue` still `nil`, then do nothing.

func parser(_ parser: XMLParser, foundCharacters string: String) {

    currentValue? += string

}

// end element
//
// - If we're at the end of the whole dictionary, then save that dictionary in our array
// - If we're at the end of an element that belongs in the dictionary, then save that value in the dictionary


func parser(_ parser: XMLParser, didEndElement elementName: String, namespaceURI: String?, qualifiedName qName: String?) {

    if elementName == recordKey {

        results.append(currentDictionary)
        currentDictionary = nil

    } else if dictionaryKeys.contains(elementName) {

        currentDictionary[elementName] = currentValue
        currentValue = nil

    }
}

// Just in case, if there's an error, report it. (We don't want to fly blind here.)

func parser(_ parser: XMLParser, parseErrorOccurred parseError: Error) {

    print(parseError)

    currentValue = nil
    currentDictionary = nil
    results = nil

}

将附加到字典didEndElement时在方法上发现错误。currentDictionaryresults

func parser(_ parser: XMLParser, didEndElement elementName: String, namespaceURI: String?, qualifiedName qName: String?) {

    if elementName == recordKey {

        results.append(currentDictionary)    // Line with Error
        currentDictionary = nil

    } else if dictionaryKeys.contains(elementName) {

        currentDictionary[elementName] = currentValue
        currentValue = nil

    }
}

请帮我解决这个问题。我正在使用在 Swift 中从 URL 解析 XML 中提供的完全相同的代码，它们似乎没有任何问题。我做错了什么吗？

Answer 1

您的代码从未真正初始化，results因此您第一次尝试使用它时，您是在尝试强制解包一个nil可选值。那很糟。并且没有理由将其声明为隐式解包的可选项。

你需要改变：

var results: [[String: String]]!

到：

var results = [[String: String]]()

您还需要删除该行：

results = nil

从你的parser(_:parseErrorOccurred:)方法。

如果您宁愿results是可选的，那么您可以对代码进行以下更改：

将声明更改results为：

var results: [[String: String]]? = [[String: String]]()

改变：

results.append(currentDictionary)

到：

results?.append(currentDictionary)

你把results = nil线路留在parser(_:parseErrorOccurred:).