Parsing XML in Swift 4

Question

I'm new to XML parsing in Swift and I found this code on Parsing XML from URL in Swift but I get a EXC_BAD_INSTRUCTION error when I try running the code. The description of the error reads: fatal error: unexpectedly found nil while unwrapping an Optional value

This is my simple XML file:

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

The following code creates an XMLParser object and parses the XML file located in my Documents.

// 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)
}

Here I implement the XMLParserDelegate methods and define my dictionaries:

// 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

}

The error is found on the didEndElement method when the currentDictionary is appended to the results dictionary.

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

    }
}

Please help me solve this issue. I'm using the exact same code provided on Parsing XML from URL in Swift and they don't seem to have any issues. Am I doing anything wrong?

Solution 1

Your code never actually initializes results so the first time you try to use it you are trying to force-unwrap an nil optional value. That's bad. And there's no reason to declare it as an implicitly unwrapped optional.

You need to change:

var results: [[String: String]]!

to:

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

You will also need to remove the line:

results = nil

from your parser(_:parseErrorOccurred:) method.

If you would rather have results be optional then you can make the following changes to your code:

Change the declaration of results to:

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

Change:

results.append(currentDictionary)

to:

results?.append(currentDictionary)

And you leave the results = nil line in parser(_:parseErrorOccurred:).

Solution 2

rmaddy has correctly diagnosed the problem (+1). The results was never initialized.

I would suggest leaving your code largely as is, but merely add a parserDidStartDocument method that initializes results as follows:

func parserDidStartDocument(_ parser: XMLParser) {
    results = [[:]]
}