XML结构错误。您将如何从XML文件中读回？您的数据现在已变得非结构化。
使用下面的代码。它很长，因为它是XM（可悲的）

func xmlAttributeWithName(name: String, value: String) -> NSXMLNode {
    let node = NSXMLNode(kind: .AttributeKind)
    node.name = name
    node.stringValue = value

    return node
}

// Your data
let points = [
    (x: 50.0, y: 50.0),
    (x: 300.0, y: 200.0),
    (x: 200.0, y: 400.0),
    (x: 300.0, y: 300.0)
]

let mapElement = NSXMLElement(name: "map")
for (index, p) in points.enumerate() {
    let pointElement = NSXMLElement(name: "ucs_point")
    pointElement.attributes = [
        xmlAttributeWithName("id", value: "\(index)"),
        xmlAttributeWithName("x", value: "\(p.x)"),
        xmlAttributeWithName("y", value: "\(p.y)")
    ]

    mapElement.addChild(pointElement)
}

let exactMapElement = NSXMLElement(name: "exactmap")
exactMapElement.addChild(mapElement)

let prefsElement = NSXMLElement(name: "prefs")
prefsElement.addChild(exactMapElement)

let xmlDocument = NSXMLDocument(rootElement: prefsElement)
xmlDocument.characterEncoding = "utf-8"
xmlDocument.version = "1.0"

let xmlOptions = NSXMLNodePrettyPrint | NSXMLNodeCompactEmptyElement
let xmlString = xmlDocument.XMLStringWithOptions(xmlOptions)
try! xmlString.writeToFile("/path/to/file.xml", atomically: true, encoding: NSUTF8StringEncoding)

结果：

<?xml version="1.0" encoding="utf-8" standalone="no"?>
<prefs>
    <exactmap>
        <map>
            <ucs_point id="0" x="50.0" y="50.0"/>
            <ucs_point id="1" x="300.0" y="200.0"/>
            <ucs_point id="2" x="200.0" y="400.0"/>
            <ucs_point id="3" x="300.0" y="300.0"/>
        </map>
    </exactmap>
</prefs>