Swift 3 version of Document.swift from https://snackcoffee.me/2015/08/27/osx-document-based-app-swift/

Document.swift

import Cocoa

class Document: NSDocument {

    @IBOutlet weak var textField: NSTextField!

    var content = ""
    
    override init() {
        super.init()
        // Add your subclass-specific initialization here.
    }

    override class func autosavesInPlace() -> Bool {
        return true
    }

    override var windowNibName: String? {
        // Returns the nib file name of the document
        // If you need to use a subclass of NSWindowController or if your document supports multiple NSWindowControllers, you should remove this property and override -makeWindowControllers instead.
        return "Document"
    }
    
    override func windowControllerDidLoadNib(_ windowController: NSWindowController) {
        super.windowControllerDidLoadNib(windowController)
        textField.stringValue = content
    }

    override func read(from url: URL, ofType typeName: String) throws {
        content = try String(contentsOf: url, encoding: String.Encoding.utf8)
    }
    
    override func write(to url: URL, ofType typeName: String) throws {
        try textField.stringValue.write(to: url, atomically: false, encoding: String.Encoding.utf8)
    }
}