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Mp3ID3Tagger: a native macOS app to edit the ID3 tag of your mp3 files written using RxSwift and RxCocoa

chicio profile image Fabrizio Duroni Originally published at ใƒปUpdated on ใƒป12 min read

The third of a short series of post in which I describe my two latest project: ID3TagEditor and
Mp3ID3Tagger. In this post I will talk about Mp3ID3Tagger, a macOS application to edit id3 tag of your mp3 files.

In this previous post I described the reason why I develop Mp3ID3Tagger, a macOS app to edit the id3 tag of your mp3 files that leverage on the power of ID3TagEditor. Below you can find the app logo.

MP3ID3Tagger macOS app RxSwift

So how did I develop MP3ID3Tagger? I was about to start the development following the classic approach to develop an app on every Apple OS: Model View Controller and plain Swift. But then I though: "This is the perfect project to test one of the last programming technique I recently learned: Reactive Programming/Reactive Extensions with RxSwift and RxCocoa!!!!!! In this way I can also try to use a different architectural pattern: the Model View ViewModel (MVVM)" ๐Ÿ˜Ž.
What kind of architectural pattern is the MVVM? What are Reactive Programming, Reactive Extensions, RxSwift and RxCocoa???

Let's start from the first one. The MVVM is an architectural pattern invented by the Microsoft software engineers Ken Cooper and Ted Peters. As for other architecture patterns I described in the past, the MVVM is useful to clearly separate the UI development from the business logic. The main components of the MVVM are:

  • the Model, that usually represents the business logic of the application.
  • the View, as in the other architectural pattern, the view is the structure, layout, and appearance of what a user sees on the screen.
  • the View model, that usually represents an abstraction of the view exposing public properties and commands.
  • the Binder interprets bindings defined in the View, observes the View Model for changes in state and updates the View and finally observes the View for changes in state and updates the View Model.

From the definition above we see that the MVVM needs something to bind the view to the view model in a platform independent way. This is why we need RxSwift, RxCocoa
and Reactive Extensions (usually called ReactiveX). What are they? Let's see some quote for the definitions:

Reactive Extensions (also known as ReactiveX or Rx) is a set of tools allowing imperative programming languages to operate on sequences of data regardless of whether the data is synchronous or asynchronous. It provides a set of sequence operators that operate on each item in the sequence. .... ReactiveX is API for asynchronous programming with
observable streams ... RxSwift is the Swift version of ReactiveX (Rx) .... RxCocoa is a framework that helps make Cocoa APIs used in iOS and OS X easier to use with reactive techniques ....

The main components of RxSwift are:

  • Observables, that are something which emit notifications of change, and Observers, that are something which subscribe to an Observable, in order to be notified when it has changed
  • Subjects, that are entity that act both as an Observable and as an Observer
  • Operator, that are basically functions that work on Observable and return Observable

So RxSwift and RxCocoa let us create an abstraction from the platform specific UI implementation and let us implement our ViewModel by working in an event-driven way: the ViewModel only works with streams of data that comes from Observable and Subjects of RxSwift.

RxCocoa gives us an abstraction over Cocoa and Cocoa Touch specific components and let us work with generic observable UI component. This basically means that:

  • RxSwift and RxCocoa are our Binder of the MVVM
  • the Various View and View Controllers are the View of the MVVM
  • the ID3TagEditor will be the Model of the MVVM
  • the ViewModel will connect the View and the ID3TagEditor Model in a platform UI independent way

With this architecture we can also think about using the same Model and ViewModel on different platform. So if in the future I will develop an iOS version of Mp3ID3Tagger, I will only have to develop the View part.
So let's start to see how I implemented Mp3ID3Tagger, the app subject of this post. Let's start from the UI to see how MP3ID3Tagger does look like.
The app has only one screen where the user can input its the data he/she want to insert into the tag. There is a button on the left to select the cover and all the textual/numeric values on the left. The values that could be set from a list are implemented as NSPopUpButton components.

MP3ID3Tagger interface

The first building block is the ViewModel base class. This class is useful to centralize the setup of a disposeBag. The DisposeBag itโ€™s an RxSwift component that keeps a reference to all the Disposable you add to it. The Observable are Disposable, so you can add them to it to have an ARC-like behaviour: when the DisposeBag will be released all the Disposable instances it keeps will be released as well. So by having the ViewModel base class all the ViewModel will have a disposeBag by default where they will add their disposables. As we have seen before the app just one screen, so there's just one ViewModel subclass to represent that screen, the Mp3ID3TaggerViewModel class. This class has 4 properties:

  • id3TagReader, of type ID3TagReader. This class has the responsibility to read a tag from an mp3 file when an openAction occurs. So ID3TagReader will be a subscriber of the openAction observable.
  • id3TagWriter, of type ID3TagWriter. This class has the responsibility to save a new tag to the mp3 file currently opened (the last openAction value) when a saveAction occurs. So ID3TagWriter will be a subscriber of the saveAction observable.
  • form, of type Form. This class has the responsibility to fill the fields of the form on the UI with values of the ID3tag read by the id3TagReader when an mp3 file has been opened. It has also the responsibility to collect the data contained in the form so that the id3TagWriter can write them when a saveAction occurs.
  • saveResult, of type PublishSubject<Bool>. This subject publishes the result of a save action made by the id3TagWriter.
class Mp3ID3TaggerViewModel: ViewModel {
    let id3TagReader: ID3TagReader
    let id3TagWriter: ID3TagWriter
    let form: Form
    let saveResult: PublishSubject<Bool>

    init(openAction: Observable<String>, saveAction: Observable<Void>) {
        self.id3TagReader = ID3TagReader(id3TagEditor: ID3TagEditor(), openAction: openAction)
        self.id3TagWriter = ID3TagWriter(id3TagEditor: ID3TagEditor(), saveAction: saveAction)
        self.form = Form()
        self.saveResult = PublishSubject<Bool>()
        super.init() { [unowned self] id3Tag in
            self.form.fillFields(using: id3Tag)

        id3TagWriter.write(input: Observable.combineLatest(form.readFields(), openAction)) { result in

Now we can see the details of all these collaborators of our view model. Let's start from the ID3TagReader.
This class keeps a reference to an instance of the ID3TagEditor. Its main function is read(_ finish: @escaping (ID3Tag?) -> ()). In this function there is the subscribe to the openAction observable received at construction time (passed by the Mp3ID3TaggerViewModel). Each new value received from the openAction is a path to a new mp3 file.
This path is passed to the ID3TagEditor instance that read of the ID3 tag of the song. If everything goes well, the tag is returned to the caller by using the finish closure. If you remember the Mp3ID3TaggerViewModel class, in this finish closure the form class is called that execute the fill of the fields (we will see below how it does this operation).

class ID3TagReader {
    private let id3TagEditor: ID3TagEditor
    private let openAction: Observable<String>
    private let disposeBag: DisposeBag

    init(id3TagEditor: ID3TagEditor, openAction: Observable<String>) {
        self.id3TagEditor = id3TagEditor
        self.openAction = openAction
        self.disposeBag = DisposeBag()

    func read(_ finish: @escaping (ID3Tag?) -> ()) {
        openAction.subscribe(onNext: { [unowned self] path in
            do {
                let id3Tag = try path)
            } catch {
        }).disposed(by: disposeBag)

Then we have the ID3TagWriter class. Like the ID3TagReader, this class keeps a reference to an instance of the ID3TagEditor. Its main function is write(input: Observable<(ID3Tag, String)>, _ finish: @escaping (Bool) -> ()). This function takes two parameters:

  • input of type Observable<(ID3Tag, String)>. This is an observable on a tuple composed by the path of an mp3 file and an ID3 tag
  • finish of type (Bool) -> ()

Inside this function there's the subscription to the saveAction observable received at construction time from the Mp3ID3TaggerViewModel class. This observable is combined with the input observable received as parameter and described above and a new subscription to the result of the combination is created: each time we receive a path to an mp3 file, an ID3 tag and a save action is triggered the ID3TagEditor instance is used to write the ID3 tag to the mp3 file. The called of the write function of the ID3TagWriter is notified of the result of the operation by calling the finish operation.

class ID3TagWriter {
    private let id3TagEditor: ID3TagEditor
    private let saveAction: Observable<Void>
    private let disposeBag: DisposeBag

    init(id3TagEditor: ID3TagEditor, saveAction: Observable<Void>) {
        self.id3TagEditor = id3TagEditor
        self.saveAction = saveAction
        self.disposeBag = DisposeBag()

    func write(input: Observable<(ID3Tag, String)>, _ finish: @escaping (Bool) -> ()) {
            .subscribe(onNext: { [unowned self] event in
                do {
                    try self.id3TagEditor.write(tag: event.0, to: event.1)
                } catch {
            .disposed(by: disposeBag)

Now let's see the Form class and its collaborators. This class has 5 collaborators. Each collaborator represents a subset of the form fields. This fields are represented as Variable subject of the specific type of the fields. In this way we are able to publish new values (by using the value property) to this observable and at the same time observe their values. In fact in this class there are two functions:

  • readFields(), that creates an observable from the fields observables by combining them using the Rx operator combineLatest
  • fillFields(using id3Tag: ID3Tag?), that sets the value of the fields observables with the received id3 tag (read by the ID3TagReader when an mp3 file has been opened)

Below you can find the Form class with all the implementations also for its collaborators. In this way it's easy to note what I stated above: the set of all the Variable fields of this classes matches the set of the UI components that we saw in the screenshot of the app that you saw above. One last important thing to note: the class AttachedPictureField forces the type of the attached picture to be saved to FrontCover. In this way the ID3TagEditor will write the ID3 tag with the correct data to display the album cover on my renault clio!!! ๐Ÿ˜Œ

class Form {
    let basicSongFields: BasicSongFields
    let versionField: VersionField
    let trackPositionInSetFields: TrackPositionInSetFields
    let genreFields: GenreFields
    let attachedPictureField: AttachedPictureField

    init() {
        self.basicSongFields = BasicSongFields()
        self.versionField = VersionField()
        self.trackPositionInSetFields = TrackPositionInSetFields()
        self.genreFields = GenreFields()
        self.attachedPictureField = AttachedPictureField()

    func readFields() -> Observable<ID3Tag> {
        return Observable.combineLatest(
        ) { (version, basicFields, trackPositionInSet, genre, image) -> ID3Tag in
            return ID3Tag(
                version: version,
                artist: basicFields.artist,
                albumArtist: basicFields.albumArtist,
                album: basicFields.album,
                title: basicFields.title,
                year: basicFields.year,
                genre: genre,
                attachedPictures: image,
                trackPosition: trackPositionInSet

    func fillFields(using id3Tag: ID3Tag?) {
        fillBasicFieldsUsing(id3Tag: id3Tag)
        fillVersionFieldUsing(id3Tag: id3Tag)
        fillTrackPositionFieldsUsing(id3Tag: id3Tag)
        fillGenreFieldsUsing(id3Tag: id3Tag)
        fillAttachedPictureUsing(id3Tag: id3Tag)

    private func fillBasicFieldsUsing(id3Tag: ID3Tag?) {
        basicSongFields.title.value = id3Tag?.title
        basicSongFields.artist.value = id3Tag?.artist
        basicSongFields.album.value = id3Tag?.album
        basicSongFields.albumArtist.value = id3Tag?.albumArtist
        basicSongFields.year.value = id3Tag?.year

    private func fillVersionFieldUsing(id3Tag: ID3Tag?) {
        if let version = id3Tag?.properties.version.rawValue {
            versionField.version.value = Int(version)

    private func fillTrackPositionFieldsUsing(id3Tag: ID3Tag?) {
        if let trackPosition = id3Tag?.trackPosition {
            trackPositionInSetFields.trackPosition.value = String(trackPosition.position)
            fillTotalTracksFieldUsing(id3Tag: id3Tag)

    private func fillTotalTracksFieldUsing(id3Tag: ID3Tag?) {
        if let totalTracks = id3Tag?.trackPosition?.totalTracks {
            trackPositionInSetFields.totalTracks.value = String(totalTracks)

    private func fillGenreFieldsUsing(id3Tag: ID3Tag?) {
        if let genre = id3Tag?.genre {
            genreFields.genreIdentifier.value = genre.identifier?.rawValue
            genreFields.genreDescription.value = genre.description

    private func fillAttachedPictureUsing(id3Tag: ID3Tag?) {
        if let validAttachedPictures = id3Tag?.attachedPictures, validAttachedPictures.count > 0 {
            attachedPictureField.attachedPicture.value = ImageWithType(data: validAttachedPictures[0].art,
                                                                       format: validAttachedPictures[0].format)


typealias BasicSongFieldsValues = (title: String?, artist: String?, album: String?, albumArtist: String?, year: String?)

class BasicSongFields {
    let title: Variable<String?>
    let artist: Variable<String?>
    let album: Variable<String?>
    let albumArtist: Variable<String?>
    let year: Variable<String?>

    init() {
        self.title = Variable<String?>(nil)
        self.artist = Variable<String?>(nil)
        self.album = Variable<String?>(nil)
        self.albumArtist = Variable<String?>(nil)
        self.year = Variable<String?>(nil)

    func observe() -> Observable<BasicSongFieldsValues> {
        return Observable.combineLatest(
        ) { title, artist, album, albumArtist, year in
            return BasicSongFieldsValues(title: title,
                                         artist: artist,
                                         album: album,
                                         albumArtist: albumArtist,
                                         year: year)


class VersionField {
    let version: Variable<Int?>
    let validVersion: Observable<ID3Version>

    init() {
        self.version = Variable<Int?>(3)
        self.validVersion = version.asObservable().map { (versionSelected) -> ID3Version in
            return ID3Version(rawValue: UInt8(versionSelected ?? 0)) ?? .version3


class TrackPositionInSetFields {
    let trackPosition: Variable<String?>
    let totalTracks: Variable<String?>
    let trackPositionInSet: Observable<TrackPositionInSet?>

    init() {
        self.trackPosition = Variable<String?>(nil)
        self.totalTracks = Variable<String?>(nil)
        self.trackPositionInSet = Observable.combineLatest(
        ) { (trackPosition, totalTracks) -> TrackPositionInSet? in
            if let validTrackPositionAsString = trackPosition,
                let validTrackPosition = Int(validTrackPositionAsString) {
                return TrackPositionInSet(position: validTrackPosition,
                                          totalTracks: TrackPositionInSetFields.convertToNumber(totalTracks: totalTracks))
            return nil

    private static func convertToNumber(totalTracks: String?) -> Int? {
        if let validTotalTracks = totalTracks {
            return Int(validTotalTracks)
        return nil


class GenreFields {
    let genreIdentifier: Variable<Int?>
    let genreDescription: Variable<String?>
    let genre: Observable<Genre?>

    init() {
        self.genreIdentifier = Variable<Int?>(nil)
        self.genreDescription = Variable<String?>(nil)
        self.genre = Observable.combineLatest(
        ) { (genreIdentifier, genreDescription) -> Genre? in
            if let validGenre = genreIdentifier,
                let validId3Genre = ID3Genre(rawValue: validGenre) {
                return Genre(genre: validId3Genre, description: genreDescription)
            return nil


class AttachedPictureField {
    let attachedPicture: Variable<ImageWithType?>

    init() {
        self.attachedPicture = Variable<ImageWithType?>(nil)

    func observeAttachPictureCreation() -> Observable<[AttachedPicture]?> {
        return attachedPicture
            .map({ imageWithType in
                if let validImageWithType = imageWithType {
                    return [AttachedPicture(art:,
                                            type: .FrontCover,
                                            format: validImageWithType.format)]
                } else {
                    return nil

Now it's time to see the view controller of the app that basically corresponds to the View of the MVVM. Its name is Mp3ID3TaggerViewController. This controller will implement a protocol I defined: the BindableView protocol. This protocol represents the View part in the MVVM architecture. This protocol must be implemented only by subclasses of the NSViewController.
The protocol contains a property and a function. The viewModel forces the class (the View) to have a property that represents its ViewModel. The function bindViewModel is where the View and the View model are bound together. The bindViewModel must be called inside one the lifecycle methods of the NSViewController.

protocol BindableView where Self: NSViewController {
    associatedtype ViewModelType
    var viewModel: ViewModelType! { get set }
    func bindViewModel()

If we look at the implementation of the bindViewModel method, we can see where something "magical" is happening ๐Ÿ”ฎ: an instance of Mp3ID3TaggerViewModel class is created and the UI components that represents the various field of the form are bounded to the view model fields by using the custom operator <->.
So this operator let us define the what is called two way binding or bidirectional binding using RxSwift:

  • each Variable field of the view model is bounded to a field on the UI. This basically means that each value we set a in the value property of a Variable field will be displayed on the UI Cocoa specific field.

  • each value inserted in the UI Cocoa specific field will be set in the corresponding Variable field on the view model.

In this way the View Model is completely decoupled from the View part (in this case the NSViewController). This means that we can reuse the same ViewModel to create other versions of Mp3ID3Tagger for other platforms. This is absolutely fantastic! ๐Ÿ˜โ˜บ๏ธ. Last but not least in the controller we have also some other functions:

  • open(_ sender: Any?) and save(_ sender: Any?) that manage the open an mp3 file and save of the same file
  • bindSaveAction() that observe the result of a save action
  • openImage(imageUrl: URL) and bindAttachedPictureField() that manage the bind and the subscription to an open action of an image to be used as front cover for the id3 tag.
infix operator <-> : DefaultPrecedence

func <-> <T>(property: ControlProperty<T>, variable: Variable<T>) -> Disposable {
    let bindToUIDisposable = variable.asObservable()
        .bind(to: property)
    let bindToVariable = property
        .subscribe(onNext: { n in
            variable.value = n
        }, onCompleted:  {

    return CompositeDisposable(bindToUIDisposable, bindToVariable)


class Mp3ID3TaggerViewController: NSViewController, BindableView {
    private let disposeBag: DisposeBag = DisposeBag()
    private let openAction: PublishSubject<String> = PublishSubject<String>()
    private let saveAction: PublishSubject<Void> = PublishSubject<Void>()
    private let stringToID3ImageExtensionAdapter = StringToID3ImageExtensionAdapter()
    var viewModel: Mp3ID3TaggerViewModel!
    @IBOutlet weak var versionPopUpbutton: NSPopUpButton!
    @IBOutlet weak var fileNameLabel: NSTextField!
    @IBOutlet weak var titleTextField: NSTextField!
    @IBOutlet weak var artistTextField: NSTextField!
    @IBOutlet weak var albumTextField: NSTextField!
    @IBOutlet weak var albumArtistField: NSTextField!
    @IBOutlet weak var yearTextField: NSTextField!
    @IBOutlet weak var trackPositionTextField: NSTextField!
    @IBOutlet weak var totalTracksTextField: NSTextField!
    @IBOutlet weak var genrePopUpMenu: NSPopUpButton!
    @IBOutlet weak var genreDescriptionTextField: NSTextField!
    @IBOutlet weak var imageSelectionButton: NSButton!

    override func viewDidLoad() {

    func bindViewModel() {
        viewModel = Mp3ID3TaggerViewModel(openAction: openAction.asObservable(), saveAction: saveAction.asObservable())
        (titleTextField.rx.text <-> viewModel.form.basicSongFields.title).disposed(by: disposeBag)
        (artistTextField.rx.text <-> viewModel.form.basicSongFields.artist).disposed(by: disposeBag)
        (albumTextField.rx.text <-> viewModel.form.basicSongFields.album).disposed(by: disposeBag)
        (albumArtistField.rx.text <-> viewModel.form.basicSongFields.albumArtist).disposed(by: disposeBag)
        (yearTextField.rx.text <-> viewModel.form.basicSongFields.year).disposed(by: disposeBag)
        (versionPopUpbutton.rx.selectedItemTag <-> viewModel.form.versionField.version).disposed(by: disposeBag)
        (trackPositionTextField.rx.text <-> viewModel.form.trackPositionInSetFields.trackPosition).disposed(by: disposeBag)
        (totalTracksTextField.rx.text <-> viewModel.form.trackPositionInSetFields.totalTracks).disposed(by: disposeBag)
        (genrePopUpMenu.rx.selectedItemTag <-> viewModel.form.genreFields.genreIdentifier).disposed(by: disposeBag)
        (genreDescriptionTextField.rx.text <-> viewModel.form.genreFields.genreDescription).disposed(by: disposeBag)

    private func bindAttachedPictureField() {
            .filter({ $0 != nil })
            .subscribe(onNext: { self.imageSelectionButton.image = NSImage(data: $0!.data) })
            .disposed(by: disposeBag)
        imageSelectionButton.rx.tap.subscribe(onNext: { tap in
            NSOpenPanel.display(in: self.view.window!,
                                fileTypes: ["png", "jpg", "jpeg"],
                                title: "Select an Image file",
                                onOkResponse: self.openImage)
        }).disposed(by: disposeBag)

    private func bindSaveAction() {
            .subscribe(onNext: { (result) in
                let alert = NSAlert()
                alert.addButton(withTitle: "Ok")
                alert.messageText = result ? "Mp3 saved correctly!" : "Error during save!"
                alert.beginSheetModal(for: self.view.window!, completionHandler: nil)
            .disposed(by: disposeBag)

    private func openImage(imageUrl: URL) {
        if let image = try? Data(contentsOf: imageUrl) {
            let type = self.stringToID3ImageExtensionAdapter.adapt(format: imageUrl.pathExtension)
            self.viewModel.form.attachedPictureField.attachedPicture.value = ImageWithType(data: image, format: type)
            self.imageSelectionButton.image = NSImage(data: image)

    @IBAction func open(_ sender: Any?) {
        NSOpenPanel.display(in: self.view.window!,
                            fileTypes: ["mp3"],
                            title: "Select an MP3 file",
                            onOkResponse: {
                                self.fileNameLabel.stringValue = $0.lastPathComponent

    @IBAction func save(_ sender: Any?) {

We're done with Mp3ID3Tagger. I hope you liked my architectural choices and how I developed it by leveraging the power of RxSwift and RxCocoa ๐Ÿ˜Ž๐Ÿ˜Œ. Obviously don't forget to see the official Mp3ID3Tagger repo

and obviously to download the Mp3ID3Tagger app from this link and use it!!! ๐Ÿ’—๐Ÿ’–

Originally published at on May 9, 2018.


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