NSOpenPanel's and NSSavePanel's Block-Based API is Superior

I got angry at NSOpenPanel the other day for visually blocking NSAlerts, and for staying in the way when the debugger hits a breakpoint. Silly me, I used ye olde runModal.

When you use the block-based API of NSSavePanel or its decendant NSOpenPanel, all of a sudden they will display on top, but only per application. Also, alerts seem to have a higher display level in these cases.

Of course now the client code has to be designed differently, too. Instead of waiting for a return calue, you pass a completion handler.

I don’t think that’s too bad, though:

func addMonitorUsing(createMonitor: CreationStrategy) {

    pickedURLs() { URLs in

        let newMonitors = URLs.map({ NewMonitor(URL: $0, fileID: FileID()) })


private func pickedURLs(completion: ([LocalURL]) -> Void) {

    return filePicker.selectedFilesWithExtensions(.Some(supportedFileExtensions), 
        completion: completion)

Why, of course I wrapped the NSOpenPanel call in its own type:

enum AllowedFileExtensions {
    case Any
    case Some([String])

    func preparePanel(panel: NSSavePanel) {
        panel.allowedFileTypes = self.allowedFileTypes

    private var allowedFileTypes: [String]? {
        switch self {
        case .Any: return .None
        case let .Some(types): return types

class SelectFiles {

    init() { }

    func selectedFilesWithExtensions(fileExtensions: AllowedFileExtensions,
        completion: ([LocalURL]) -> Void) {
        let panel = NSOpenPanel()
        panel.canChooseDirectories = false
        panel.canChooseFiles = true
        panel.allowsMultipleSelection = true
        panel.beginWithCompletionHandler { response in
            guard response == NSFileHandlingPanelOKButton else {
            let selectedFileURLs = panel.URLs.map() { LocalURL(URL: $0) }

As a side-note, see how the AllowedFileExtensions enum helped me set up the panel while making the intent clear?

Initially, I commented the selectedFilesWithExtensions method thus:

/// Show open file panel and return results as `LocalURLs`.
/// - parameter fileExtensions: `nil` indicates "all file types"
/// - returns: Collection of selected files; nil if cancelled.

But now I don’t need to, because the types are self-documenting: the AllowedFileExtensions enum provides easy to understand options, and since the completion block is not called when nothing is selected, I don’t even have to document a nil case for that, either.

When you allow nil, you essentially allow either all of the return type’s values or something else entirely. I always try to get rid of it and make APIs simpler.

Now the app runs great, the user experience is better than before, and the panels get out of my way while debugging. I’m pretty happy with that now.

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