Eric Roller's Development Blog

SwiftUI would be impossible without the preview canvas where our view model can be inspected live. When using Core Data, however, I don't want to preview actual data; I want to be able to supply custom preview-specific data.

To do this, I am creating an in-memory persistent store with hard-coded data:

import SwiftUI
import ShopCore    // Private framework
import CoreData

// …


struct ContentView_Preview: PreviewProvider {
    static var previews: some View {
        return ContentView(store: previewStore)

    static let previewStore: ShopKeeper = {
        // Create a dedicated instance, without loading from Core Data
        let store = ShopKeeper()

        // Create the preview coordinator
        let context = PreviewCoordinator.shared.viewContext

        let previewShop =
            ShopModel.entityName(), into: context)
            as! ShopModel

        // Data to be shown in the preview canvas:
        previewShop.name = "Preview Shop"
        previewShop.comment = """
This is a dynamically created preview shop that
is stored in an in-memory \"persistent\" store.
        // …

        try? context.save()

        // Show just our preview store
        store.list = [ previewStore ]
        return store

And this is how the in-memory persistent store is set up for the preview:

private class PreviewCoordinator {
    static let shared = PreviewCoordinator()

    let objectModel: NSManagedObjectModel
    let storeCoordinator: NSPersistentStoreCoordinator
    let viewContext: NSManagedObjectContext

    init() {
        objectModel = NSManagedObjectModel()
        objectModel.entities = [

        storeCoordinator = 

        viewContext = NSManagedObjectContext(concurrencyType:

        do {
            try storeCoordinator.addPersistentStore(ofType:
                NSInMemoryStoreType, configurationName: nil, at: nil,
                options: nil)
            viewContext.persistentStoreCoordinator = storeCoordinator
        } catch {


To make this work, our managed object needs to be able supply its entity (class) name:

extension NSManagedObject {
    public class func entityName() -> String {
        // Typically: "MyApp.MyModel"
        let moduleAndClassName = NSStringFromClass(object_getClass(self)!)
        // Return just: "MyModel"
        return String(moduleAndClassName.split(separator: ".").last!)

I cannot load the Core Data model description, since it is located in an embedded framework. Therefore, the entity property cannot be used; instead the ShopModel needs to supply a preview-specific entity description:

import CoreData


extension ShopModel {
    // Returns the entity description, mirroring what is defined
    // in the .xcdatamodeld file for Core Data.
    // Beware: To be kept in sync with the .xcdatamodeld !!!
    public class func previewDescription() -> NSEntityDescription {
        let entity = NSEntityDescription()

        entity.name = entityName()
        entity.managedObjectClassName =
            "ShopCore." + (entity.name ?? "none")
        entity.renamingIdentifier = entity.name

        entity.properties = [
            NSAttributeDescription(name: "id",
                type: .UUIDAttributeType, optional: true),
            NSAttributeDescription(name: "name",
                defaultValue: "New Shop"),
            // …


The most critical part turned out to be the managedObjectClassName - it needs to be set correctly or else the as! ShopModel cast will fail. Normally, the class name would be something like "ShopApp.ShopModel", but in my case, where the ShopModel is declared in an embedded "ShopCore" framework, it would be "ShopCore.ShopModel". To eliminate these conflicts, it is a good idea to manually set the Module (to "ShopCore") for each object in the .xcdatamodeld editor.

To streamline the above code, I also added a convenience initialiser, with defaults that are specific to my use case:


extension NSAttributeDescription {
    // Used below to create properties for the entity description.
    convenience public init(name: String,
        type: NSAttributeType = .stringAttributeType,
        defaultValue: Any? = nil, optional: Bool = false) {


        self.name = name
        self.renamingIdentifier = name
        self.isOptional = optional
        self.defaultValue = defaultValue
        self.attributeType = type

        switch type {
        case .stringAttributeType:
            self.attributeValueClassName = "NSString"
        case .dateAttributeType:
            self.attributeValueClassName = "NSDate"
        case .UUIDAttributeType:
            self.attributeValueClassName = "NSUUID"
            self.attributeValueClassName = "NSNumber"


Update History

[2020-08-14] Now using a constant managedObjectClassName prefix. [2020-08-09] Renamed entityDescription() to previewDescription(), clarifying why entity cannot be used. Moved previewStore into the ContentView_Preview struct, avoiding the use of a global valiable. Moved convenience init() to be last.

Quite often I find myself in the Terminal and want to open Xcode with the project in the current directory:

% open CurrentProject.xcodeproj

This works just fine, of course, but with the following alias (added to ~/.cshrc), I don't need to know the name of the project, assuming there is always only one project in the current directory:

% alias xcode '\ls | grep xcodeproj | xargs open'
% xcode

N.B. The use of \ls ignores the alias that I have for 'ls'. Instead of ls *.xcodeproj, the pipe via grep avoids errors like "ls: No match.".

P.S. Knowing the name of the project is actually not the problem; since there are multiple items named "Current" or "CurrentProject" in the same folder, the issue is that I can't just type open SHIFT+C<TAB><CR> (where pressing TAB autocompletes the name); instead, I have to type: open SHIFT+C<TAB>SHIFT+P<TAB>.x<TAB><CR>.


The Xcode text editor invocation tool xed can achieve much the same thing; just type:

% xed .

Trying to update Xcode 4.0.1 from within the Mac AppStore application, I was greeted with this error message:

You have updates available for other accounts Sign in to (null) to update applications for that account.

There exist numerous forum entries on Apple's discussion board on that subject and the recommended solution is to trash the "Install Xcode" application and then to re-launch the AppStore, selecting to re-install Xcode.

I trashed "Install Xcode" but AppStore produced the same message. Relaunched one additional time and it allowed me to update (note: not install). However, when it was finished, there was no new "Install Xcode" in the Applications folder!

It turns out, the AppStore had located and updated a different copy of the software on a separate drive where I had made a backup, even though I had renamed it to "Install Xcode-4.0.1". Its info window revealed that it had been modified and that its version number now was 4.0.2.

Installation from that modified backup worked correctly.

The old backup could be restored from the version that I had put into the trash.

Searched for it online and found this solution, which worked fine for me (Xcode 3.0):

defaults write com.apple.xcode  PBXCustomTemplateMacroDefinitions  \
        '{ ORGANIZATIONNAME = "tredje design"; }'