
## Preview Error

When you're using CoreData in a SwiftUI `View`, you'll get erros when resuming the preview with the following code.

```swift
struct ContentView: View {
  @Environment(\.managedObjectContext) var moc

  var body: some View {
    Text("hello World")
  }
}

struct ContentView_Previews: PreviewProvider {
  static var previews: some View {
    ContentView()
  }
}

```

The reason is that your preview `ContentView()` does not have a managed object context.

## Solution

One solution I found is to create a wrapper view which adds test data to the managed object context and passes it back to the view.

```swift
struct PreviewCoreDataWrapper<Content: View>: View {
  let content: (NSManagedObjectContext) -> Content

  var body: some View {
    let managedObjectContext = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext

    let todo = Todo(context: managedObjectContext)
    todo.title = "I Am Legend"

    return self.content(managedObjectContext)
  }

  init(@ViewBuilder content: @escaping (NSManagedObjectContext) -> Content) {
    self.content = content
  }
}

```

Now, you can embed every view which needs access to the managed object context within the `PreviewCoreDataWrapper` view
and access the `managedObjectContext` to pass it into the enviroment.

```swift
struct MoviesView_Previews: PreviewProvider {
  static var previews: some View {
    PreviewCoreDataWrapper { managedObjectContext in
      MoviesView().environment(\.managedObjectContext, managedObjectContext)
    }
  }
}
```

Now you can resume your preview and it should render just fine without any errors.

---

<small>
  If you have feedback or questions, please feel free to get in touch via Twitter.
</small>


The SwiftUI preview errors when using CoreData managed object context.

SwiftUI Preview with CoreData


As you might already know, I love coding and designing.
I couldn't help myself but use the powerful SwiftUI framework and build an iOS game with playful animations.

# Learning Resources

I started looking into SwiftUI the day it was released. I started with Apple's own tutorials.

A couple of days later, I discovered [Hacking with Swift](https://www.hackingwithswift.com) on Twitter.
There, I learned a lot about SwiftUI and its API, but couldn't find as much as I wanted about animations and transitions.
Meanwhile you can find a complete list called [SwiftUI By Example](https://www.hackingwithswift.com/quick-start/swiftui).

To dive deeper into animations and transitions, I went back to my trusted design tutorial source [Design+Code](https://designcode.io/).

# Design

I stared designing in Sketch to get a feeling of how the game would be. After some iterations, I had something I could start coding in Xcode.

<img src="/contents/project/rapilo/sketch-screens.png?v1" class="block my-8 shadow-xl min-h-48 mx-auto rounded-lg" />

## Colors

Besides the gray colors, I used the default iOS colors `Color.pink`, `Color.orange`, `Color.green` and `Color.blue`.

## Icons

I used the new iOS icons font introduced with iOS 13 called _SF Symbols_.
The leaderboard is the only icon I designed myself, which is exported as _PDF_ to be used as a vector.

# Developement

After exporting the one icon and the gray colors to the `Assets.xcassets`, I was ready to start coding in Xcode.

I strongly believe that if a UI feels wrong, it's not going to be used. That's why I started with the UI and animations first.
It took me some time until I had something I felt was the right amount of animations and types of transitions.

<img src="/contents/project/rapilo/playing.gif?v1" class="block my-8 shadow-xl w-64 min-h-48 mx-auto rounded-lg" />

After the UI and animations, I added the state management using the _Combine_ framework.
Adding the leaderboard by using Game Center, was the last bit i did before I started beta testing.

# Beta Tester

If you want to be a beta tester, just sign up [right here](https://testflight.apple.com/join/WwxTW39J).
Don't forget to give me your feedback. It helps me to improve the game so you can have even more fun.


An iOS game build with SwiftUI using custom animations and transitions.

#swiftui

Don't miss out.