Swiftui Previews - Unexpected Data

SwiftUI Previews - unexpected data

Why Preview of my ItemView.swift view shows a mixture of example data I have provided + Core Data models created in Live Preview?

1. the below line initiates context to your persistent data, so makes available all stored core data object to be fetched

    let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
   

2. the below line creates a new object in the context created above, so it is in context (though to stored) and available for fetching alongside of others stored.

    let testItem = Item.init(context: context)
   

Thus fetching object from this context ...

    return ItemView(filter: false)
        .environment(\.managedObjectContext, context)

... makes all - stored and created - available in Preview.

SwiftUI - too many previews

Replace

var body: some View {
    // I think problem is here
    ForEach(items, id: \.self) {item in
        Text("\(item.date ?? Date())")
    }       
}

with

var body: some View {

    /// add VStack here!
    VStack {
        ForEach(items, id: \.self) {item in
            Text("\(item.date ?? Date())")
        }       
    }
}

Without a VStack or some other container, your body consists of multiple views. This makes Xcode generate a preview for each of them... see here for a similar question.

How to fix multiple SwiftUI previews

You'll need to put both Texts in a VStack or similar.

struct ContentView: View {
    var body: some View {
        VStack {
            Text("Example title")
            Text("Example title 2")
        }
    }
}

It seems that Xcode automatically adds a new preview for each separate view returned in the body var (It used to throw an error, but I think Xcode 12 changed some things).

Error when using Canvas to preview SwiftUI view

Clean Build Folder, then kill Xcode, start Xcode, do a full Build may (sometimes) fix the problems. Then the whole circus starst again.
I think it is a bit of a shame on Apple. Most frustrating of all (for me at least) is that documentation is virtually non existing. It is all trial and error. Mostly error.

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