Dynamic Predicates with Core Data in SwiftUI

Create a reusable generic SwiftUI view to wrap dynamic Core Data content

Photo by Clément H on Unsplash

With the release of iOS 13 Beta 5, Apple gave developers a way forward with using Core Data with SwiftUI but provide little in the way of usage details:

• NSManagedObject now conforms to ObservableObject. The new @FetchRequestproperty wrapper can drive views from the results of a fetch request, and managedObjectContext is now included in the environment. (50280673)

While this change was welcomed, it wasn’t clear (after playing around with these new APIs for a bit) how one was supposed to fetch anything other than a predetermined set of data in your SwiftUI view.

The problem in a nutshell:

How do we dynamically change the NSPredicate in the above example without using self?

The answer is to wrap the fetch request in another view and pass the predicate down to that view!

We can create a containing view for the fetch request and initialize the containing view with the predicate like so:

But we can do better. What if you want to reuse this view in multiple lists in your application?

The answer is to build a wrapper view with Swift generics!

TL;DR — Just show me the codes:

The above code uses Swift generics and @ViewBuilder to create a reusable SwiftUI component for any fetched objects.

Lets implement this in our previous example and create a list of Students:

We now have a way to dynamically replace the predicate when the button action is fired. And because SwiftUI will redraw our view’s body when the @State property is changed, our list will update!

That’s it! Go forth and fetch data…