Offering a default worth for a SwiftUI Binding – Donny Wals

Generally in SwiftUI apps I’ll discover that I’ve a mannequin with an elective worth that I’d wish to go to a view that requires a non elective worth. That is particularly the case once you’re utilizing Core Knowledge in your SwiftUI apps and use auto-generated fashions.

Think about the next instance:

class SearchService: ObservableObject {
  @Revealed var outcomes: [SearchResult] = []
  @Revealed var question: String?
}

Let me begin by acknowledging that sure, this object might be written with a question: String = "" as a substitute of an elective String?. Sadly, we don’t at all times personal or management the fashions and objects that we’re working with. In these conditions we is perhaps coping with optionals the place we’d moderately have our values be non-optional. Once more, this may be very true when utilizing generated code (like once you’re utilizing Core Knowledge).

Now let’s think about using the mannequin above within the following view:

struct MyView: View {
  @ObservedObject var searchService: SearchService

  var physique: some View {
      TextField("Question", textual content: $searchService.question)
  }
}

This code won’t compile as a result of we have to go a binding to a non elective string to our textual content area. The compiler will present the next error:

Can’t convert worth of sort ‘Binding<String?>’ to anticipated argument sort ‘Binding‘

One of many methods to repair that is to supply a customized occasion of Binding that may present a default worth in case question is nil. Making it a Binding<String> as a substitute of Binding<String?>.

Defining a customized binding

A SwiftUI Binding occasion is nothing greater than a get and set closure which might be known as every time anyone tries to learn the present worth of a Binding or after we assign a brand new worth to it.

Right here’s how we will create a customized binding:

Binding(get: {
  return "Hiya, world"
}, set: { _ in
  // we will replace some exterior or captured state right here
})

The instance above basically recreates Binding‘s .fixed which is a binding that can at all times present the identical pre-determined worth.

If we have been to jot down a customized Binding that permits us to make use of $searchService.question to drive our TextField it might look a bit like this:

struct MyView: View {
  @ObservedObject var searchService: SearchService

  var customBinding: Binding<String> {
    return Binding(get: {
      return searchService.question ?? ""
    }, set: { newValue in
      searchService.question = newValue
    })
  }

  var physique: some View {
    TextField("Question", textual content: customBinding)
  }
}

This compiles, and it really works nicely, but when now we have a number of occurrences of this example in our codebase, it might be good if had a greater approach of scripting this. For instance, it might neat if we might write the next code:

struct MyView: View {
  @ObservedObject var searchService: SearchService

  var physique: some View {
    TextField("Question", textual content: $searchService.question.withDefault(""))
  }
}

We will obtain this by including an extension on Binding with a way that’s accessible on present bindings to elective values:

extension Binding {
  func withDefault<T>(_ defaultValue: T) -> Binding<T> the place Worth == Elective<T> {
    return Binding<T>(get: {
      self.wrappedValue ?? defaultValue
    }, set: { newValue in
      self.wrappedValue = newValue
    })
  }
}

The withDefault(_:) perform we wrote right here might be known as on Binding situations and in essence it does the very same factor as the unique Binding already did. It reads and writes the unique binding’s wrappedValue. Nevertheless, if the supply Binding has nil worth, we offer our default.

What’s good is that we will now create bindings to elective values with a reasonably simple API, and we will use it for any type of elective information.