Swiftui - How do I initialize an observedObject using an environmentobject as a parameter?

Question

I'm not sure if this is an antipattern in this brave new SwiftUI world we live in, but essentially I have an @EnvironmentObject with some basic user information saved in it that my views can call.

I also have an @ObservedObject that owns some data required for this view.

When the view appears, I want to use that @EnvironmentObject to initialize the @ObservedObject:

struct MyCoolView: View { 

    @EnvironmentObject userData: UserData
    @ObservedObject var viewObject: ViewObject = ViewObject(id: self.userData.UID)  

    var body: some View { 
            Text("\(self.viewObject.myCoolProperty)")
    } 
}

Unfortunately I can't call self on the environment variable until after initialization:

"Cannot use instance member 'userData' within property initializer; property initializers run before 'self' is available."

I can see a few possible routes forward, but they all feel like hacks. How should I approach this?

I tried that and got a somewhat strange error: Property wrappers are not yet supported on local properties Basically its saying I cant create an @ObservedObject in an init method. — snarik
– snarik, Commented Dec 17, 2019 at 3:45

Martijn Pieters · Accepted Answer · 2022-08-23 13:38:31Z

32

Here is the approach (the simplest IMO):

struct MyCoolView: View {
    @EnvironmentObject var userData: UserData

    var body: some View {
        MyCoolInternalView(ViewObject(id: self.userData.UID))
    }
}

struct MyCoolInternalView: View {
    @EnvironmentObject var userData: UserData
    @ObservedObject var viewObject: ViewObject

    init(_ viewObject: ViewObject) {
        self.viewObject = viewObject
    }

    var body: some View {
            Text("\(self.viewObject.myCoolProperty)")
    }
}

edited Aug 23, 2022 at 13:38

Martijn Pieters

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answered Dec 17, 2019 at 5:40

Asperi

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2 Comments

snarik Over a year ago

This is perfect. The MyCoolView was actually a child to a 'home' view where I declared the ObservedObject. Thanks!

BobiSad Over a year ago

But what if you want to manipulate the userData inside ViewObject without an entirely new ViewObject being created every time?

Pavel Holec · Accepted Answer · 2023-08-28 08:15:52Z

0

One way to initialize ObservableObject that depends on environment (key or object) is to pass the necessary environment from a parent view via a view init that will itself create the StateObject using wrappedValue: initializer.

Solution

struct FirstScreen: View {
  @Environment(\.fooService) var fooService
  
  var body: some View {
    content
      .sheet(...) {
        SecondScreen(
          // Here we DO have access to environment
          fooService: fooService
        )
      }
  }
}

struct SecondScreen: View {
  @StateObject var viewModel: ViewModel

  init(fooService: FooService) {
    // The ObservableObject initialisation is encapsulated here
    self._viewModel = .init(
      // The autoclosure initializer needs to be used
      wrappedValue: .init(fooService: fooService)
    )
  }
}

Disclosure

Following is a link to my article describing the solution in more detail:

ObservableObject initialisation using Environment

edited Aug 28, 2023 at 8:15

answered Aug 24, 2023 at 9:26

Pavel Holec

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1 Comment

Adrian Mole Over a year ago

As I commented in another one of your answers, you must explicitly disclose any affiliation you have with sites you link.

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