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

问题

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?

回答1:

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)")
    }
}

回答2:

instead of creating a subview you can add a dummy initializer to your "ViewObject" so that you can call it before calling the real initializer

struct MyCoolView: View { 

    @EnvironmentObject userData: UserData
    @ObservedObject var viewObject: ViewObject

    init() {
        viewObject = ViewObject()
        defer {
            viewObject = ViewObject(id: self.userData.UID)
        }
    }

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

for the record i haven't tested it

回答3:

Here is an easy way to to it:

struct MyCoolView: View {
    @EnvironmentObject var userData: UserData

    var body: some View {
        Observe(obj: ViewObject(id: userData.UID)) { viewObject in
             Text("\(viewObject.myCoolProperty)")
        }
    }
}

With this helper that makes it work:

struct Observe<T: ObservableObject, V: View>: View {
    @ObservedObject var obj: T
    let content: (T) -> V
    var body: some View { content(obj) }
}

来源：https://stackoverflow.com/questions/59367218/swiftui-how-do-i-initialize-an-observedobject-using-an-environmentobject-as-a

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