SwiftUI open view with data or only with a reference to Firestore document

问题

I need to open a view called PackageDetails.

There are two cases in which this view could be opened.

1. If you tap a specific Package from a list of packages.

I have an array of all the packages. So I'm passing the specific struct from this list and I'm opening this package view.PackageDetail(package: package). I already have all the data I need so I do not need to make an extra call to Firestore to fetch this document.

2. Package view can be opened by a link from some other view in this app.

In this case I do not have all the details of this specific Package but I have just a reference to the Firestore document. I am using this reference to make a query as soon as this view appears and get the details about this Package PackageDetail(package: packageReference)

The ways I'm doing this is by declaring Package as an Optional @State var package: PackageModel? and onAppear I'm checking if the Package is null or not.

.onAppear {
    if let package = self.package {
        // package data exists
    } else {
        // fetch it from firestore
    }
}

The question is: Am I doing this right? Is this the best approach?

回答1:

The Package is not view state, so @State, by design does not fit here. Instead it is preferable to use view model pattern, so the approach (scratchy) might be like below:

class PackageViewModel: ObservableObject {
    @Published var packageReference: URL?
    @Published var package: PackageModel?

    init(package: PackageModel? = nil, reference: URL? = nil) {
        self.package = package
        self.packageReference = reference
    }

    private func loadPackage() {
        // do load here from 'self.packageReference' and in callback assign package created from result
        DispatchQueue.main.async {
            self.package = package
        }
    }
}

struct PackageView: View {
    @ObservedObject var vm: PackageViewModel

    var body: some View {
        VStack {
            // some main part here

            if vm.package != nil {
                PackageDetail(package: self.vm.package)
            }
        }
        .onAppear {
            if self.vm.package == nil {
                self.vm.loadPackage()
            }
        }
    }
}

来源：https://stackoverflow.com/questions/60688040/swiftui-open-view-with-data-or-only-with-a-reference-to-firestore-document

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