[Office Hours] Mutability of Medication List

[EthanRakutoBell]

In what area do you have a technical challenge?

Xcode, Simulator & Previews

Description

Upon progressing to the "Add Medication" sheet, I am having difficulty updating the given patient's medication list upon prescribing a new medication. It appears the root of the issue lies in the mutability of the patient variable- and passing in the patient as Binding through different view in the NavigationLink. Please see snippets of code from each level of the navigation stack.

@Observable
class PatientList: ObservableObject { // used @published and made PatientList an ObservableObject
    @Published var patients: [Patient] = [
        Patient(firstName: "John", lastName: "Doe", DOB: DOB(DOB: DateComponents(year: 1999, month: 11, day: 8)), height: 200, weight: 75, bloodType: BloodType(typeBlood: "O+"), medications: [Medication1])
    ]
}

struct PatientListView: View {
 @ObservedObject var patientList = PatientList()
    var body: some View {
        NavigationStack {
            List($patientList.patients) { $patient in
                NavigationLink(value: $patient) {
                ...
        .navigationTitle("All Patients")
         .navigationDestination(for: Patient.self) { patient in PatientDetailView(patient: $patient) // error- cannot find $patient in [scope]

struct PatientDetailView: View {
    @State private var showAddMedicationView: Bool = false
    @Binding var patient: Patient
    var body: some View {
    ...
    Button(action: {self.showAddMedicationView.toggle()}) { Text("Add Medication") }
                .font(.headline)
                .foregroundColor(.white)
                .padding()
                .background(Color.blue)
                .cornerRadius(10)
        }
        .sheet(isPresented: $showAddMedicationView) {
            AddMedicationView(patient: $patient, showAddMedicationView: $showAddMedicationView) // opens add medication sheet

struct AddMedicationView: View {
    @Binding var patient: Patient
     var body: some View {
     ...
     Button(action: { if checkInput() {
                let newMedication =  Medication(name: medicationName, dosage: medicationDosage, route: medicationRoute,        frequency: medicationFrequency, duration: medicationDuration)
                patient.prescribeMedication(med: newMedication) // (error: call can throw- but not marked with try)
                self.showAddMedicationView.toggle()
            }

Reproduction

I have been changing @binding and @observable on patient objects just to see what works- so I am now a little lost with what is a standard way to pass in a mutable variable.

Expected behavior

Prescribe the medication to the patient- so that upon returning to the detail view, the medication list is updated.

Additional context

Thank you for the help!

Code of Conduct

• I agree to follow this course's Code of Conduct and Contributing Guidelines

[PSchmiedmayer]

Hi @EthanRakutoBell,

Thank you for reaching out and asking this question!

Let's go through some of the elements step by step. Most of the elements that you are encountering are due to your usage (or rather not usage) of @Observable and the usage of Bindings. Some of these things can be challenging to get started, so let's go through them step-by-step.

It seems like you are mixing elements between the old way observation in Swift was handled and the new @Observable macro. We would encourage you to use @Observable, but a lot of examples are out there, and almost all LLMs still output stuff in the old syntax and mechanism. Here is a refresher on @Observable. The following WWDC video also provides a nice visual explanation of the mechanism.

The SwiftUI basics video might also be a great help to understand the fundamentals of SwiftUI and @Observable and the underlying mechanisms I am referring to.

I think it might also be helpful to follow this article to explain to you how you can translate old ObservableObject code that you might find somewhere on to @Observable.
You will see that you don't have to use @Published anymore, conforming to ObservableObject is not necessary, and @ObservedObject is not needed.

struct Patient: Hashable, Identifiable {
    let id = UUID()
    var name: String
}

@Observable
class PatientManager {
    var patients: [Patient] = []
}

To ensure that you obtain bindable elements from your list, please revisit the usage of @Bindable to ensure that you can pass a patient down to a child view. You will need this to bind the list of patients to a @Bindable reference to Patients to pass them down in the child views where you can utilize @Binding.

Secondly, the mechanism of utilizing a NavigationLink with a value attached to it might make things more complicated than they have to be. NavigationLink also has an initializer that allows you to pass in a label and a destination view. Constructing this right in your list might be easier than using navigationDestination.

struct ContentView: View {
    var patientList = PatientManager()
    
    var body: some View {
        @Bindable var patientList = patientList
        List($patientList.patients) { patientBinding in
            NavigationLink(
                destination: {
                    PatientDetailView(patient: patientBinding)
                },
                label: {
                    Text(patientBinding.wrappedValue.name)
                }
            )
        }
    }
}

struct PatientDetailView: View {
    @Binding var patient: Patient
    
    var body: some View {
        Text("Detail view for patient: \(patient.name)")
    }
}

As you can see in the simplified example above, you can easily bind the patientList to an @Bindable inside your computed body property and eventually pass the patientBinding to the detail view.

You can always obtain the wrapped value of the Binding property wrapper using the .wrappedValue property to access its underlying wrapped value. I would highly recommend the Assignment 1 readings for some context about this, including the properties article also covering property wrappers.

[EthanRakutoBell]

Thank you so much! This really helps clear things up.

[PSchmiedmayer]

Happy to help; good luck with the rest of the assignment 🚀