QueryProperty on ViewModel gets assigned to variable too late in .NET MAUI?

Question

I am calling a page from my main page and I am using QueryProperty to pass a string. I want to evaluate this string and create a different list of objects depending on which string was passed so that I can reuse this same page for different things.

The problem is that when EnterValueViewModel runs and I want to check my passed string, the value is still null. How can I run the if statement after the value has been assigned?

[QueryProperty("TypeOfCaller", "TypeOfCaller")]


public partial class EnterValueViewModel : ObservableObject
{
    [ObservableProperty]
    string typeOfCaller;

    [ObservableProperty]
    ObservableCollection<string> listOfObjects;

    public EnterValueViewModel()
    {
        if (typeOfCaller == "PermitHolder")
        {
            listOfObjects = new ObservableCollection<string>(PermitHolderRepository.GetPermitHolders().Select(p => p.Name));
        }
        else if (typeOfCaller == "Company")
        {
            listOfObjects = new ObservableCollection<string>(CompanyRepository.GetCompanies().Select(p => p.Name));
        }
        else if (typeOfCaller == "Facility")
        {
            listOfObjects = new ObservableCollection<string>(FacilityRepository.GetFacilities().Select(p => p.Name));
        }
    }

I tried putting the if statement inside a method, but i don't know when to call it. I need an event that executes after the page is loaded but before the user can interact with it.

Answer 1

To execute code when an observable property changes, use one of the auto-generated methods for the property.

https://learn.microsoft.com/en-us/dotnet/communitytoolkit/mvvm/generators/observableproperty

Also, why are you using an ObservableCollection if you're going to recreate it every time? Just create the collection once and update it as required.

public partial class EnterValueViewModel : ObservableObject
{
    [ObservableProperty]
    string typeOfCaller;

    public ObservableCollection<string> ListOfObjects {get;} = new ObservableCollection<string>();

    partial void OnTypeOfCallerChanged(string newValue)
    {
        ListOfObjects.Clear();

        List<string> newObjects;

        if (newValue == "PermitHolder")
        {
            newObjects = PermitHolderRepository.GetPermitHolders().Select(p => p.Name)).ToList();
        }
        else if (newValue == "Company")
        {
            newObjects = CompanyRepository.GetCompanies().Select(p => p.Name)).ToList();
        }
        else if (newValue == "Facility")
        {
            newObjects = FacilityRepository.GetFacilities().Select(p => p.Name)).ToList();
        }
        else
        {
            return;
        } 

        foreach(string newObject in newObjects)
        {
            ListOfObjects.Add(newObject);
        } 
    } 
}