Using ApplyQueryAttributes Method with Constructor

Question

How can I use the ApplyQueryAttributes Method of IQueryAttributable to retrieve a value for a parameter in my viewmodel? The problem is that the method I want to use the parameter for is in the constructor and ApplyQueryAttributes has not executed before this method in the constructor is called. Is there a way to fire off ApplyQueryAttributes before the constructor, or should I call the method from within ApplyQueryAttributes? What is the best practice? Here is an example of my code:

The gist of the matter is I am trying to pass parameters from one viewmodel to another, but the method that uses the parameter is in the constructor so it fires before ApplyQueryAttibutes where I get the parameter value.

public void ApplyQueryAttributes(IDictionary<string, object> query)
{
    Package myPackage = query["MyPackage"] as Package;
    Id = myPackage.packageId;
    // My Method
    RefreshListData();
}

Answer 1

Hello,

should I call the method from within ApplyQueryAttributes?

Yes, you can implement INotifyPropertyChanged interface in your ViewModel, INotifyPropertyChanged provides the ability for a class to raise the PropertyChanged event whenever one of its properties changes. The data binding mechanism in .NET MAUI attaches a handler to this PropertyChanged event so it can be notified when a property changes and keep the target updated with the new value.

For example, you bind the Name in the Label like <Label Text="{Binding Name}"/>. When MyViewModel's constructor is exeuted, Label is empty. After executing ApplyQueryAttributes, Name has a value. Label' Text will be changed at runtime.

public class MyViewModel: IQueryAttributable, INotifyPropertyChanged
{
    public event PropertyChangedEventHandler? PropertyChanged;
     public void OnPropertyChanged([CallerMemberName] string name = "") =>
       PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));

   public MyViewModel()
    {
    }

   private string name;
   public string Name
    {
        get { return name; }
        set { name = value; OnPropertyChanged(); }
    }
    public void ApplyQueryAttributes(IDictionary<string, object> query)
    {
        Name = query["name"] as string;
       
    }
}

For more details about Data binding and MVVM, please check this document:Data binding and MVVM - .NET MAUI | Microsoft Learn

Best Regards,

Leon Lu

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