How do you pass an object from one view model to another in Xamarin-CodePudding

I'm working on a Xamarin.Forms project. I have a ViewModel for displaying a list of objects, and another for editing the objects. In my ListViewModel, I have a Command that takes a selected object and ideally sends it to the EditViewModel. I am getting an error when trying to build the project that says...

Views\EditObjectPage.xaml(9,10): XamlC error XFC0004: Missing default constructor for "MobileApp.ViewModels.EditViewModel"

This is the starting point of my offending code in my Views\EditObjectPage.xaml

             .....
             x:Class="MobileApp.Views.EditObjectPage"
             xmlns:viewmodel="clr-namespace:MobileApp.ViewModels">
    
    <!--Binding Context-->
    <ContentPage.BindingContext>
        <viewmodel:EditViewModel />  //offending line
    </ContentPage.BindingContext>

In the code behind I have a constructor that takes an object from a command in my ListViewModel (Note: I have tried adding an empty default constructor but nothing changed)

namespace MobileApp.Views
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class EditObjectPage : ContentPage
    {
        public EditObjectPage(Object object)
        {
            InitializeComponent();
        }
    }
}

The command in my ListViewModel

public ICommand EditObjectCommand { get; set; }   //declaration of property
public ListViewModel()
{
    ...........
    EditObjectCommand = new Command(async (o) => await EditObject(o));
}
private async Task EditObject(object o)
{
    SelectedObject = o as Object;  //SelectedObject is a working property
    await Application.Current.MainPage.Navigation.PushAsync(new 
    EditObjectPage(SelectedTerm));
}

And my EditViewModels Constructor is

 public EditViewModel(Object object)
{
   _modifiedObject = object; //_modifiedObject is a private property of this class
   ....
}

I am new to MVVM and Xamarin, and am considering just trying to make my edit and add views and viewmodels the same by just overloading the constructor in the code behind, but I don't actually know if that would work. I realize that this might be a spaghetti code way of doing things, so any input or links would be much appreciated.

CodePudding user response：

Seems missing MVVM Default Constructor

public ListViewModel(INavigation navigation)
{
}

CodePudding user response：

You can just try to add a non-parameter constructor in the EditViewModels.cs, such as:

public EditViewModel() {}

And when you create the instance of it, you can still use the new EditViewModel(Object object) method.

Update

Sorry for my careless, adding a non-parameter constructor can just the make the error disappear, but the viewmodel will never get the object because xmal create the instance of it by the non-parameter constructor.

So just as Marco Beninca says, you need to create the instance of the viewmodel in the page's constructors, and also set the binding context there.

CodePudding user response：

The issue here is the the xaml lines:

<ContentPage.BindingContext>
    <viewmodel:EditViewModel />  //offending line
</ContentPage.BindingContext>

use the default constructor for the view model. The easiest way to solve this is to move the creation of the ViewModel insto the View constructor

namespace MobileApp.Views
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class EditObjectPage : ContentPage
    {
        private EditViewModel _model = null;

        public EditObjectPage(Object obj)
        {
            InitializeComponent();
            _model = new EditViewModel(obj);
            this.BindingContext = _model;
        }
    }
}