Is instantiating a class in VM, MVVM compliant ? How else if so? - xaml

Question

I have a contentpage and a ContentView with the content property bound to the view model
MainPage:
`
<?xml version="1.0" encoding="utf-8" ?>
<ContentPage x:Class="MvvM.Views.MainPage"
xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:local="clr-namespace:MvvM.Views"
xmlns:vm="clr-namespace:MvvM.ViewModels">
<!-- ViewModel BindingContext -->
<ContentPage.BindingContext>
<vm:MainViewModel />
</ContentPage.BindingContext>
<Grid>
<Grid.RowDefinitions>
<!-- Header Row -->
<RowDefinition Height="50" />
<!-- ContentView Row -->
<RowDefinition Height="*" />
</Grid.RowDefinitions>
<!-- Header -->
<Grid Grid.Row="0"
BackgroundColor="CornflowerBlue"
VerticalOptions="FillAndExpand">
<!-- Button On Header -->
<Button Command=""
Text="Page Switch"
VerticalOptions="Center">
<Button.GestureRecognizers>
<TapGestureRecognizer Command="TapGestureCommand" />
</Button.GestureRecognizers>
</Button>
</Grid>
<!-- Content Container -->
<Grid Grid.Row="1" VerticalOptions="Center">
<ContentView Content="{Binding DisplayPage}" />
</Grid>
</Grid>
</ContentPage>
`
ViewModel:
`using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Text;
using MvvM.Views;
using Xamarin.Forms;
namespace MvvM.ViewModels
{
public class MainViewModel :INotifyPropertyChanged
{
public MainViewModel()
{
DisplayPage = new Views.MainPage();
}
private ContentPage _displayPage;
public ContentPage DisplayPage
{
get { return _displayPage; }
set
{
if (value != _displayPage)
{
_displayPage = value;
}
}
}
private ContentView _contentToDisplayView;
public ContentView SelectedView
{
get => _contentToDisplayView;
set
{
_contentToDisplayView = value;
OnPropertyChanged();
}
}
public Command TapGestureCommand
{
get
{
return new Command(TapGesture);
}
}
private void TapGesture()
{
_contentToDisplayView = new RedView();
_displayPage.Content = _contentToDisplayView.Content;
OnPropertyChanged();
}
#region PropertyChangedHandler
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
#endregion
}
}`
and the second page called "RedPage" want to access the content from
`
<?xml version="1.0" encoding="utf-8" ?>
<ContentView x:Class="MvvM.Views.RedView"
xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:vm="clr-namespace:MvvM.ViewModels"
BindingContext="vm:MainViewModel">
<ContentView.Content>
<Grid Width="*"
Height="*"
BackgroundColor="Red" />
</ContentView.Content>
</ContentView> `
The outcome I want is the ContentView content on the RedPage to be displayed in the mainpage contentview.
is creating an instance of the redpage in the view model MVVM complaint ? (I feel that this would tightly bind view to view model?)
how else can i get the content property on red page into the view model ?(cant bind it and sets elements in it as you can only set content property once)

Answer 1

Ideally you would want the ViewModel not to know anything about the View and vice versa, so from that perspective this is not something you would want.
To overcome this, you would want ViewModel-to-ViewModel navigation. So, you just specify to which ViewModel you want to go and the associated View will be loaded. You can implement this manually, and depending on your chosen implementation you would have some way of resolving a View that is linked to that ViewModel.
One way to do this would be by naming conventions and reflection. This means you name all your pages like:
MyPage
YourPage
OurPage
And all the ViewModels like:
MyPageModel
YourPageModel
OurPageModel
Then with reflection you can simply strip off the "Model" suffix and resolve the page from there. Note that I use the Page and PageModel naming, but of course this works for View and ViewModel as well. After you do, you will still have to account for the navigation to and from this views, is it modal or not, etc.
While you can implement all of this manually it would probably be worth while to look into a MVVM framework. The method I just described is how FreshMvvm does this for instance. But there are other good frameworks out there like Prism, Exrin, MvvmCross, etc.