Sharing the same view model across multiple views about template10 HOT 3 CLOSED

I use a DI container in my case Unity and Locator pattern of MVVM Light.
Register the ViewModel in locator constructor

unityContainer.RegisterType<MainViewModel>(new ContainerControlledLifetimeManager());

new ContainerControlledLifetimeManager() - important part provides one instance

Getting the ViewModel from Locator.

public MainViewModel Main => ServiceLocator.Current.GetInstance<MainViewModel>();

Locator as a resource app.xaml (actually a BootStrapper from T10)

<common:BootStrapper
    x:Class="Dab.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:Dab"
    xmlns:common="using:Template10.Common"
    RequestedTheme="Dark">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="Styles/Custom.xaml"/>
                <ResourceDictionary Source="Styles/DataTemplates.xaml"/>
                <ResourceDictionary Source="Styles/Mdl2.xaml"/>
            </ResourceDictionary.MergedDictionaries>
            <vm:ViewModelLocator x:Key="Locator" xmlns:vm="using:Dab.ViewModel" />
        </ResourceDictionary>
    </Application.Resources>
</common:BootStrapper>

The ViewModel as the DataContext for the Model

<view:MainPageBase
    x:Class="Dab.View.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:view="using:Dab.View"
    mc:Ignorable="d"
    DataContext="{Binding Path=Main, Source={StaticResource Locator}}">
...

from template10.

Let's start with this: typically, in MVVM, there is a one to one for view and viewmodel. This means you would never use the MainPageViewModel, for example, for the DetailsPage even if the data were identical. This is true for several reasons, the least of which is convention.

If what you mean, however, is how to reuse the viewmodel in multiple instances of the same view, then there are several techniques for this. Using inversion of control, a dependency injection container can be instructed to treat a viewmodel as a singleton or to create a new one every time.

In addition, it's worth pointing out that ViewModelBase now has a property in Template10 called SessionState that allows you to persist instances of a viewmodel in memory while you either 1) pass a pointer to it using parameters or 2) use a second instance for a point in time, then return to it.

So, how do you reuse the same viewmodel for multiple types of views? I would recommend that you do not. If you feel there is too much code duplication then use subclassing so you can maintain the overall good practice of one to one viewmodels without much duplication. If you mean reusing a view for a second instance of a view you can instruct your container to handle that and store a reference to the previous viewmodel in SessionState if you want to keep it.

from template10.

I found this question, because I have same problem, same dilemma. What If I have 2 Views (let's say LoginView_Part1, LoginView_Part2). Now I have 2 VM's, one for each View. But I'm thinking to combine the VM's into one VM, because the Login view is only splitted into 2 views. And some data from first View I need in second View. Should I send them as parameter in Navigate method (so I would have 2 VM), or should I create one VM shared for the Views?

from template10.