Xamarin forms TabbedPage View Model called multiple time - mvvm

Question

I have implemented Tabbedpage using ViewModel but my ViewModel constructor call 4 times because I create 4 tabs, I also used prism for ViewModel binding.
Below is a design file
<?xml version="1.0" encoding="UTF-8"?>
<TabbedPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:prism="clr-namespace:Prism.Mvvm;assembly=Prism.Forms"
xmlns:material="clr-namespace:XF.Material.Forms.UI;assembly=XF.Material"
xmlns:ffimageloading="clr-namespace:FFImageLoading.Forms;assembly=FFImageLoading.Forms"
xmlns:ffTransformations="clr-namespace:FFImageLoading.Transformations;assembly=FFImageLoading.Transformations"
prism:ViewModelLocator.AutowireViewModel="True"
xmlns:ios="clr-namespace:Xamarin.Forms.PlatformConfiguration.iOSSpecific;assembly=Xamarin.Forms.Core"
xmlns:extended="clr-namespace:Xamarin.Forms.Extended;assembly=Xamarin.Forms.Extended.InfiniteScrolling"
xmlns:customcontrols="clr-namespace:QuranicQuizzes.CustomControls"
xmlns:local="clr-namespace:QuranicQuizzes.Views" NavigationPage.HasNavigationBar="True"
x:Class="QuranicQuizzes.Views.DashboardPage">
<NavigationPage.TitleView>
<StackLayout Orientation="Horizontal" HorizontalOptions="FillAndExpand">
<Label Text="Dashboard" TextColor="White" HorizontalTextAlignment="Center" HorizontalOptions="CenterAndExpand" VerticalTextAlignment="Center" FontFamily="{StaticResource QuranFontBold}" FontSize="Medium" />
<StackLayout Orientation="Horizontal">
<material:MaterialMenuButton x:Name="Menus" ButtonType="Text" Image="list" TintColor="White" BackgroundColor="Transparent" CornerRadius="24" Choices="{Binding Actions}" MenuSelected="MaterialMenuButton_MenuSelected" />
</StackLayout>
</StackLayout>
</NavigationPage.TitleView>
<local:HomeTabPage/>
<local:QuizzesTabPage/>
<local:LiveGameTabPage/>
<local:AssignmentTabPage/>
</TabbedPage>
Below is my code
public partial class DashboardPage : TabbedPage
{
private DashboardPageViewModel vm;
public DashboardPage()
{
try
{
InitializeComponent();
vm = BindingContext as DashboardPageViewModel;
}
catch (Exception ex)
{
}
}
}
Below is my ViewModel
public class DashboardPageViewModel : ViewModelBase
{
INavigationService _navigationService;
IClientAPI _clientAPI;
Dashboards dashboard;
public DashboardPageViewModel(INavigationService navigationService, IClientAPI clientAPI) : base(navigationService)
{
_navigationService = navigationService;
_clientAPI = clientAPI;
if (CrossConnectivity.Current.IsConnected)
{
var StartDate = DateTime.Now.AddDays(-7).ToString("yyyy-MM-dd");
var Enddate = DateTime.Now.ToString("yyyy-MM-dd");
if (dashboard == null)
{
dashboard = new Dashboards();
getDashboardData(StartDate, Enddate);
}
}
}
}

Answer 1

I see what you're trying to do. You want to initialise your vm instance so that you can access you vm from your view.
Instead of doing this:
vm = BindingContext as DashboardPageViewModel;
what we can do is change the type of the existing BindingContext property by doing this:
public partial class DashboardPage
{
new DashboardPageViewModel BindingContext
{
get => (DashboardPageViewModel) base.BindingContext;
set => base.BindingContext = value;
}
public DashboardPage()
{
InitializeComponent();
}
}
now you can just access BindingContext.DoSomething because its type is now DashboardPageViewModel.
Now that's sorted out, your viewmodel should not be being called 4 times! Something is wrong here. Here is a checklist of things to do that may be causing the constructor being called 4 times as not a lot more info was provided.
Try removing <NavigationPage.TitleView> segment.
Make sure you are navigating to DashboardPage.
Make sure that each individual TabbedPage has it's own viewmodel.
Try removing prism:ViewModelLocator.AutowireViewModel="True"and manually adding the viewmodel to the TabbedPage.
Finally constructors should be able to run very fast and should only be used for assigning variables or instantiation or very quick operations. What you could maybe do is separate the code in your VM:
public class DashboardPageViewModel : ViewModelBase
{
IClientAPI _clientAPI;
Dashboards dashboard;
public DashboardPageViewModel(INavigationService navigationService, IClientAPI clientAPI) : base(navigationService)
{
_clientAPI = clientAPI;
}
public void Init()
{
if (CrossConnectivity.Current.IsConnected)
{
var StartDate = DateTime.Now.AddDays(-7).ToString("yyyy-MM-dd");
var Enddate = DateTime.Now.ToString("yyyy-MM-dd");
if (dashboard == null)
{
dashboard = new Dashboards();
getDashboardData(StartDate, Enddate);
}
}
}
}
and then in your view you could add this method:
protected override void OnBindingContextChanged()
{
base.OnBindingContextChanged();
if(BindingContext == null)
{
return;
}
BindingContext.Init();
}
I hope this really helps you.
NB: All this code was written on the fly and never compiled, there may be some errors.