Fetching data from the internet after a page opens in Xamarin forms

Issue

I am having some performance issues with my code. I am working with basic MVVM in Xamarin forms project, and I want to fetch data from the internet when someone navigates to another page. Below is what I have done;

This is how I am navigating to another page via Command; (To be honest I don't really know if this method of navigation has some performance penalties)

if (Application.Current.MainPage.Navigation.NavigationStack.Last().GetType() != typeof(SubcategoryPage))
{
   await Application.Current.MainPage.Navigation.PushAsync(new SubcategoryPage());
}

Here, I am passing Id of a particular category to get its corresponding subcategories

public SubcategoryPage(int id)
{
    InitializeComponent();
    this.BindingContext  = new SubcategoryPageViewModel(id);
}

In the constructor of the SubcategoryPageViewModel, I used the Id to fetch the data online like so;

public SubcategoryPageViewModel(int id)
{
    SubcategoryLoader(id);
}

Below is the method that is fetching the data from the internet through my DataService class. The code below work well in getting the data from the internet;

private async Task SubcategoryLoader(int id)
{
   try
   {
       var subCategories = await SubcategoryDataService.GetSubcategories(id);

       if (subCategories.code == 1) // StatusCode = Successful 
       {
            SubCategories = subCategories.document;
       }
       else
       {
           await Application.Current.MainPage.DisplayAlert("Oops!","Something went wrong", "Ok");
       }
   }
   catch (Exception ex)
   {
       await Application.Current.MainPage.DisplayAlert("Oops!", ex.Message, "Ok");
   }
}

Now my problem is that the SubcategoryPage doesn't open until the online service is over which leads to a serious lags. So what I want to have happen is to open the SubcategoryPage before the internet services happen.

Please any assistance is appreciated.

Solution

• Move it to OnAppearing method .

• Add a simple check .

protected override void OnAppearing()
{
    base.OnAppearing();

    if(BindingContext == null)
    {
         BindingContext = new SubcategoryPageViewModel(id);
    }            
}

Answered By - ColeX - MSFT