c# - Passing data between view in xamarin forms using mvvm

Question

I'm trying to navigate between pages and bind data at same time.

This is what I have tried :

public ICommand GetIdeasCommand
{
    get
    {
        return new Command(async () =>
        {
            Ideas = await _apiServices.GetIdeasAsync();
            await Application.Current.MainPage.Navigation.PushAsync(new IdeasSinglePage(Ideas));
        });
    }
}

It is supposed the Ideas is a list of arrays I get from the json. But this approach is not helping me since I get a blank page. Also if I call this function inside the page everything is fine. This post gave me an idea : How to pass a parameter from one Page to another Page in Xamarin.Forms?

My view :

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         x:Class="Ideas.Pages.IdeasSinglePage"
         xmlns:vm="clr-namespace:Ideas.ViewModel;assembly=Ideas"
          Title="My Page">

<ContentPage.BindingContext>
    <vm:IdeasViewModel/>
</ContentPage.BindingContext>

        <ListView.ItemTemplate>
            <DataTemplate>
                <ViewCell>
                    <StackLayout Padding="20, 10">
                        <Label Text="{Binding Ideas}"
                               FontSize="12"
                               TextColor="RoyalBlue"/>
                    </StackLayout>
                </ViewCell>
            </DataTemplate>
        </ListView.ItemTemplate>
    </ListView>

Code behind :

public partial class IdeasSinglePage : ContentPage
{
    public IdeasSinglePage(List<Models.Ideas> ideas)
    {

      InitializeComponent();
      this.BindingContext = new IdeasSinglePage(ideas); //the app breaks here
    }
}

Thanks.

Answer 1

Your problem is obvious, you are passing the data to the ContentPage but you do nothing with it. Generally speaking passing a parameter from one ViewModel to another ViewModel is a very simple problem.

Here is an illustration without XAML:

public class MyFirstPage : ContentPage
{
  public MyFirstPage()
  {
    this.BindingContext = new MyFirstPageViewModel();
  }
}

public class MyFirstPageViewModel : INotifyPorpertyChanged
{
  public ICommand<List<string>> DownloadDataCmd { get; }

  public MyFirstPageViewModel()
  {
    DownloadDataCmd = new Command<List<string>>(async () => {
        var data = await dataService.DownloadData();
        await navService.PushAsync(new MySecondPage(data));
    });
  }
}

public class MySecondPage : ContentPage
{
  public MySecondPage(List<string> downloadedData)
  {
    this.BindingContext = new MySecondPageViewModel(downloadedData);
  }
}

public class MySecondPageViewModel : INotifyPropertyChanged
{
  public List<string> Data { get; }
  public MySecondPageViewModel(List<string> downloadedData)
  {
     // Do whatever is needed with the data
     Data = downloadedData;
  }
}

Now, looking at this solution there are few questions:
1. Why not to download the data directly on the second page?
2. Why not to store the data in cache or db if you need it across the app?