Xamarin shell initializing API Controller-CodePudding

I have a running XF App with a lot of database accesses through an Azure API. So far all was running quite well. Due to layout changes I changed to a shell based navigation. I worked through the whole stuff but faces a huge problem. My app.xaml.cs is loding a lot of controllers:

Public partical class App : Xamarin.Forms.Application, INotifyPropertyChanged {public static CampaignController CampaignController { get; private set; }}

And in OnStart()

CampaignController = new CampaignController(new RestService());

In public App() I load the AppShell() as Mainpage

MainPage = new AppShell();

This is my AppShell.xaml

<Shell xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         xmlns:local="clr-namespace:EY365OCMobileApp"
         x:Class="EY365OCMobileApp.AppShell">
<ShellContent Route="WelcomePage" ContentTemplate="{DataTemplate local:WelcomePage}">
    
</ShellContent>

The WelcomePage.xaml.cs looks like this:

protected async override void OnAppearing()
    {
        try
        {
            base.OnAppearing();
            Campaigns = await App.CampaignController.GetCampaignsAsync(364840001);
            CarouselView.ItemsSource = Campaigns;
            BindingContext = Campaigns;
        }
        catch (Exception ex)
        {
            await CreateNewBug.CreateANewBug(ex.Message,"Error in Module "   ex.Source,"\nMessage---\n{ 0}"   ex.Message   "\nInnerException---\n{ 0}"   ex.InnerException   "\nStackTrace---\n{ 0}"   ex.StackTrace   "\nTargetSite---\n{ 0}"   ex.TargetSite);
            ToastOptions toastOptions = Message.ShowMessage("An error was raised and a new bug created in our system.", "error");
            await this.DisplayToastAsync(toastOptions);
        }
    }

This line creates an error:

            Campaigns = await App.CampaignController.GetCampaignsAsync(364840001);

Object reference not set to an instance of an object.

When I debug it seems that the initialization of the app.xaml.cs doesn't run though because it jumps to the AppShell.xaml.cs. When I move the code from App.xaml.cs to AppShell.xaml.cs it brings the same error. Any idea how to initialize the rest-service controller in a shell environment of Xamarin?

CodePudding user response：

You can lazy load the controller whenever you use it .

App

CampaignController _controller;
Public CampaignController controller{ 
   get
   {      
      if(_controller == null) 
      {
          _controller = new CampaignController(new RestService());
      }
      return _controller;  
   }
 }

WelcomePage

Campaigns = await (App.Current as App).controller.GetCampaignsAsync(364840001);
CarouselView.ItemsSource = Campaigns;