Download and display images from sharepoint (Xamarin.Forms PCL) - sharepoint

Question

I'm currently developping an app based on Sharepoint REST API and Microsoft Graph. My app is composed mainly with a Tabbed Page, and on the first tab I want to display News from sharepoint. To do so I want to get images from the news, but using the url of the images as Image Source returns me "forbidden" error. So, I decided to find a way to download those images. I've been looking for a way for few days now, and found nothing...
Does anyone knows a way to do that, or why do I have a forbidden access error ?
I bring some precisions, I'm using an HttpClient to authenticate to the tenant with REST API :
client = new HttpClient();
Uri mUri = new Uri(App.ReturnUri);
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
var data = await DependencyService.Get<IAuthenticator>()
.Authenticate(App.LoginAuthority, App.RestResourceUri, App.ClientId, mUri);
App.AuthenticationResult = data;
client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", App.AuthenticationResult.AccessToken);
From that, I hope someone knows if there is a way to download images, or to access to url in order to use them as Source. I succeeded in retrieving URLs but not displaying images, knowing that I can retrieve any other data from sharepoint
Thanks for any help,

Answer 1

The Image component can use a URL as its data, but that requires images that aren't behind an authentication which is probably the case with your SharePoint images. That is also why it's returning you a Forbidden error.
var webImage = new Image { Aspect = Aspect.AspectFit };
webImage.Source = ImageSource.FromUri(new Uri("https://xamarin.com/content/images/pages/forms/example-app.png"));
To add the authentication layer for SharePoint you would probably be able to implement your own UriImageSource implementation. In the end an ImageSource only needs a Stream object so getting that from either the internet or local storage would work.
To download and store images you will have to write code for both platforms specifically because local storage works differently. Check out this blog post as a starting point combined with your own piece of code.
https://blog.falafel.com/xamarin-forms-storing-and-retrieving-photos/