Xamarin Forms: Open downloaded file in default Android/iOS application - xamarin.forms

Question

I am using Xamarin Forms. I would like to download jpg file (it is done) and then open that jpg in default application on Android/iOS (opening photo browser with this photo). Of course photo is single example, I would like to open any file in default application.
I found several solutions native-only but my application is designed to be cross-platform. I though that I can use Launcher from Xamarin.Essentials package but apparently I can't.
How to achieve this?

Answer 1

You can have a try with Xamarin.Essentials: Launcher:
var fn = "File.txt";
var file = Path.Combine(FileSystem.CacheDirectory, fn);
File.WriteAllText(file, "Hello World");
await Launcher.OpenAsync(new OpenFileRequest
{
File = new ReadOnlyFile(file)
});

Answer 2

I found several solutions native-only
Opening something in another app is quite close to the system for a mobile application and there are some things to consider, which dependend on the platform. Usually, mobile apps run in a sandbox with very limited access to the surrounding system. Particularly this means that, if you downloaded a file to the sandbox of your app, other apps (which native viewers are), aren't allowed to access the file.
On Android, you can copy the file to a shared space (see Application.Context.GetExternalFilesDir(null)) and then open it. This might be possible with Essentials, but I'm not quite sure, but since we're on the Android platform anyway now, you could create an intent now anyway.
On iOS you create controllers from within your app (for example the QLPreviewController to preview the file) that may access items in your sandbox. Depending on the type of controller (e.g. UIActivityViewController) they may open other apps.
How to use this platform-independently?
Since you are programming a platform independent app, you'll have to take care that the correct class is called to the platform dependent work. There are several options how you can achieve this
Use the DependencyService
Use a real dependency injection framework
Use an abstract base class with initialization in the platform dependent projects
DependencyService
To use the Xamarin.Forms DependencyService you need two things
An interface for the functionality you'd like to implement
One implementation per platform
Assuming you hvae a simple interface to share a file
public IShareFile
{
void ShareFile(string fileName);
}
you can implement an implementation of this interface on each platform and add the DependencyAttribute to the assembly. e.g. for iOS:
[assembly: Dependency(typeof(MyApp.iOS.DeviceOrientationService))]
namespace MyApp.iOS
{
public class ShareFile : IShareFile
{
public void Share(string fileName)
{
// implementation goes here
}
}
}
The general scaffold is the same for Android, albeit the implementation differs.
Using a real dependency injection framework
Basically it's pretty much the same. You can skip the DependencyAttribute, though. In order to make the implementation available you'll have to get hold of the DI container from your platform specific code, which might be tricky. This might be an overshoot for a single dependency, but if you're using a DI container anyway and there are X dependencies, it might be worth the effort.
Using an abstract base class
Add an abstract base class to your project
public abstract class ShareFile
{
public static ShareFile Instance { get; protected set; }
public abstract void Share(string fileName);
}
and in your implementation in the platform specific project, you add an Init() method
internal class ShareFileImpl : ShareFile
{
public static void Init()
{
ShareFile.Instance = new ShareFileImpl();
}
public void Share(string fileName)
{
// implementation goes here
}
}
This init method must be called from your platform specific code. Most likely during initialization. The implementation can then be accessed via its abstraction from your platform independent code (of course you'll see only the abstraction, public methods added to ShareFileImpl won't be visible from your platform independent code).
ShareFile.Instance.Share(fileName);
A combination of the abstract class approach and dependency injection is also conceivable. When registering your classes in the DI framework, you could register the platform instance like
container.RegisterInstance<ShareFile>(ShareFile.Instance);
This way you can make use of the DI container features (e.g. constructor injection), while keeping the hassles of using the DI container from your platform specific project away from you. The drawback is, that you'll still have to call ShareFileImpl.Init() from your platform specfic code.