Implementação de SHARE no xamarin forms

IShare.cs

//interface do projeto core
public interface IShare
{
    Task Show(string title, string message, string filePath);
}

ShareAndroid.cs

//Classe do projeto android
[assembly: Xamarin.Forms.Dependency(typeof(Share))]
namespace FindenApp.Droid.Helpers
{
    public class Share : FindenApp.Helpers.IShare
    {
        private readonly Context _context;
        public Share()
        {
            //_context = Android.App.Application.Context;
            _context = Application.Context;
        }

        public Task Show(string title, string message, string filePath)
        {
            var extension = filePath.Substring(filePath.LastIndexOf(".") + 1).ToLower();
            var contentType = string.Empty;

            // verifica a extensão do arquivo
            switch (extension)
            {
                case "pdf":
                    contentType = "application/pdf";
                    break;
                case "png":
                    contentType = "image/png";
                    break;
                default:
                    contentType = "text/plain";
                    break;
            }

            var intent = new Intent(Intent.ActionSend);
            intent.SetType(contentType);
            //FILE
            //intent.PutExtra(Intent.ExtraStream, Uri.Parse("file://" + filePath));
            //URL
            intent.AddFlags(ActivityFlags.ClearWhenTaskReset);
            //intent.PutExtra(Intent.ExtraText, string.Empty);
            intent.PutExtra(Intent.ExtraText, filePath);
            intent.PutExtra(Intent.ExtraSubject, message);

            var chooserIntent = Intent.CreateChooser(intent, title ?? string.Empty);
            chooserIntent.SetFlags(ActivityFlags.ClearTop);
            chooserIntent.SetFlags(ActivityFlags.NewTask);
            _context.StartActivity(chooserIntent);

            return Task.FromResult(true);
        }
    }
}

ShareiOs.cs

//Classe do projeto iOs
[assembly: Dependency(typeof(Share))]
namespace FindenApp.iOS.Helpers
{
    public class Share : FindenApp.Helpers.IShare
    {
        // MUST BE CALLED FROM THE UI THREAD
        public async Task Show(string title, string message, string filePath)
        {
            var items = new NSObject[] { NSObject.FromObject(title), NSUrl.FromFilename(filePath) };
            var activityController = new UIActivityViewController(items, null);
            var vc = GetVisibleViewController();

            NSString[] excludedActivityTypes = null;

            if (excludedActivityTypes != null && excludedActivityTypes.Length > 0)
                activityController.ExcludedActivityTypes = excludedActivityTypes;

            if (UIDevice.CurrentDevice.CheckSystemVersion(8, 0))
            {
                if (activityController.PopoverPresentationController != null)
                {
                    activityController.PopoverPresentationController.SourceView = vc.View;
                }
            }
            await vc.PresentViewControllerAsync(activityController, true);
        }

        UIViewController GetVisibleViewController()
        {
            var rootController = UIApplication.SharedApplication.KeyWindow.RootViewController;

            if (rootController.PresentedViewController == null)
                return rootController;

            if (rootController.PresentedViewController is UINavigationController)
            {
                return ((UINavigationController)rootController.PresentedViewController).TopViewController;
            }

            if (rootController.PresentedViewController is UITabBarController)
            {
                return ((UITabBarController)rootController.PresentedViewController).SelectedViewController;
            }

            return rootController.PresentedViewController;
        }
    }
}