Building the ExtendedContentPage

Add a new folder called UI inside the FileStorage project, add in a new file called ExtendedContentPage.cs, and implement the following:

public class ExtendedContentPage : ContentPage
  {
    #region Private Properties
      private ViewModelBase _model;
    #endregion
    #region Constructors
      public ExtendedContentPage(ViewModelBase model)
      {
        _model = model;
        _model.Alert -= HandleAlert;
        _model.Alert += HandleAlert;
      }
    #endregion
    #region Private Methods
      private async void HandleAlert(object sender, string message) 
      {
        await DisplayAlert("FileStorage", message, "OK");
      }
    #endregion 
  }

The _model property is used to reference the view-model of each page as every view-model inherits the ViewModelBase class. When the page is created, we register the HandleAlert function to the view-model Alert event. Every time this function is called, it will call the DisplayAlert...