The Adapter Pattern

"The Adapter Pattern converts the interface of a class into another interface the clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces." - Head First Design Patterns

Head First Design Patterns (Head First) by Elisabeth Freeman, Eric Freeman, Bert Bates, Kathy Sierra

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If you're trying to build a presentation layer that is platform agnostic, you might turn to adapters for simple UI controls. For example in ASP.NET there is a drop down list control, and in Win Forms there is the Combo Box control. A quick an easy adapter to the two controls might look like:

  public interface IDropDownListAdapter 
  {
      void BindTo( IEnumerable<IDropDownListItem> pairs );
      IDropDownListItem SelectedItem { get; }
  }

Each drop down list item might look like:

  public interface IDropDownListItem 
  {
      string Text { get; }
      string Value { get; }
  }

A concrete implementation for a Win Forms application might look like:

  public class DesktopDropDownList : IDropDownListAdapter 
  {
      public DesktopDropDownList( ComboBox dropDown ) 
      {
          _dropDown = dropDown;
          _pairs = new Dictionary<string, IDropDownListItem>( );
      }

      public void BindTo( IEnumerable< IDropDownListItem > pairs ) 
      {
          if ( pairs != null ) 
          {
              _pairs = new Dictionary< string, IDropDownListItem >( );

              foreach ( IDropDownListItem pair in pairs ) 
              {
                  _dropDown.Items.Add( pair.Text );
                  _pairs.Add( pair.Text, pair );
              }
              _dropDown.SelectedIndex = 0;
          }
      }

      public IDropDownListItem SelectedItem 
      {
          get { return !string.IsNullOrEmpty( _dropDown.Text ) ? _pairs[ _dropDown.Text ] : null; }
      }

      private ComboBox _dropDown;
      private IDictionary< string, IDropDownListItem > _pairs;
  }

A concrete implementation for a ASP.NET application might look like:

  public class WebDropDownList : IDropDownListAdapter 
  {
      public WebDropDownList( DropDownList dropDown ) 
      {
          _dropDown = dropDown;
      }

      public void BindTo( IEnumerable< IDropDownListItem > pairs ) 
      {
          if ( pairs != null ) 
          {
              foreach ( IDropDownListItem pair in pairs ) 
              {
                  _dropDown.Items.Add( new ListItem( pair.Text, pair.Value ) );
              }
          }
      }

      public IDropDownListItem SelectedItem 
      {
          get { return new DropDownListItem( _dropDown.SelectedItem.Text, _dropDown.SelectedItem.Value ); }
      }

      private DropDownList _dropDown;
  }

And voila... we can write a presentation layer that binds data to an IDropDownListAdapter, without having to be specific to WinForms, ASP.NET, WPF, Silverlight etc.

  public Presenter( IView view ) 
  {
      _view = view;
  }

  public void Initialize( ) 
  {
      IList< IDropDownListItem > items = new List< IDropDownListItem >( );
      items.Add( new DropDownListItem( "Yes", "1" ) );
      items.Add( new DropDownListItem( "No", "2" ) );
      _view.AreYouHappy.BindTo( items );
  }

DesignPatterns.Adapter.zip (4.56 KB)