Selecting a tile and issuing a command

To do this, the Tiles program needs a selection class, TTilesSelection, to indicate a selected tile, and a command, TChangeColorCommand, that acts on a TTilesSelection. To cause a tile to change color, the following happens:

1. The MouseDown function creates a TTilesSelection on the tile chosen by the user, and instantiates a TChangeColorCommand.
   
2. MouseDown binds the selection and command together and passes them to the document.
   
3. The document executes the command on the selection, causing the model to be modified.
   
4. Finally, changing the model causes the view to be updated--the Presentation framework includes a default notification mechanism that ensures that anytime a command is executed the view is notified. The default behavior of the view is to invoke InvalidateAll, which causes the framework to call DrawContents.
   

When the document executes a command, it also saves the command/selection pair in an undo/redo history log. If the user chooses the Undo feature, the framework retrieves the command and selection, and requests the command to undo its effect on the selection. Similarly, if the user chooses the Redo feature after undoing a command, the framework retrieves the command selection and requests the command to redo the operation.

To implement this behavior, this step shows you how to:

• Create the TTilesSelection class
• Create the TChangeColorCommand class
• Modify TTilesModel::CreateSelection so that it creates a new TTilesSelection as the default selection on the model (currently it creates a "dummy" selection, TGUIModelSelectionFor<TTilesModel>)
• Modify TTilesView::MouseDown so that it creates the TChangeColorCommand/TTilesSelection pair