step-12 Deleting an Item

* Adding a CommandHandler to remove selected items from the current
StringListContext
* Tying that CommandHandler to the
"core.editor.entities.Commands.DELETE" Command, meaning items can be
deleted by pressing the Delete Key or navigating to "Edit/Delete" in the
MenuBar.

src/HelloWorldExtension.as

@@ -8,14 +8,17 @@ package
 	import core.appEx.resources.CommandHandlerFactory;
 	import core.appEx.resources.FileType;
 	import core.appEx.resources.KeyBinding;
+	import core.appEx.validators.ContextSelectionValidator;
 	import core.appEx.validators.ContextValidator;
 	import core.editor.CoreEditor;
 	import core.editor.core.IGlobalViewContainer;
+	import core.editor.entities.Commands;
 	import core.editor.icons.CoreEditorIcons;
 	import core.editor.resources.ActionFactory;
 	import core.editor.resources.EditorFactory;
 
 	import helloWorld.commandHandlers.AddStringCommandHandler;
+	import helloWorld.commandHandlers.DeleteStringCommandHandler;
 	import helloWorld.commandHandlers.MyCommandHandler;
 	import helloWorld.contexts.HelloWorldContext;
 	import helloWorld.contexts.StringListContext;
@@ -28,16 +31,20 @@ package
 			CoreApp.resourceManager.addResource( new FactoryResource( HelloWorldContext, "Hello World" ) );
 			CoreApp.resourceManager.addResource( new EditorFactory( StringListContext, "String List", "strlist", CoreEditorIcons.Text ) );
 
-			CoreApp.resourceManager.addResource( new ActionFactory( IGlobalViewContainer, Commands.MY_COMMAND, "My Action", "myActions", "Actions/myActions", CoreEditorIcons.Resource ) );
-			CoreApp.resourceManager.addResource( new ActionFactory( StringListContext, Commands.ADD_STRING, "Add String", "myActions" ) );
+			CoreApp.resourceManager.addResource( new ActionFactory( IGlobalViewContainer, helloWorld.entities.Commands.MY_COMMAND, "My Action", "myActions", "Actions/myActions", CoreEditorIcons.Resource ) );
+			CoreApp.resourceManager.addResource( new ActionFactory( StringListContext, helloWorld.entities.Commands.ADD_STRING, "Add String", "myActions" ) );
 
-			CoreApp.resourceManager.addResource( new KeyBinding( Commands.MY_COMMAND, 77, KeyModifier.CTRL ) );
+			CoreApp.resourceManager.addResource( new KeyBinding( helloWorld.entities.Commands.MY_COMMAND, 77, KeyModifier.CTRL ) );
 
-			CoreApp.resourceManager.addResource( new CommandHandlerFactory( Commands.MY_COMMAND, MyCommandHandler ) );
-			var commandHandlerFactory:CommandHandlerFactory = new CommandHandlerFactory( Commands.ADD_STRING, AddStringCommandHandler );
+			CoreApp.resourceManager.addResource( new CommandHandlerFactory( helloWorld.entities.Commands.MY_COMMAND, MyCommandHandler ) );
+			var commandHandlerFactory:CommandHandlerFactory = new CommandHandlerFactory( helloWorld.entities.Commands.ADD_STRING, AddStringCommandHandler );
 			commandHandlerFactory.validators.push( new ContextValidator( CoreEditor.contextManager, StringListContext ) );
 			CoreApp.resourceManager.addResource( commandHandlerFactory );
 
+			commandHandlerFactory = new CommandHandlerFactory( core.editor.entities.Commands.DELETE, DeleteStringCommandHandler );
+			commandHandlerFactory.validators.push( new ContextSelectionValidator( CoreEditor.contextManager, StringListContext, String ) );
+			CoreApp.resourceManager.addResource( commandHandlerFactory );
+
 			CoreApp.resourceManager.addResource( new FileType( "String List File", "strlist", CoreEditorIcons.Text ) );
 		}
 	}

src/helloWorld/commandHandlers/DeleteStringCommandHandler.as

@@ -0,0 +1,44 @@
+package helloWorld.commandHandlers
+{
+	import core.app.operations.ChangePropertyOperation;
+	import core.app.operations.RemoveItemOperation;
+	import core.app.operations.UndoableCompoundOperation;
+	import core.appEx.core.commandHandlers.ICommandHandler;
+	import core.editor.CoreEditor;
+
+	import helloWorld.contexts.StringListContext;
+
+	public class DeleteStringCommandHandler implements ICommandHandler
+	{
+		public function DeleteStringCommandHandler()
+		{
+		}
+
+		public function execute(parameters:Object):void
+		{
+			// Grab a reference to the Context we're interested in
+			var context:StringListContext = CoreEditor.contextManager.getLatestContextOfType(StringListContext);
+
+			// Because we could be deleting multiple items, it's worth creating a CompoundOperation.
+			// Even though we add each individual 'RemoveItemOperation' to this compound, removing
+			// all these items will be seen as a single Operation, and so calling the UNDO Command
+			// will add all the items back in one go.
+			var compoundOperation:UndoableCompoundOperation = new UndoableCompoundOperation();
+			compoundOperation.label = "Delete Item(s)";
+
+			var selectedItems:Array = context.selection.source;
+			for each ( var item:String in selectedItems )
+			{
+				var removeItemOperation:RemoveItemOperation = new RemoveItemOperation( item, context.dataProvider );
+				compoundOperation.addOperation(removeItemOperation);
+			}
+
+			// Even though we have removed the items from the dataProvider, the Context's selection still contains them.
+			// We need to make sure that the Context's selection is set to [].
+			var clearSelectionOperation:ChangePropertyOperation = new ChangePropertyOperation( context.selection, "source", [] );
+			compoundOperation.addOperation(clearSelectionOperation);
+
+			context.operationManager.addOperation(compoundOperation);
+		}
+	}
+}