Ui: Make the ResultDisplay red on command failure

src/main/java/seedu/address/commons/events/ui/NewResultAvailableEvent.java

@@ -8,9 +8,11 @@
 public class NewResultAvailableEvent extends BaseEvent {
 
     public final String message;
+    public final boolean isError;
 
-    public NewResultAvailableEvent(String message) {
+    public NewResultAvailableEvent(String message, boolean isError) {
         this.message = message;
+        this.isError = isError;
     }
 
     @Override

src/main/java/seedu/address/ui/CommandBox.java

@@ -98,14 +98,14 @@ private void handleCommandInputChanged() {
             // process result of the command
             commandTextField.setText("");
             logger.info("Result: " + commandResult.feedbackToUser);
-            raise(new NewResultAvailableEvent(commandResult.feedbackToUser));
+            raise(new NewResultAvailableEvent(commandResult.feedbackToUser, false));
 
         } catch (CommandException | ParseException e) {
             initHistory();
             // handle command failure
             setStyleToIndicateCommandFailure();
             logger.info("Invalid command: " + commandTextField.getText());
-            raise(new NewResultAvailableEvent(e.getMessage()));
+            raise(new NewResultAvailableEvent(e.getMessage(), true));
         }
     }
 

src/main/java/seedu/address/ui/ResultDisplay.java

@@ -6,6 +6,7 @@
 
 import javafx.beans.property.SimpleStringProperty;
 import javafx.beans.property.StringProperty;
+import javafx.collections.ObservableList;
 import javafx.fxml.FXML;
 import javafx.scene.control.TextArea;
 import javafx.scene.layout.Region;
@@ -17,6 +18,8 @@
  */
 public class ResultDisplay extends UiPart<Region> {
 
+    private static final String ERROR_STYLE_CLASS = "error";
+
     private static final Logger logger = LogsCenter.getLogger(ResultDisplay.class);
     private static final String FXML = "ResultDisplay.fxml";
 
@@ -35,6 +38,32 @@ public ResultDisplay() {
     private void handleNewResultAvailableEvent(NewResultAvailableEvent event) {
         logger.info(LogsCenter.getEventHandlingLogMessage(event));
         displayed.setValue(event.message);
+
+        if (event.isError) {
+            setStyleToIndicateCommandFailure();
+        } else {
+            setStyleToDefault();
+        }
+    }
+
+    /**
+     * Sets the {@code ResultDisplay} style to use the default style.
+     */
+    private void setStyleToDefault() {
+        resultDisplay.getStyleClass().remove(ERROR_STYLE_CLASS);
+    }
+
+    /**
+     * Sets the {@code ResultDisplay} style to indicate a failed command.
+     */
+    private void setStyleToIndicateCommandFailure() {
+        ObservableList<String> styleClass = resultDisplay.getStyleClass();
+
+        if (styleClass.contains(ERROR_STYLE_CLASS)) {
+            return;
+        }
+
+        styleClass.add(ERROR_STYLE_CLASS);
     }
 
 }