Added noOpUndoManager (#881)

Author: Jugen

richtextfx/src/main/java/org/fxmisc/richtext/util/UndoUtils.java

@@ -7,7 +7,9 @@
 import org.fxmisc.richtext.model.TextChange;
 import org.fxmisc.undo.UndoManager;
 import org.fxmisc.undo.UndoManagerFactory;
-import org.reactfx.EventStream;
+import org.reactfx.value.Val;
+
+import javafx.beans.value.ObservableBooleanValue;
 
 import java.time.Duration;
 import java.util.List;
@@ -35,6 +37,37 @@ public static <PS, SEG, S> UndoManager defaultUndoManager(GenericStyledArea<PS,
                 : plainTextUndoManager(area);
     }
 
+    /**
+     * Constructs an UndoManager with no history
+     */
+    public static UndoManager noOpUndoManager() {
+        return new UndoManager() {
+
+            private final Val<Boolean> alwaysFalse = Val.constant(false);
+
+            @Override public boolean undo() { return false; }
+            @Override public boolean redo() { return false; }
+            @Override public Val<Boolean> undoAvailableProperty() { return alwaysFalse; }
+            @Override public boolean isUndoAvailable() { return false; }
+            @Override public Val<Boolean> redoAvailableProperty() { return alwaysFalse; }
+            @Override public boolean isRedoAvailable() { return false; }
+            @Override public boolean isPerformingAction() { return false; }
+            @Override public boolean isAtMarkedPosition() { return false; }
+
+            // not sure whether these may throw NPEs at some point
+            @Override public Val nextUndoProperty() { return null; }
+            @Override public Val nextRedoProperty() { return null; }
+            @Override public ObservableBooleanValue performingActionProperty() { return null; }
+            @Override public UndoPosition getCurrentPosition() { return null; }
+            @Override public ObservableBooleanValue atMarkedPositionProperty() { return null; }
+
+            // ignore these
+            @Override public void preventMerge() { }
+            @Override public void forgetHistory() { }
+            @Override public void close() { }
+        };
+    }
+
     /* ********************************************************************** *
      *                                                                        *
      * UndoManager Factory Methods                                            *