fix undo && redo selection

packages/notus/lib/notus.dart

 library notus;
 
 export 'src/document.dart';
+export 'src/history.dart';
 export 'src/document/attributes.dart';
 export 'src/document/block.dart';
 export 'src/document/leaf.dart';

packages/notus/lib/src/document.dart

   NotusDocument.fromDelta(Delta delta)
       : assert(delta != null),
         _heuristics = NotusHeuristics.fallback,
+        _history = NotusHistory(),
         _delta = delta {
     _loadDocument(_delta);
   }
       throw StateError('Compose produced inconsistent results. '
           'This is likely due to a bug in the library. Tried to compose change $change from $source.');
     }
-    _controller.add(NotusChange(before, change, source));
+    // _controller.add(NotusChange(before, change, source));
+    final notusChange = NotusChange(before, change, source);
+    _controller.add(notusChange);
+    if (history != true) _history?.handleDocChange(notusChange);
   }
 
   //
     }
   }
 
-  void undo() {
-    assert(_history != null);
-    _history?.undo(this);
+  Delta undo() {
+    return _history?.undo(this);
   }
 
-  void redo() {
-    assert(_history != null);
-    _history?.redo(this);
+  Delta redo() {
+    return _history?.redo(this);
   }
 }

packages/notus/lib/src/history.dart

 
   NotusHistory(
       {this.ignoreChange = false,
-      this.interval = 400,
+      this.interval = 800,
       this.maxStack = 100,
       this.userOnly = false,
       this.lastRecorded = 0});
     }
   }
 
-  void _change(NotusDocument doc, List<Delta> source, List<Delta> dest) {
-    if (source.length == 0) return;
+  Delta _change(NotusDocument doc, List<Delta> source, List<Delta> dest) {
+    if (source.length == 0) return Delta();
     Delta delta = source.removeLast();
     Delta base = doc.toDelta();
     Delta inverseDelta = delta.invert(base);
     this.ignoreChange = true;
     doc.compose(delta, ChangeSource.local, history: true);
     this.ignoreChange = false;
+    return delta;
   }
 
-  void undo(NotusDocument doc) {
-    _change(doc, stack.undo, stack.redo);
+  Delta undo(NotusDocument doc) {
+    return _change(doc, stack.undo, stack.redo);
   }
 
-  void redo(NotusDocument doc) {
-    _change(doc, stack.redo, stack.undo);
+  Delta redo(NotusDocument doc) {
+    return _change(doc, stack.redo, stack.undo);
   }
 }
 

packages/zefyr/lib/src/widgets/buttons.dart

       return () => editor.closeKeyboard();
     } else if (action == ZefyrToolbarAction.showKeyboard) {
       return () => toolbar.closeOverlay();
+    } else if (action == ZefyrToolbarAction.redo && editor.controller.document.history.stack.redo.isNotEmpty) {
+      return () => editor.redo();
+    } else if (action == ZefyrToolbarAction.undo && editor.controller.document.history.stack.undo.isNotEmpty) {
+      return () => editor.undo();
     }
 
     return null;

packages/zefyr/lib/src/widgets/controller.dart

   }
 
   void undo() {
-    document.undo();
-    updateSelection(TextSelection.collapsed(offset: document.length));
+    Delta change = document.undo();
+    updateSelection(TextSelection.collapsed(offset: change.transformPosition(_selection.extentOffset, force: false)));
   }
 
   void redo() {
-    document.redo();
-    updateSelection(TextSelection.collapsed(offset: document.length));
+    Delta change = document.redo();
+    updateSelection(TextSelection.collapsed(offset: change.transformPosition(_selection.extentOffset, force: true)));
   }
   
   /// Returns style of specified text range.