Fix Undo throw Offset cannot be negative

lib/models/documents/document.dart

@@ -155,14 +155,18 @@ class Document {
     _history.handleDocChange(change);
   }
 
-  void undo() {
-    _history.undo(this);
+  bool undo() {
+    return _history.undo(this);
   }
 
-  void redo() {
-    _history.redo(this);
+  bool redo() {
+    return _history.redo(this);
   }
 
+  get hasUndo => _history.hasUndo;
+
+  get hasRedo => _history.hasRedo;
+
   static Delta _transform(Delta delta) {
     Delta res = Delta();
     for (Operation op in delta.toList()) {

lib/models/documents/history.dart

@@ -6,6 +6,10 @@ import 'document.dart';
 class History {
   final HistoryStack stack = HistoryStack.empty();
 
+  get hasUndo => stack.undo.isNotEmpty;
+
+  get hasRedo => stack.redo.isNotEmpty;
+
   /// used for disable redo or undo function
   bool ignoreChange;
 
@@ -80,8 +84,10 @@ class History {
     }
   }
 
-  void _change(Document doc, List<Delta> source, List<Delta> dest) {
-    if (source.length == 0) return;
+  bool _change(Document doc, List<Delta> source, List<Delta> dest) {
+    if (source.length == 0) {
+      return false;
+    }
     Delta delta = source.removeLast();
     Delta base = doc.toDelta();
     Delta inverseDelta = delta.invert(base);
@@ -90,14 +96,15 @@ class History {
     this.ignoreChange = true;
     doc.compose(delta, ChangeSource.LOCAL);
     this.ignoreChange = false;
+    return true;
   }
 
-  void undo(Document doc) {
-    _change(doc, stack.undo, stack.redo);
+  bool undo(Document doc) {
+    return _change(doc, stack.undo, stack.redo);
   }
 
-  void redo(Document doc) {
-    _change(doc, stack.redo, stack.undo);
+  bool redo(Document doc) {
+    return _change(doc, stack.redo, stack.undo);
   }
 }
 

lib/widgets/controller.dart

@@ -43,14 +43,20 @@ class QuillController extends ChangeNotifier {
   }
 
   void undo() {
-    document.undo();
+    if (document.undo()) {
       notifyListeners();
     }
+  }
 
   void redo() {
-    document.redo();
+    if (document.redo()) {
       notifyListeners();
     }
+  }
+
+  get hasUndo => document.hasUndo;
+
+  get hasRedo => document.hasRedo;
 
   replaceText(int index, int len, Object data, TextSelection textSelection) {
     assert(data is String || data is Embeddable);

lib/widgets/toolbar.dart

@@ -537,28 +537,57 @@ class HistoryButton extends StatefulWidget {
 }
 
 class _HistoryButtonState extends State<HistoryButton> {
+  Color _iconColor;
+  ThemeData theme;
+
   @override
   Widget build(BuildContext context) {
-    final theme = Theme.of(context);
-    final iconColor = theme.iconTheme.color;
+    theme = Theme.of(context);
+    _setIconColor();
+
     final fillColor = theme.canvasColor;
+    widget.controller.changes.listen((event) async {
+      _setIconColor();
+    });
     return QuillIconButton(
       highlightElevation: 0,
       hoverElevation: 0,
       size: 32,
-      icon: Icon(widget.icon, size: 18, color: iconColor),
+      icon: Icon(widget.icon, size: 18, color: _iconColor),
       fillColor: fillColor,
       onPressed: _changeHistory,
     );
   }
 
+  void _setIconColor() {
+    if (widget.undo) {
+      setState(() {
+        _iconColor = widget.controller.hasUndo
+            ? theme.iconTheme.color
+            : theme.disabledColor;
+      });
+    } else {
+      setState(() {
+        _iconColor = widget.controller.hasRedo
+            ? theme.iconTheme.color
+            : theme.disabledColor;
+      });
+    }
+  }
+
   void _changeHistory() {
     if (widget.undo) {
+      if (widget.controller.hasUndo) {
         widget.controller.undo();
+      }
     } else {
+      if (widget.controller.hasRedo) {
         widget.controller.redo();
       }
     }
+
+    _setIconColor();
+  }
 }
 
 class IndentButton extends StatefulWidget {