adjust cursor after undo/redo

lib/models/documents/document.dart

@@ -155,11 +155,11 @@ class Document {
     _history.handleDocChange(change);
   }
 
-  bool undo() {
+  Tuple2 undo() {
     return _history.undo(this);
   }
 
-  bool redo() {
+  Tuple2 redo() {
     return _history.redo(this);
   }
 

lib/models/documents/history.dart

@@ -84,11 +84,22 @@ class History {
     }
   }
 
-  bool _change(Document doc, List<Delta> source, List<Delta> dest) {
+  Tuple2 _change(Document doc, List<Delta> source, List<Delta> dest) {
     if (source.length == 0) {
-      return false;
+      return new Tuple2(false, 0);
     }
     Delta delta = source.removeLast();
+    // look for insert or delete
+    int len = 0;
+    List<Operation> ops = delta.toList();
+    for(var i = 0; i < ops.length; i++){
+      if (ops[i].key == Operation.insertKey){
+        len = ops[i].length;
+      }
+      else if (ops[i].key == Operation.deleteKey){
+        len = ops[i].length * -1;
+      }
+    }
     Delta base = Delta.from(doc.toDelta());
     Delta inverseDelta = delta.invert(base);
     dest.add(inverseDelta);
@@ -96,14 +107,14 @@ class History {
     this.ignoreChange = true;
     doc.compose(delta, ChangeSource.LOCAL);
     this.ignoreChange = false;
-    return true;
+    return new Tuple2(true, len);
   }
 
-  bool undo(Document doc) {
+  Tuple2 undo(Document doc) {
     return _change(doc, stack.undo, stack.redo);
   }
 
-  bool redo(Document doc) {
+  Tuple2 redo(Document doc) {
     return _change(doc, stack.redo, stack.undo);
   }
 }

lib/widgets/controller.dart

@@ -43,16 +43,20 @@ class QuillController extends ChangeNotifier {
   }
 
   void undo() {
-    if (document.undo()) {
-      _handleHistoryChange();
+    Tuple2 tup = document.undo();
+    if (tup.item1){
+      _handleHistoryChange(tup.item2);
     }
   }
 
-  void _handleHistoryChange() {
-    if (this.selection.extentOffset >= document.length) {
-      // cursor exceeds the length of document, position it in the end
+  void _handleHistoryChange(int len) {
+    if (len != 0) {
+    // if (this.selection.extentOffset >= document.length) {
+      // // cursor exceeds the length of document, position it in the end
+      // updateSelection(
+      //     TextSelection.collapsed(offset: document.length), ChangeSource.LOCAL);
       updateSelection(
-          TextSelection.collapsed(offset: document.length), ChangeSource.LOCAL);
+          TextSelection.collapsed(offset: this.selection.baseOffset + len), ChangeSource.LOCAL);
     } else {
       // no need to move cursor
       notifyListeners();
@@ -60,8 +64,9 @@ class QuillController extends ChangeNotifier {
   }
 
   void redo() {
-    if (document.redo()) {
-      _handleHistoryChange();
+    Tuple2 tup = document.redo();
+    if (tup.item1){
+      _handleHistoryChange(tup.item2);
     }
   }