MarkdownToolbar: Remove old block tag

If a text starts with '# ' and we click on the '- ' button, it should
remove the old tag '#' and replace it with '- '. Before this it would
have done something like this '- #'

lib/widgets/markdown_toolbar.dart

@@ -85,6 +85,22 @@ class MarkdownToolBar extends StatelessWidget {
   }
 }
 
+final _allowedBlockTags = [
+  '# ',
+  '## ',
+  '### ',
+  '#### ',
+  '##### ',
+  '###### ',
+  '- ',
+  '* ',
+];
+
+final _allowedBlockRegExps = [
+  RegExp('- \[[xX ]\] '),
+  RegExp('\d+\. '),
+];
+
 TextEditingValue modifyCurrentLine(
   TextEditingValue textEditingValue,
   String char,
@@ -112,13 +128,46 @@ TextEditingValue modifyCurrentLine(
     lineEndPos = text.length;
   }
 
+  // Check if line already has a block tag
+  for (var blockTag in _allowedBlockTags) {
+    if (text.startsWith(blockTag, lineStartPos)) {
+      var newVal = _removeFromLine(text, cursorPos, lineStartPos, blockTag);
+      if (blockTag == char) {
+        return newVal;
+      }
+      return modifyCurrentLine(newVal, char);
+    }
+  }
+
+  for (var blockTagRegExp in _allowedBlockRegExps) {
+    var match = blockTagRegExp.matchAsPrefix(text, lineStartPos);
+    if (match != null) {
+      var blockTag = match.group(0);
+      var newVal = _removeFromLine(text, cursorPos, lineStartPos, blockTag);
+      if (blockTag == char) {
+        return newVal;
+      }
+      return modifyCurrentLine(newVal, char);
+    }
+  }
+
   //print('Line Start: $lineStartPos');
   //print('Line End: $lineEndPos');
   //print('Line: ${text.substring(lineStartPos, lineEndPos)}');
 
-  // Check if already present
+  return TextEditingValue(
-  if (text.startsWith(char, lineStartPos)) {
+    text: text.replaceRange(lineStartPos, lineStartPos, char),
-    //print('Removing `$char`');
+    selection: TextSelection.collapsed(offset: cursorPos + char.length),
+  );
+}
+
+TextEditingValue _removeFromLine(
+  String text,
+  int cursorPos,
+  int lineStartPos,
+  String char,
+) {
+//print('Removing `$char`');
   var endOffset = cursorPos;
   //print("End Offset: $endOffset");
   if (endOffset > lineStartPos) {
@@ -133,12 +182,6 @@ TextEditingValue modifyCurrentLine(
     text: text.replaceFirst(char, '', lineStartPos),
     selection: TextSelection.collapsed(offset: endOffset),
   );
-  }
-
-  return TextEditingValue(
-    text: text.replaceRange(lineStartPos, lineStartPos, char),
-    selection: TextSelection.collapsed(offset: cursorPos + char.length),
-  );
 }
 
 TextEditingValue modifyCurrentWord(

test/markdown_toolbar_test.dart

@@ -104,6 +104,26 @@ void main() {
     );
   });
 
+  test("Replaces h1 with h2", () {
+    _testLine(
+      before: 'Hi\n# Hello Darkness\nFire',
+      beforeOffset: 4,
+      after: 'Hi\n## Hello Darkness\nFire',
+      afterOffset: 6,
+      char: '## ',
+    );
+  });
+
+  test("Replaces h2 with list", () {
+    _testLine(
+      before: 'Hi\n## Hello Darkness\nFire',
+      beforeOffset: 5,
+      after: 'Hi\n- Hello Darkness\nFire',
+      afterOffset: 5,
+      char: '- ',
+    );
+  });
+
   //
   // Word based
   //
@@ -252,5 +272,5 @@ void main() {
     _testPrevWord(text, 0, 0);
   });
 
-  // Test that clicking on h2 removes h1 and then adds h2
+  // Test that if some text is selected then it should be modified
 }