add indent button on press

lib/models/documents/attribute.dart

@@ -116,6 +116,25 @@ class Attribute<T> {
   // "attributes":{"list":"ordered"}
   static Attribute<String> get ol => ListAttribute('ordered');
 
+  // "attributes":{"indent":1"}
+  static Attribute<int> get indentL1 => IndentAttribute(level: 1);
+
+  // "attributes":{"indent":2"}
+  static Attribute<int> get indentL2 => IndentAttribute(level: 2);
+
+  // "attributes":{"indent":3"}
+  static Attribute<int> get indentL3 => IndentAttribute(level: 3);
+
+  static Attribute<int> getIndentLevel(int level) {
+    if (level == 1) {
+      return indentL1;
+    }
+    if (level == 2) {
+      return indentL2;
+    }
+    return indentL3;
+  }
+
   bool get isInline => scope == AttributeScope.INLINE;
 
   bool get isBlockExceptHeader => blockKeysExceptHeader.contains(key);

lib/widgets/toolbar.dart

@@ -638,7 +638,24 @@ class _IndentButtonState extends State<IndentButton> {
       size: 32,
       icon: Icon(widget.icon, size: 18, color: iconColor),
       fillColor: fillColor,
-//      onPressed: ,
+      onPressed: () {
+        final indent = widget.controller.getSelectionStyle().attributes[Attribute.indent.key];
+        if (indent == null) {
+          if (widget.isIncrease) {
+            widget.controller.formatSelection(Attribute.indentL1);
+          }
+          return;
+        }
+        if (indent.value == 1 && !widget.isIncrease) {
+          widget.controller.formatSelection(Attribute.clone(Attribute.indentL1, null));
+          return;
+        }
+        if (widget.isIncrease) {
+          widget.controller.formatSelection(Attribute.getIndentLevel(indent.value + 1));
+          return;
+        }
+        widget.controller.formatSelection(Attribute.getIndentLevel(indent.value - 1));
+      },
     );
   }
 }