flutter-quill/doc/attribute_introduction.md

What is an Attribute

An attribute is a property or characteristic that can be applied to text or a section of text within the editor to change its appearance or behavior. Attributes allow the user to style the text in various ways.

How do attributes work?

An Attribute is applied to selected segments of text in the editor. Each attribute has an identifier and a value that determines how it should be applied to the text. For example, to apply bold to a text, an attribute with the identifier "bold" is used. When text is selected and an attribute is applied, the editor updates the visual representation of the text in real time.

Scope of an Attribute

The attributes has an Scope that limit where start and end the Attribute. The Scope is called as AttributeScope. It has these options to be selected:

enum AttributeScope {
  inline, // just the selected text will apply the attribute (like: bold, italic or strike)
  block, // all the paragraph will apply the attribute (like: Header, Alignment or CodeBlock)
  embeds, // the attr will be taked as a different part of any paragraph or line, working as a block (By now not works as an inline)
  ignore, // the attribute can be applied, but on Retain operations will be ignored
}

How looks a Attribute

The original Attribute class that you need to extend from if you want to create any custom attribute looks like:

class Attribute<T> {
  const Attribute(
    this.key,
    this.scope,
    this.value,
  );

  /// Unique key of this attribute.
  final String key;
  final AttributeScope scope;
  final T value;
}

The key of any Attribute must be unique to avoid any conflict with the default implementations.

Why Attribute class contains a Generic as a value

This is the same reason why we can create Block styles, Inline styles and Custom styles. Having a Generic type value let us define any value as we want to recognize them later and apply it.

Example of an default attribute

Inline Scope:

class BoldAttribute extends Attribute<bool> {
  const BoldAttribute() : super('bold', AttributeScope.inline, true);
}

Block Scope:

class HeaderAttribute extends Attribute<int?> {
  const HeaderAttribute({int? level})
      : super('header', AttributeScope.block, level);
}

If you want to see an example of an embed implementation you can see it here

Example of a Custom Inline Attribute

The Attribute

import 'package:flutter_quill/flutter_quill.dart';

const String highlightKey = 'highlight';
const AttributeScope highlightScope = AttributeScope.inline;

class HighlightAttr extends Attribute<bool?> {
  HighlightAttr(bool? value) : super(highlightKey, highlightScope, value);
}

Where should we add this HighlightAttr?

On QuillEditor or QuillEditorConfigurations doesn't exist a param that let us pass our Attribute implementations. To make this more easy, we can use just customStyleBuilder param from QuillEditorConfigurations, that let us define a function to return a TextStyle. With this, we can define now our HighlightAttr

The editor

QuillEditor.basic(
      controller: controller,
      configurations: QuillEditorConfigurations(
        customStyleBuilder: (Attribute<dynamic> attribute) {
          if (attribute.key.equals(highlightKey)) {
            return TextStyle(color: Colors.black, backgroundColor: Colors.yellow);
          }
          //default paragraph style
          return TextStyle();
        },
      ),
);

Then, it should look as: