Setting Up Custom Flutter Theming Library

Step 1: Defining a Theme Extension

Define a theme extension for your application. This should include all the colors, and typography customizable parameters for your application. This is also the reference to know what style properties are customizable.

Approach

We leverage on the ThemeExtension class, an interface tht defines custom additions to a ThemeData object

To use, subclass ThemeExtension, define a number of fields (e.g. Colors and TextStyle), and implement the copyWith and lerp methods. The latter will ensure smooth transitions of properties when switching themes.

Color class - dart:ui library - Dart API

API docs for the Color class from the dart:ui library, for the Dart programming language.

copyWith method - ThemeExtension class - material library - Dart API

API docs for the copyWith method from the ThemeExtension class, for the Dart programming language.

lerp method - ThemeExtension class - material library - Dart API

API docs for the lerp method from the ThemeExtension class, for the Dart programming language.

Extended Class Implemntation Example (SisitechThemeExtension)

class SisitechThemeExtension extends ThemeExtension<SisitechThemeExtension> {
  final Color? backgroundColor;
  final TextStyle? titleStyle;
  final TextStyle? subTitleStyle;
  final TextStyle? descriptionStyle;

  const SisitechThemeExtension(
      {this.backgroundColor,
      this.titleStyle,
      this.subTitleStyle,
      this.descriptionStyle});

  @override
  SisitechThemeExtension copyWith(
      {Color? backgroundColor,
      TextStyle? titleStyle,
      TextStyle? subTitleStyle,
      TextStyle? descriptionStyle}) {
    return SisitechThemeExtension(
      backgroundColor: backgroundColor ?? this.backgroundColor,
      titleStyle: titleStyle ?? this.titleStyle,
      subTitleStyle: subTitleStyle ?? this.subTitleStyle,
      descriptionStyle: descriptionStyle ?? this.descriptionStyle,
    );
  }

  @override
  SisitechThemeExtension lerp(
      ThemeExtension<SisitechThemeExtension>? other, double t) {
    if (other is! SisitechThemeExtension) {
      return this;
    }
    return SisitechThemeExtension(
      backgroundColor: Color.lerp(backgroundColor, other.backgroundColor, t),
      titleStyle: TextStyle.lerp(titleStyle, other.titleStyle, t),
      subTitleStyle: TextStyle.lerp(subTitleStyle, other.subTitleStyle, t),
      descriptionStyle:
          TextStyle.lerp(descriptionStyle, other.descriptionStyle, t),
    );
  }
}

Step 2: Using the ThemeExtension in your Library Components

In order to get an instance of your customTheme anywhere in your code, use:

var sisitechTheme = Theme.of(context).extension<SisitechThemeExtension>();

Step 3: Setting default values for your customTheme

class OnboardingPageWidget extends StatelessWidget {
  final String title;
  final String description;
  final String imagePath;
  final Widget? child;

  const OnboardingPageWidget({
    Key? key,
    required this.title,
    required this.description,
    required this.imagePath,
    this.child,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    if (child != null) {
      return child!;
    }
    var onBoardingTheme =
        Theme.of(context).extension<SisitechOnboardingTheme>();

    // Applying  Default Styles
    var titleStyle =
        onBoardingTheme?.titleStyle ?? Theme.of(context).textTheme.titleMedium;

    return Text(
              title,
              style: titleStyle,
    )
  }
}

library sistch_fl_lib;

import 'package:flutter/material.dart';
import 'package:sistch_fl_lib/theme_extension.dart';

class SistchCustomWidget extends StatelessWidget {
  const SistchCustomWidget({super.key});

  @override
  Widget build(BuildContext context) {
    var sisitechTheme = Theme.of(context).extension<SisitechThemeExtension>();

    return Container(
      padding: const EdgeInsets.all(10),
      decoration: BoxDecoration(
          color: sisitechTheme?.backgroundColor,
          borderRadius: const BorderRadius.all(Radius.circular(10))),
      child: Column(
        children: [
          Text(
            "Title",
            style: sisitechTheme?.titleStyle,
          ),
          Text(
            "Sub Title With Description",
            style: sisitechTheme?.subTitleStyle,
          ),
          Text(
            "Footer",
            style: sisitechTheme?.descriptionStyle,
          ),
        ],
      ),
    );
  }
}

Step 5: Usage in your project

There are two methods:

Define the custom extension as part of your themeData in your main.dart

  darkTheme: ThemeData(
        brightness: Brightness.dark,
        extensions: const <ThemeExtension<dynamic>>[
          SisitechThemeExtension(
            backgroundColor: Colors.black12,
            titleStyle: TextStyle(fontSize: 16, fontWeight: FontWeight.bold),
          ),
        ],
        /* dark theme settings */
      ),
      theme: ThemeData(
        extensions: const <ThemeExtension<dynamic>>[
          SisitechThemeExtension(
            backgroundColor: Colors.blue,
            titleStyle: TextStyle(fontSize: 16, fontWeight: FontWeight.bold),
            ),
        ],
      )

Extend a theme anywhere inside any component using copyWith() which can be used to override the default theme in yout main app.

Theme(
  data: Theme.of(context).copyWith(extensions: [
    SisitechThemeExtension(
      backgroundColor: Colors.brown,
      titleStyle: TextStyle(
          fontSize: 16,
          fontWeight: FontWeight.bold,
          color: Theme.of(context).primaryColor),
    )
  ]),
  child: SistchCustomWidget(),
)

Step 6: Complete Example

import 'package:flutter/material.dart';
import 'package:sistch_fl_lib/sistch_fl_lib.dart';
import 'package:sistch_fl_lib/theme_extension.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      darkTheme: ThemeData(
        brightness: Brightness.dark,
        extensions: const <ThemeExtension<dynamic>>[
          SisitechThemeExtension(
            backgroundColor: Colors.black12,
            titleStyle: TextStyle(fontSize: 16, fontWeight: FontWeight.bold),
          ),
        ],
        /* dark theme settings */
      ),
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.red),
        useMaterial3: true,
        extensions: const <ThemeExtension<dynamic>>[
          // SisitechThemeExtension(
          //   backgroundColor: Colors.blue,
          //   titleStyle: TextStyle(fontSize: 16, fontWeight: FontWeight.bold),
          // ),
        ],
      ),
      // themeMode: ThemeMode.dark,
      themeMode: ThemeMode.light,
      home: Scaffold(
        body: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            const SistchCustomWidget(),
            const SizedBox(
              height: 10,
            ),
            Theme(
                data: Theme.of(context).copyWith(extensions: [
                  SisitechThemeExtension(
                    backgroundColor: Colors.brown,
                    titleStyle: TextStyle(
                        fontSize: 16,
                        fontWeight: FontWeight.bold,
                        color: Theme.of(context).primaryColor),
                  )
                ]),
                child: SistchCustomWidget())
          ],
        ),
      ),
    );
  }
}