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Starting with Flutter: A simple guide for Provider

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・5 min read

In this tutorial you will learn the basic of the Provider package, we will build a simple app that will change its main color scheme using Provider.

But first, what is Provider?

What is Provider?

Provider is one of the many state management options when using Flutter. It's one of the first state manager recommended by Flutter itself and one of the simplest. If you're new to state management check out the official pages that describes the different state managers and how/when you should use it https://flutter.dev/docs/development/data-and-backend/state-mgmt/options.

You may ask, what can Provider do? The answer is simple, and the power of the Provider package is in its simplicity:

Providers allow to not only expose a value, but also create/listen/dispose it.

From: https://pub.dev/packages/provider

When you place a Provider widget in your widget tree all the Childs of the Provider will have access to the values exposed by it.
For this tutorial we will use a particular type of Provider: ChangeNotifierProvider. It adds the possibility to listen to changes in the provider and it automatically re-builds the widgets when the provider needs to.

Adding dependencies

The first thing that we need to do is add our dependencies:

dependencies:
  flutter:
    sdk: flutter
  flutter_colorpicker: ^0.4.0
  provider: ^5.0.0
  cupertino_icons: ^1.0.2
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Let's write some code

Then we will need to define our Provider, create a new dart file called theme_provider.dart with this content:

import 'package:flutter/material.dart';

class ThemeProvider extends ChangeNotifier {
  Color mainColor = Colors.blue;

  void changeThemeColor(Color color) {
    mainColor = color;
    notifyListeners();
  }
}
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As you can see our ThemeProvider class is simple but has a lot of things that needs explanation:

  • It extends ChangeNotifier, a class that "provides change notification to its listeners". So you can listen to an instance of a ChangeNotifier and being notified when it changes.
  • It exposes the mainColor value which starts as the material blue.
  • We will use the changeThemeColor function to change the mainColor.
  • When the mainColor changes the class will notify its listener using notifyListeners().

Now we will need to add the Provider widget in our widget tree, in our case all the children of the app should have access to the main color of the application, so we sill need to wrap the App widget in a Provider widget (in particular a ChangeNotifierProvider because we're using a ChangeNotifier). ChangeNotifierProvider needs a create parameter, it's the function that will be called to create our ThemeProvider.

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider<ThemeProvider>(
      create: (context) => ThemeProvider(),
      child: Consumer<ThemeProvider>(
        builder: (context, themeProvider, child) => MaterialApp(
          title: 'Flutter Provider Demo',
          theme: ThemeData(
            primarySwatch: Colors.blue,
            appBarTheme: AppBarTheme(brightness: Brightness.dark),
          ),
          home: MainScaffold(),
        ),
      ),
    );
  }
}
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Now we can access the mainColorof our ThemeProvider, to do so we need to use a Consumer widget.

The Consumerwidget has a builder function that is called whenever the Provider needs to (basically when notifyListeners is called). The builder function has 3 parameters:

  • context: the context of this build
  • themeProvider: our themeProvided declared above. If the provider package doesn't find a parent with the correct Provider type it will throw an Exception.
  • child: you can optionally pass a child widget to the Consumer, that will be passed to the builder function in this parameter. This is here for optimization, if you have a big widget that doesn't need the value exposed by the provider you can pass it as child and use it in the builder function.

Here's an example of a text that change color based on the mainColor of our ThemeProvider:

Consumer<ThemeProvider>(
              builder: (context, themeProvider, child) => Text(
                'Text',
                style: Theme.of(context).textTheme.headline2?.copyWith(
                      color: themeProvider.mainColor,
                    ),
              ),
            )
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Show the color picker

How do we change the main color of our app? We will use the flutter_colorpicker package that we've added to our dependencies at the beginning, showing it as an alert.

This time we will not need to listen to changes to our ThemeProvider class so we can access the provider without a consumer like this.

ThemeProvider themeProvider = Provider.of<ThemeProvider>(context, listen: false);
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and then show the color picker:

  void _showColorPicker(BuildContext context) {
    ThemeProvider themeProvider =
        Provider.of<ThemeProvider>(context, listen: false);
    showDialog(
      context: context,
      builder: (context) => AlertDialog(
        titlePadding: const EdgeInsets.all(0.0),
        contentPadding: const EdgeInsets.all(0.0),
        content: Wrap(
          children: [
            ColorPicker(
              pickerColor: themeProvider.mainColor,
              onColorChanged: (color) => themeProvider.changeThemeColor(color),
            ),
          ],
        ),
        actions: [
          TextButton(
            onPressed: () {
              Navigator.of(context).pop();
            },
            child: Text('Close'),
          )
        ],
      ),
    );
  }
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As you can see when the color changes (onColorChange) we're calling the changeThemeColor of our provider that will change the color and trigger a rebuild with notifyListeners.

Why don't you just call setState?

The obvious question that could come to your mind is "Why don't you just use a StatefulWidget with setState?". Here are some of the reasons:

  • One of the main reasons to prefer Provider over Statefulwidgets is that, using Provider, you will rebuild only the widgets that needs that value (the Consumers) while the other will not be rebuilt. Instead when you call setState the whole build function of the widget will be called.
  • You can use the values exposed by providers in other widgets without passing them. In our case, if you need to push a new Scaffold you can still use the mainColor with the Consumer because it will be a child of the Provider
  • You can have different Providers, that do different things using a MultiProvider
  • The allocation and disposal of objects is managed by the package and the objects are created lazily when they are needed.

Wrap Up

Here's a complete example of a Scaffold that uses the Theme:

import 'package:flutter/material.dart';
import 'package:flutter_colorpicker/flutter_colorpicker.dart';
import 'package:provider/provider.dart';
import 'package:theme_provider/theme_provider.dart';

class MainScaffold extends StatelessWidget {
  const MainScaffold({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: _ThemedAppBar(
        title: Text('Theme provider example'),
        actions: [
          IconButton(
            icon: Icon(Icons.colorize),
            onPressed: () => _showColorPicker(context),
          ),
        ],
      ),
      body: Padding(
        padding: const EdgeInsets.all(20.0),
        child: Column(
          mainAxisAlignment: MainAxisAlignment.spaceAround,
          children: [
            Consumer<ThemeProvider>(
              builder: (context, themeProvider, child) => Text(
                'Text',
                style: Theme.of(context).textTheme.headline2?.copyWith(
                      color: themeProvider.mainColor,
                    ),
              ),
            ),
            Consumer<ThemeProvider>(
              builder: (context, themeProvider, child) => Text(
                'Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur.',
                style: Theme.of(context).textTheme.bodyText2?.copyWith(
                      color: themeProvider.mainColor,
                    ),
              ),
            ),
            Consumer<ThemeProvider>(
              builder: (context, themeProvider, child) => Container(
                height: 100,
                width: 100,
                decoration: BoxDecoration(
                  color: themeProvider.mainColor,
                  borderRadius: BorderRadius.all(
                    Radius.circular(50),
                  ),
                ),
              ),
            ),
            Consumer<ThemeProvider>(
              builder: (context, themeProvider, child) => Slider(
                activeColor: themeProvider.mainColor,
                inactiveColor: themeProvider.mainColor.withOpacity(0.5),
                value: 0,
                onChanged: (newValue) {},
              ),
            ),
            Consumer<ThemeProvider>(
              builder: (context, themeProvider, child) => Switch(
                activeColor: themeProvider.mainColor,
                value: true,
                onChanged: (newValue) {},
              ),
            ),
          ],
        ),
      ),
    );
  }

  void _showColorPicker(BuildContext context) {
    ThemeProvider themeProvider =
        Provider.of<ThemeProvider>(context, listen: false);
    showDialog(
      context: context,
      builder: (context) => AlertDialog(
        titlePadding: const EdgeInsets.all(0.0),
        contentPadding: const EdgeInsets.all(0.0),
        content: Wrap(
          children: [
            ColorPicker(
              pickerColor: themeProvider.mainColor,
              onColorChanged: (color) => themeProvider.changeThemeColor(color),
            ),
          ],
        ),
        actions: [
          TextButton(
            onPressed: () {
              Navigator.of(context).pop();
            },
            child: Text('Close'),
          )
        ],
      ),
    );
  }
}

class _ThemedAppBar extends StatelessWidget with PreferredSizeWidget {
  final Widget? title;
  final List<Widget>? actions;

  final Size preferredSize;

  _ThemedAppBar({
    Key? key,
    this.title,
    this.actions,
  })  : preferredSize = Size.fromHeight(kToolbarHeight),
        super(key: key);

  @override
  Widget build(BuildContext context) {
    return Consumer<ThemeProvider>(
      builder: (context, themeProvider, child) => AppBar(
        title: title,
        actions: actions,
        backgroundColor: themeProvider.mainColor,
      ),
    );
  }
}

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To build an AppBar with the backgroundColor as our mainColor we had to build a custom widget (the _ThemedAppBar that you see at the bottom) because the Scaffold AppBar needs to be a PreferredSizeWidget so we couldn't use the usual Consumer.

Wrapping all up

Article written by the awesome Lorenzo

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