skeletonizer

---

• Introduction
• Basic usage
• The need for fake data
• Annotations
• Creating Skeletons manually
• Customization
• Resources

Introduction

What are skeleton loaders?

UI skeleton loading is a technique used to enhance user experience during web or app loading. It involves displaying a simplified, static version of the user interface while the actual content is being fetched. This placeholder UI gives the illusion of instant loading and prevents users from perceiving long loading times.

Motivation

Creating a skeleton-layout for your screens doesn't only feel like a duplicate work but things can go out of sync real quick, when updating the actual layout we often forget to update the corresponding skeleton-layout.

How does it work?

As the name suggests, skeletonizer will reduce your already existing layouts into mere skeletons and apply painting effects on them, typically a shimmer effect. It automatically does the job for you, in addition skeleton annotations can be used to change how specific widgets are skeletonized.

Basic usage

Simply wrap your layout with Skeletonizer widget or SliverSkeletonizer | Skeletonizer.sliver for sliver widgets.

Skeletonizer(
  enabled: _loading,
  child: ListView.builder(
    itemCount: 7,
    itemBuilder: (context, index) {
      return Card(
        child: ListTile(
          title: Text('Item number $index as title'),
          subtitle: const Text('Subtitle here'),
          trailing: const Icon(Icons.ac_unit),
        ),
      );
    },
  ),
)

Skeletonizer with default config

Note: all the following shimmer effects are disturbed by the gif optimization

Click here for HD video demo

Skeletonizer with no containers

Skeletonizer(ignoreContainers: true)

The need for fake data

In order for skeletonizer to work it actually needs a layout, but in most cases the layout would need data to shape, e.g the following ListView will not render anything unless users is populated, because if users is empty we have no layout which means we have nothing to skeletonize.

Skeletonizer(
  enabled: _loading,
  child: ListView.builder(
    itemCount: users.length,
    itemBuilder: (context, index) {
      return Card(
        child: ListTile(
          title: Text(users[index].name),
          subtitle: Text(users[index].jobTitle),
          leading: CircleAvatar(
            radius: 24,
            backgroundImage: NetworkImage(users[index].avatar),
          ),
        ),
      );
    },
  ),
)

So the key here is to provide fake data for the layout to shape until the real data is fetched form the server, and we would have such a setup in our build method:

 
  if (_loading) {
    final fakeUsers = List.filled(7, User(
        name: 'User name',
        jobTitle: 'Developer',
        avatar: ''
      ),
    );
    return Skeletonizer(
      child: UserList(users: fakeUsers),
    );
  } else {
    return UserList(users: realUsers);
  }  

or by utilizing the enabled flag

  {
  final users = _loading ? List.filled(
      7, const User(
      name: 'User name',
      jobTitle: 'Developer',
      avatar: ''
  ) : realUsers;
  );
  return Skeletonizer(
    enabled: _loading,
    child: UserList(users: users),
  );
 

Note: you can use BoneMock to easily generate fake text for your mock data

 
 
    final fakeUsers = List.filled(7, User(
        name: BoneMock.name,
        jobTitle: BoneMock.words(2),
        email: BoneMock.email,
        createdAt: BoneMock.date, 
      ),
    );
     

Now we have our layout but one issue remains, if you run the above example you'll get an error in your console stating that an invalid url was passed to NetworkImage which is legit because our fake avatar url is an empty string, in such cases we need to make sure NetworkImage is not in our widget tree when skeletonizer is enabled and we do that by using a skeleton annotation called Skeleton.replace ..read more about annotations below.

Skeletonizer(
  enabled: _loading,
  child: ListView.builder(
    itemCount: users.length,
    itemBuilder: (context, index) {
      return Card(
        child: ListTile(
          title: Text(users[index].name),
          subtitle: Text(users[index].jobTitle),
            leading: Skeleton.replace(
            width: 48, // width of replacement
            height: 48, // height of replacement
            child: CircleAvatar(
            radius: 24,
            backgroundImage: NetworkImage(users[index].avatar),
          ),
        ),
      );
    },
  ),
)

or you can do it directly like follows:

Skeletonizer(
  enabled: _loading,
  child: ListView.builder(
    itemCount: users.length,
    itemBuilder: (context, index) {
      return Card(
        child: ListTile(
          title: Text(users[index].name),
          subtitle: Text(users[index].jobTitle),
            leading: CircleAvatar(
            radius: 24,
            backgroundImage: _loading ? null : NetworkImage(users[index].avatar),
          ),
        ),
      ),);
    },
  ),
)

Note: you can also check wither a skeletonizer is enabled inside descendent widgets using:

Skeletonizer.of(context).enabled;

Annotations

We can use annotations to change how some widgets should be skeletonized, skeleton annotations have no effect on the real layout as they only hold information for skeletonizer to use when it's enabled.

Skeleton.ignore

Widgets annotated with Skeleton.ignore will not be skeletonized

Card(
  child: ListTile(
    title: Text('The title goes here'),
    subtitle: Text('Subtitle here'),
    trailing: Skeleton.ignore( // the icon will not be skeletonized
      child: Icon(Icons.ac_unit, size: 40),
    ),
  ),
)

Skeleton.leaf

Widgets annotated with Skeleton.leaf marks containers as leafs, leafs are painted using the shader paint.

Skeleton.leaf(
   child : Card(
    child: ListTile(
        title: Text('The title goes here'),
        subtitle: Text('Subtitle here'),
        trailing: Icon(Icons.ac_unit, size: 40),
      ),
  )
)

Skeleton.keep

Widgets annotated with Skeleton.keep will not be skeletonized but will be painted as is

Card(
  child: ListTile(
    title: Text('The title goes here'),
    subtitle: Text('Subtitle here'),
    trailing: Skeleton.keep( // the icon will be painted as is
      child: Icon(Icons.ac_unit, size: 40),
    ),
  ),
)

Skeleton.shade

Widgets annotated with Skeleton.shade will not be skeletonized but will be shaded by a shader mask.

Note: CustomPainter widgets need to be explicitly annotated with Skeleton.shade to be shaded.

Card(
  child: ListTile(
    title: Text('The title goes here'),
    subtitle: Text('Subtitle here'),
    trailing: Skeleton.shade( // the icon will be shaded by shader mask
      child: Icon(Icons.ac_unit, size: 40),
    ),
  ),
)

Skeleton.replace

Widgets annotated with Skeleton.replace will be replaced when skeletonizer is enabled and the replacement will be skeletonized, This is good for widgets that can not render with fake data like Image.network()

Card(
  child: ListTile(
    title: Text('The title goes here'),
    subtitle: Text('Subtitle here'),
    trailing: Skeleton.replace( // the icon will be replaced when skeletonizer is enabled
        width: 50, // the width of the replacement
        height: 50, // the height of the replacement
        replacement: // defaults to a DecoratedBox
        child: Icon(Icons.ac_unit, size: 40),
  ),
)
,)

Skeleton.unite

Widgets annotated with Skeleton.unite will not be united and drawn as one big bone, this is good for when you have multiple small bones close to each other and you want to present them as one bone.

Card(
  child: ListTile(
    title: Text('Item number 1 as title'),
    subtitle: Text('Subtitle here'),
    trailing: Skeleton.unite(
      child: Row(
        mainAxisSize: MainAxisSize.min,
        children: [
          Icon(Icons.ac_unit, size: 32),
          SizedBox(width: 8),
          Icon(Icons.access_alarm, size: 32),
        ],
      ),
    ),
  ),
)
,

Skeleton.ignorePointers

Widgets annotated with Skeleton.ignorePointers will ignore pointer events when skeletonizer is enabled.

Creating Skeletons manually

Originally, Skeletonizer worked its magic by creating skeletons from real widgets. But hey, we get it – sometimes you want to do your own thing, have a different layout, or just skip the whole fake data scene. Now, you can speed things up big time by crafting custom skeletons from scratch using Bone widgets. Now, you might wonder, "What sets this apart from other shimmer libraries?" Simple – Bone widgets are your time-saving superheroes. They mimic everyday UI components you'd typically want to shimmer, such as icons, texts, and buttons and use their actual theme data to design border radius and sizing information.

Skeletonizer.zone(
    child: Card(
      child: ListTile(
        leading: Bone.circle(size: 48),  
        title: Bone.text(words: 2),
        subtitle: Bone.text(),
        trailing: Bone.icon(), 
      ),
    ),
 );

Obviously, you can create any bone shape using Bone(width, height) or use the helpers Bone.circle(size) and Bone.square(size). The cool part here is using mimic bone types like Bone.icon(), which, you guessed it, reads icon sizing information from the inherited theme. As such, Bone.text reads font size and line height from the inherited theme if not provided in the constructor. By default, it mimics 3 words (word = 5 letters), but you can easily change that. For multiline, use Bone.multiText(lines: 3).

Bone.button() and Bone.iconButton() use BoneResolver to determine their dimensions, shape, and typography. They use generic defaults when no resolver is provided. Provide a resolver to match your design system's button theme.

When opting for the manual approach, only Bone widgets get the shimmer treatment – they're the ones that'll be shaded. This means you can wrap other colorable widgets like cards, containers ..etc, without worrying about them getting shaded. which results to a seamless and perfect shimmer effect!

Customization

Loading effects

Skeletonizer has 3 different painting effects to choose from which can be customized to your liking.

Note: Loading effects are disturbed by Gif optimization, these look much better on flutter

Effects are configurable, e.g you can change the base color, highlight color, direction, and duration of ShineEffect like so:

Skeletonizer(
  effect: const ShimmerEffect(
    baseColor: Colors.grey[300],
    highlightColor: Colors.grey[100],
    duration: Duration(seconds: 1),
  ),
  child: ...
)

Animating the transition between skeleton and content (Slivers are not supported)

you can animate the transition between skeleton and content by setting enableSwitchAnimation to true, animation can be customized by providing a SwitchAnimationConfig.

SwitchAnimationConfig({
    this.duration = const Duration(milliseconds: 300),
    this.switchInCurve = Curves.linear,
    this.switchOutCurve = Curves.linear,
    this.transitionBuilder = AnimatedSwitcher.defaultTransitionBuilder,
    this.layoutBuilder = AnimatedSwitcher.defaultLayoutBuilder,
    this.reverseDuration,
}); 

Text skeleton config

You can provide a global text config options to skeletonizer widgets like

Skeletonizer(
    justifyMultiLineText: false,
    textBoneBorderRadius: TextBoneBorderRadius.fromHeightFactor(.5),
    ...
)

ignoreContainers

if you want to ignore all containers and only skeletonize their children you can set ignoreContainers to true

containersColor

if provided, all containers will be painted with this color otherwise the actual color will be

Using the inheritable Config data

Wrap a subtree with SkeletonizerConfig to provide default configuration values to all descendant Skeletonizer widgets. By default, SkeletonizerConfigData resolves a light shimmer in light mode and a dark shimmer in dark mode, using the platform brightness.

SkeletonizerConfig(
  data: const SkeletonizerConfigData(),
  child: MaterialApp(
    home: MyHomePage(),
  ),
)

Use effectResolver to supply effects for each brightness. SkeletonizerConfigData uses the platform brightness by default. When your app uses a different theme mode, provide its brightness explicitly. For example, in a Material app, use Theme.of(context).brightness from a context below MaterialApp.

MaterialApp(
  home: Builder(
    builder: (context) => SkeletonizerConfig(
      data: SkeletonizerConfigData(
        effectResolver: (brightness) => switch (brightness) {
          Brightness.light => const ShimmerEffect(),
          Brightness.dark => const ShimmerEffect.dark(),
        },
        brightness: Theme.of(context).brightness,
        justifyMultiLineText: true,
        textBorderRadius: const TextBoneBorderRadius.fromHeightFactor(.5),
        ignoreContainers: false,
      ),
      child: MyHomePage(),
    ),
  ),
)

Button bone resolvers

For the default button-bone specifications, use a BoneResolver instance:

const BoneResolver()

For a custom design system, extend BoneResolver and implement resolveButton and resolveIconButton, or construct a BoneResolver with callbacks. In a Material app, this callback resolver reads the active button themes:

SkeletonizerConfig(
  data: SkeletonizerConfigData(
    boneResolver: BoneResolver(
      button: (context, type) {
        final theme = ButtonTheme.of(context);
        final style = switch (type) {
          BoneButtonType.prominent => ElevatedButtonTheme.of(context).style,
          BoneButtonType.plain => TextButtonTheme.of(context).style,
          BoneButtonType.outlined => OutlinedButtonTheme.of(context).style,
        };
        return BoneButtonSpec(
          height: theme.height,
          width: theme.minWidth,
          shape: style?.shape?.resolve({}) ?? const StadiumBorder(),
          textStyle: style?.textStyle?.resolve({}),
        );
      },
      iconButton: (context, type) => BoneIconButtonSpec(
        shape: IconButtonTheme.of(context).style?.shape?.resolve({}),
      ),
    ),
  ),
  child: MyHomePage(),
)

Resources

• Flutter skeleton loader using skeletonizer

Support Skeletonizer

You can support skeletonizer by liking it on Pub and staring it on Github, sharing ideas on how we can enhance a certain functionality or by reporting any problems you encounter and of course buying a couple coffees will help speed up the development process.