indicator_tab_bar

A tab indicator that draws a short bar of a fixed width under the tab instead of one stretched across it, and a sliver AnimatedSwitcher for cross-fading the body each tab selects.

Nothing here imports Material. TabBar.indicator takes a plain Decoration, so the indicator works the same whether the TabBar comes from package:flutter/material.dart or from the material_ui fork — and the sliver switcher is pure flutter/widgets. No assets, no other pub.flutter-io.cn dependency.

Install

flutter pub add indicator_tab_bar

Requires Flutter 3.44.0 or newer, and so Dart 3.12.0 or newer — that is the Flutter release which ships it.

LineTabIndicator

TabBar sizes its indicator from TabBarIndicatorSize, which offers a choice between the width of the whole tab and the width of its label — both of which move as the labels do. This one is neither: indicatorWidth is the width it draws, whatever the tab underneath measures, so a row of tabs with labels of very different lengths still gets underlines of one length.

TabBar(
  isScrollable: true,
  tabAlignment: TabAlignment.start,
  indicatorSize: TabBarIndicatorSize.label,
  indicator: LineTabIndicator(
    color: theme.colorScheme.primary,
    strokeWidth: 3,
    indicatorWidth: 20,
    radius: 4,
  ),
  tabs: const [
    Tab(text: 'Spot'),
    Tab(text: 'Futures & derivatives'),
    Tab(text: 'Earn'),
  ],
)

The bar is a rounded rectangle strokeWidth tall and exactly indicatorWidth wide, placed inside the tab by alignment — bottom centre unless told otherwise — within the rect insets leaves. A tab narrower than the bar gets one that overhangs it evenly on both sides.

color	what the bar is filled with
gradient	fills it in place of color — see below — default null
strokeWidth	its height — default 3
indicatorWidth	its width, independent of the tab — default 50
radius	its corner radius, clamped to strokeWidth / 2 — default 0, square ends
alignment	where it sits in the tab — default Alignment.bottomCenter
insets	deflates the rect it is aligned within — default EdgeInsets.zero

radius is clamped because past half the height there is nothing left to round: at or above strokeWidth / 2 the ends are already semicircles. So radius: 4 on a 3-tall bar and radius: 999 on the same bar draw the same pill, and the second screenshot row above — radius: 0 — is the only shape that reads differently at that thickness.

Placement

alignment and insets decide where in the tab the bar lands, and the default pair — Alignment.bottomCenter with no insets — is the underline sitting flush on the bottom edge.

LineTabIndicator(
  color: theme.colorScheme.primary,
  indicatorWidth: 20,
  radius: 4,
  // Lift it clear of the bottom edge, so the TabBar divider shows beneath.
  insets: const EdgeInsets.only(bottom: 6),
  // Tuck it under the leading edge of the label instead of centring it;
  // the directional form flips with the reading direction.
  alignment: AlignmentDirectional.bottomStart,
)

Alignment.topCenter makes it an overline instead, for a TabBar that sits below what it selects, and Alignment.center strikes through the label.

Gradient

LineTabIndicator.gradient fills the bar with a Gradient rather than a flat colour. The gradient is laid out across the bar, not across the tab, so a LinearGradient runs over exactly indicatorWidth logical pixels no matter how wide the tab is.

LineTabIndicator.gradient(
  gradient: LinearGradient(
    colors: [theme.colorScheme.primary, theme.colorScheme.tertiary],
  ),
  indicatorWidth: 40,
  strokeWidth: 4,
  radius: 2,
)

The shader is rebuilt only when the bar moves, so a gradient indicator sliding between tabs costs one shader per frame of the slide and none while it rests.

Value semantics

Two indicators with the same fields are ==, so TabBar does not repaint on a rebuild that changes nothing, and Decoration.lerp interpolates every field — alignment, insets and gradient included — so it animates across a theme change. A lerp against null fades the fill towards transparent without moving the bar.

SliverAnimatedSwitcher

When the tab body is a sliver inside a CustomScrollView — a SliverList under a SliverAppBar, say — the plain AnimatedSwitcher cannot be used, twice over: it stacks its children in a Stack, which is a box, and it wraps them in box transitions. This keeps the same driving animation but lays the incoming sliver out on its own and fades it with a SliverFadeTransition.

CustomScrollView(
  slivers: [
    const SliverAppBar(title: Text('History'), pinned: true),
    SliverAnimatedSwitcher(
      duration: const Duration(milliseconds: 250),
      switchInCurve: Curves.easeOut,
      child: SliverList.list(
        // The key is what marks this as a *different* sliver.
        key: ValueKey(tab),
        children: rows,
      ),
    ),
  ],
)

As with AnimatedSwitcher, the swap is detected by Widget.canUpdate: two slivers of the same runtime type with the same key are the same sliver updated, and nothing animates. Give each branch its own Key when they share a type, as the SliverList above does.

transitionBuilder replaces the fade with something else — anything that returns a sliver, so SliverOpacity and friends, not FadeTransition.

Only the incoming sliver animates

Slivers cannot be overlaid — there is no sliver Stack to hold the old one over the new — so the outgoing sliver is removed the moment the swap begins, and what you see is the incoming one fading up in the space it leaves. That is why there is no switchOutCurve or reverseDuration here: there would be no outgoing animation for them to shape.

Example

example/ is a runnable app: a scrollable TabBar with the indicator under it, live controls for every field — the three metrics, the inset, the placement and the gradient — and a CustomScrollView whose body sliver is swapped per tab.

cd example && flutter run

The images above are rendered from the real widgets, so they can be regenerated whenever the package changes:

cd example && flutter test --update-goldens test/screenshots_test.dart

Licence

MIT — see LICENSE.