indicator_tab_bar library

A fixed-width tab indicator, and a sliver AnimatedSwitcher for the body each tab selects.

LineTabIndicator goes on TabBar.indicator and draws a short bar of a set width centred under the tab, rather than one stretched across the tab or across its label:

TabBar(
  indicatorSize: TabBarIndicatorSize.label,
  indicator: const LineTabIndicator(
    color: Color(0xFF2F6BFF),
    strokeWidth: 3,
    indicatorWidth: 20,
    radius: 4,
  ),
  tabs: const [Tab(text: 'Spot'), Tab(text: 'Futures')],
)

SliverAnimatedSwitcher cross-fades what that tab reveals, when the body is a sliver inside a CustomScrollView rather than a box:

SliverAnimatedSwitcher(
  child: SliverList.list(key: ValueKey(tab), children: rows),
)

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.

Classes

LineTabIndicator

A Decoration for TabBar.indicator that draws a short bar of a fixed width, by default centred under the tab, instead of one stretched across it.

SliverAnimatedSwitcher

An AnimatedSwitcher for slivers: swap child for another sliver and the new one fades in, in place, inside a CustomScrollView.

Typedefs

SliverAnimatedSwitcherTransitionBuilder = Widget Function(Widget sliver, Animation<double> animation)

Builds the transition wrapped around an incoming sliver.