Flutter Development

Flutter Development

Intro

In Flutter, everything is a widget — compose small StatelessWidgets and StatefulWidgets into screens, prefer const constructors, and keep build() methods lightweight. Pick the simplest state-management solution that fits the complexity, not the most fashionable one.

Overview

Widget architecture

Use StatelessWidget when there's no mutable state and StatefulWidget when the widget owns animations, controllers, or form input. Extract sub-widgets into top-level classes (not helper methods) so they can be const and rebuild independently. Compose by wrapping, never by extending. Name widgets by purpose (UserProfileCard), not by position or sequence.

Layout system

Row and Column are the primary axis-based layouts; Expanded and Flexible distribute space among children. Stack plus Positioned handles overlap and absolute placement. Use ListView.builder / GridView.builder for any list that may grow — they only build visible items. SizedBox for fixed gaps, Spacer for flexible gaps, Padding/Container for spacing and decoration. Reach for LayoutBuilder and MediaQuery when the layout depends on the viewport.

State management

Pick the smallest tool that fits:

• Local widget state → StatefulWidget + setState
• Shared state across a few screens → Provider or Riverpod
• Complex async flows with many events → BLoC

Riverpod is type-safe and avoids BuildContext plumbing; BLoC enforces event-driven structure for large apps. Avoid putting all state in a single global provider — split by domain so unrelated screens don't rebuild together.

Navigation with GoRouter

Define routes declaratively and navigate with context.go(...) to replace or context.push(...) to stack. Use ShellRoute / StatefulShellRoute for tabbed layouts that need to preserve state per tab. Authentication guards live in the redirect callback. Deep linking works automatically once routes are declared.

Theming

Configure theme on MaterialApp with ThemeData(useMaterial3: true, colorScheme: ColorScheme.fromSeed(...)). Read theme values via Theme.of(context) rather than hardcoding font sizes or colors. Provide a matching darkTheme and let Flutter pick based on platform brightness.

Networking and data

Use http or dio for REST. Serialize JSON via json_serializable and build_runner for type-safe parsing. Wrap APIs in repositories (UserRepository over a raw client). In the UI, render loading/error/data states explicitly with FutureBuilder or StreamBuilder — never show a blank screen while waiting.

Testing

Three layers: pure-Dart unit tests with test(), widget tests with testWidgets() and find.byType(), and full integration tests with the integration_test package. Use pumpWidget to mount and pump to advance frames. Mock external boundaries with mocktail or mockito. Use golden tests (matchesGoldenFile) for visual regression on critical screens.

Gotchas

Agent-specific failure modes — provider-neutral pause-and-self-check items:

• Nested Scaffold widgets. A Scaffold inside another Scaffold produces double app bars, broken navigation behavior, and unpredictable layout. Each route gets exactly one Scaffold at the top level.
• setState called after dispose(). Calling setState on a widget that has been removed from the tree throws a FlutterError in debug mode and causes silent bugs in release mode. Guard all async callbacks with if (!mounted) return; before calling setState.
• Forgetting to call dispose() on controllers, animations, and stream subscriptions. Objects that own platform resources or listeners are not garbage-collected automatically. Not disposing them causes memory leaks and "stream already closed" errors. Override dispose() and clean up every controller and subscription.
• Using helper methods instead of top-level StatelessWidget classes for sub-widgets. A helper method returning a Widget rebuilds on every parent rebuild regardless of whether its inputs changed. A top-level widget class enables const instantiation and independent rebuild boundaries.
• Unbounded Text or Column inside a Row. A Text widget without a size constraint in a Row throws a "RenderFlex overflowed" error. Wrap with Expanded or Flexible to constrain it, or use SizedBox with a fixed width.
• Blocking the event loop in a widget's build() method. build() is called every frame. Any synchronous heavy computation inside it drops frames and causes jank. Move expensive work into initState, an async handler, or a compute isolate.
• Using ListView() with all items materialized for a long list. ListView() with a children list builds all items up front. Use ListView.builder so only the visible items are built; this scales to thousands of items.

Full reference

Layout widget recipes

// Row — horizontal layout
Row(
  mainAxisAlignment: MainAxisAlignment.spaceBetween,
  crossAxisAlignment: CrossAxisAlignment.center,
  children: [Text('Left'), Text('Right')],
)

// Column — vertical layout
Column(
  mainAxisSize: MainAxisSize.min,  // shrink-wrap
  children: [
    Text('Title'),
    SizedBox(height: 8),
    Text('Subtitle'),
  ],
)

// Stack — overlapping children
Stack(
  children: [
    Image.network(url),
    Positioned(
      bottom: 16, left: 16,
      child: Text('Overlay text'),
    ),
  ],
)

// Wrap — flow layout with wrapping
Wrap(
  spacing: 8, runSpacing: 4,
  children: tags.map((t) => Chip(label: Text(t))).toList(),
)

Scrollable widgets

// ListView.builder — efficient for large lists
ListView.builder(
  itemCount: items.length,
  itemBuilder: (context, index) => ListTile(
    title: Text(items[index].name),
    subtitle: Text(items[index].description),
    leading: CircleAvatar(child: Text(items[index].initials)),
    trailing: Icon(Icons.chevron_right),
    onTap: () => context.push('/items/${items[index].id}'),
  ),
)

// GridView.builder
GridView.builder(
  gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
    crossAxisCount: 2, crossAxisSpacing: 8, mainAxisSpacing: 8,
  ),
  itemCount: products.length,
  itemBuilder: (context, i) => ProductCard(products[i]),
)

// CustomScrollView with slivers
CustomScrollView(
  slivers: [
    SliverAppBar(floating: true, title: Text('Feed')),
    SliverList.builder(
      itemCount: posts.length,
      itemBuilder: (context, i) => PostCard(posts[i]),
    ),
  ],
)

Form input

// TextField
TextField(
  controller: _controller,
  decoration: InputDecoration(
    labelText: 'Email',
    hintText: 'you@example.com',
    prefixIcon: Icon(Icons.email),
    border: OutlineInputBorder(),
    errorText: _hasError ? 'Invalid email' : null,
  ),
  keyboardType: TextInputType.emailAddress,
  onChanged: (value) => setState(() => _email = value),
)

// Form with validation
Form(
  key: _formKey,
  child: Column(children: [
    TextFormField(
      validator: (v) => v == null || v.isEmpty ? 'Required' : null,
    ),
    ElevatedButton(
      onPressed: () {
        if (_formKey.currentState!.validate()) { /* submit */ }
      },
      child: Text('Submit'),
    ),
  ]),
)

Async builders

// FutureBuilder — one-shot async
FutureBuilder<User>(
  future: _userFuture,
  builder: (context, snapshot) {
    if (snapshot.connectionState == ConnectionState.waiting) {
      return CircularProgressIndicator();
    }
    if (snapshot.hasError) return Text('Error: ${snapshot.error}');
    final user = snapshot.data!;
    return Text(user.name);
  },
)

// StreamBuilder — continuous updates
StreamBuilder<int>(
  stream: _counterStream,
  builder: (context, snapshot) {
    return Text('Count: ${snapshot.data ?? 0}');
  },
)

Common patterns

// Responsive layout
LayoutBuilder(
  builder: (context, constraints) {
    if (constraints.maxWidth > 600) {
      return Row(children: [Sidebar(), Expanded(child: Content())]);
    }
    return Content();  // mobile: no sidebar
  },
)

// Pull-to-refresh
RefreshIndicator(
  onRefresh: () async => await _loadData(),
  child: ListView.builder(...),
)

// Empty state
items.isEmpty
    ? Center(child: Text('No items yet'))
    : ListView.builder(
        itemCount: items.length,
        itemBuilder: (ctx, i) => ItemTile(items[i]),
      )

Material 3 navigation

NavigationBar(
  selectedIndex: _currentIndex,
  onDestinationSelected: (i) => setState(() => _currentIndex = i),
  destinations: [
    NavigationDestination(icon: Icon(Icons.home), label: 'Home'),
    NavigationDestination(icon: Icon(Icons.search), label: 'Search'),
    NavigationDestination(icon: Icon(Icons.person), label: 'Profile'),
  ],
)

Platform channels

Prefer existing packages (camera, geolocator, path_provider) over hand-rolled channels. When you must call native code, use MethodChannel for one-off calls and EventChannel for streams. Platform-specific source goes in android/ and ios/. Use Platform.isIOS / Platform.isAndroid for branching in Dart.

Performance

• Use const widgets aggressively — they short-circuit rebuilds
• ListView.builder for any non-trivial list
• Avoid setState high in the tree; it rebuilds every descendant
• Wrap expensive painters in RepaintBoundary to isolate them
• Prefer Opacity only when necessary — it forces an off-screen layer
• Cache network images with CachedNetworkImage
• Profile rebuild counts and frame times in Flutter DevTools

Common mistakes

• Nested Scaffolds — leads to double app bars and broken navigation
• setState after dispose — guard with mounted, cancel timers and stream subscriptions in dispose()
• Unbounded children in Row/Column — wrap with Expanded or Flexible
• Forgetting dispose() — controllers, animation controllers, and stream subscriptions all need explicit disposal
• Hardcoded user-facing strings — use intl or easy_localization

Further reading

• Flutter Documentation
• Flutter Widget Catalog