One of the most defining trends in modern mobile app User Experience (UX) is undoubtedly 'content-centric design.' The technique of dynamically hiding non-essential UI elements to allow users to focus on the content is no longer an option but a necessity. A prime example, commonly seen in apps like Instagram, Facebook, and modern web browsers, is the bottom tab bar (BottomNavigationBar) that disappears when scrolling down and reappears when scrolling up. This feature maximizes screen real estate and provides a much cleaner, more pleasant user experience.
If you're developing an app with Flutter, you've likely wondered how to implement such dynamic UI. It's not just about a binary 'show/hide' toggle; it's about creating a polished feature with smooth animations that accurately interprets the user's scroll intent. This article will provide a comprehensive, A-to-Z guide on implementing a 'scroll-aware bottom bar' that works perfectly in any complex scroll view. We will leverage Flutter's ScrollController, NotificationListener, and AnimationController. By the end, you won't just be copying and pasting code; you'll master the underlying principles and learn how to handle various edge cases.
1. Understanding the Core Principles: How Does It Work?
Before diving into the implementation, it's crucial to understand the core principles behind the feature we're building. The goal is simple: detect the user's scroll direction and, based on that direction, either push the BottomNavigationBar off-screen or bring it back into view.
- Detect Scroll Direction: We need to know if the user is swiping their finger up (scrolling the content down) or pulling their finger down (scrolling the content up).
- Modify UI Position: Based on the detected direction, we will move the BottomNavigationBar along the Y-axis. When scrolling down, we'll move it down by its own height to hide it off-screen. When scrolling up, we'll return it to its original position (Y=0).
- Apply a Smooth Transition: An instantaneous change in position feels jarring to the user. Therefore, we must apply an animation to make the bar slide smoothly in and out of view.
To implement these three principles, Flutter provides a set of powerful tools:
ScrollControllerorNotificationListener: These are used to listen for scroll events from scrollable widgets likeListView,GridView, orCustomScrollView. WhileScrollControllerallows for direct control over the scroll position,NotificationListenercan listen for various notifications from child scroll widgets higher up the widget tree. We will explore both but focus on implementing the more flexibleNotificationListenerapproach.userScrollDirection: This is a property of theScrollPositionobject that indicates the user's current scroll direction as one of three states:ScrollDirection.forward(scrolling up),ScrollDirection.reverse(scrolling down), andScrollDirection.idle(stopped).AnimationControllerandTransform.translate: AnAnimationControllermanages the progress of an animation (from 0.0 to 1.0) over a specific duration. By using its value to control theoffsetof aTransform.translatewidget, we can smoothly move any widget along a desired axis.
Now, let's use these tools to write the actual code.
2. Step-by-Step Implementation: From Scroll Detection to Animation
We'll start with the most basic form and gradually enhance its functionality. First, let's create a basic app structure with a scrollable screen and a BottomNavigationBar.
2.1. Basic Project Setup
Since we need to manage state, we'll start with a StatefulWidget for our main page. This page will contain a ListView with a long list of items and a BottomNavigationBar.
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';
void main() {
runApp(const MyApp());
}
class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Scroll Aware Bottom Bar',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: const HomePage(),
);
}
}
class HomePage extends StatefulWidget {
const HomePage({Key? key}) : super(key: key);
@override
State<HomePage> createState() => _HomePageState();
}
class _HomePageState extends State<HomePage> {
int _selectedIndex = 0;
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: const Text('Scroll Aware Bottom Bar'),
),
body: ListView.builder(
itemCount: 100, // Provide enough items to make the list scrollable
itemBuilder: (context, index) {
return ListTile(
title: Text('Item $index'),
);
},
),
bottomNavigationBar: BottomNavigationBar(
items: const <BottomNavigationBarItem>[
BottomNavigationBarItem(
icon: Icon(Icons.home),
label: 'Home',
),
BottomNavigationBarItem(
icon: Icon(Icons.search),
label: 'Search',
),
BottomNavigationBarItem(
icon: Icon(Icons.person),
label: 'Profile',
),
],
currentIndex: _selectedIndex,
onTap: (index) {
setState(() {
_selectedIndex = index;
});
},
),
);
}
}
The code above is a standard, plain Flutter app with no special functionality yet. Now, let's add the scroll detection logic.
2.2. Detecting the Scroll: Utilizing NotificationListener
While you could attach a ScrollController directly to the ListView and add a listener, using a NotificationListener can help keep the widget tree cleaner. You simply wrap the ListView with a NotificationListener<UserScrollNotification> widget. UserScrollNotification is particularly useful because it's only dispatched in response to a user's direct scroll action, allowing you to distinguish it from programmatic scrolling for more precise control.
First, let's add a state variable _isVisible to control the visibility of the BottomNavigationBar.
// Add inside the _HomePageState class
bool _isVisible = true;
Next, wrap the ListView with a NotificationListener and implement the onNotification callback. This callback function will be invoked every time a scroll event occurs.
// Inside the build method
// ...
body: NotificationListener<UserScrollNotification>(
onNotification: (notification) {
// When the user scrolls down (towards the end of the list)
if (notification.direction == ScrollDirection.reverse) {
if (_isVisible) {
setState(() {
_isVisible = false;
});
}
}
// When the user scrolls up (towards the start of the list)
else if (notification.direction == ScrollDirection.forward) {
if (!_isVisible) {
setState(() {
_isVisible = true;
});
}
}
// Return true to prevent the notification from bubbling up.
return true;
},
child: ListView.builder(
itemCount: 100,
itemBuilder: (context, index) {
return ListTile(
title: Text('Item $index'),
);
},
),
),
// ...
Now, the _isVisible state changes based on the scroll direction. However, there's no visible change in the UI yet. Let's use this state variable to actually move the BottomNavigationBar.
2.3. Smooth Movement with Animations
To make the BottomNavigationBar appear and disappear smoothly whenever the _isVisible state changes, we need animations. We can use AnimationController with either AnimatedContainer or Transform.translate. Here, we'll introduce the method of using AnimationController and Transform.translate with AnimatedBuilder, which is more powerful and efficient.
2.3.1. Initializing the AnimationController
Add an AnimationController to _HomePageState and initialize it in initState. Since this requires a vsync, we must add the TickerProviderStateMixin to the _HomePageState class.
// Modify the class declaration
class _HomePageState extends State<HomePage> with TickerProviderStateMixin {
// ... existing variables
late AnimationController _animationController;
late Animation<Offset> _offsetAnimation;
@override
void initState() {
super.initState();
// Initialize the animation controller
_animationController = AnimationController(
vsync: this,
duration: const Duration(milliseconds: 300), // Animation speed
);
// Initialize the offset animation
// begin: Offset.zero -> In its original position inside the screen
// end: Offset(0, 1) -> Moved down by its own height, outside the screen
_offsetAnimation = Tween<Offset>(
begin: Offset.zero,
end: const Offset(0, 1),
).animate(CurvedAnimation(
parent: _animationController,
curve: Curves.easeOut,
));
}
@override
void dispose() {
_animationController.dispose();
super.dispose();
}
// ...
}
The _animationController acts as the "engine" for our animation. We set its duration and link it to a vsync to create smooth animations synchronized with the screen's refresh rate. The _offsetAnimation is a `Tween` that translates the controller's value (0.0 to 1.0) into an Offset value that the UI can use. An Offset(0, 1) tells a widget to move down along the Y-axis by 1x its own height. (This is how `SlideTransition` works internally.)
2.3.2. Triggering the Animation on Scroll
Now, instead of calling setState in our NotificationListener, we'll control the _animationController.
// Modify the onNotification callback
onNotification: (notification) {
if (notification.direction == ScrollDirection.reverse) {
// Scrolling down -> Hide the bar
_animationController.forward(); // Animates towards the 'end' state (hidden)
} else if (notification.direction == ScrollDirection.forward) {
// Scrolling up -> Show the bar
_animationController.reverse(); // Animates towards the 'begin' state (visible)
}
return true;
},
Here, _animationController.forward() drives the animation from its beginning to its end (making the bar disappear), while reverse() does the opposite. We can add checks like _animationController.isCompleted or isDismissed to prevent redundant calls.
2.3.3. Applying the Animation to the UI with SlideTransition
Finally, we wrap our BottomNavigationBar with a SlideTransition widget to apply the animation to the UI.
// Modify the bottomNavigationBar part of the build method
// ...
bottomNavigationBar: SlideTransition(
position: _offsetAnimation,
child: BottomNavigationBar(
// ... existing BottomNavigationBar code
),
),
Let's refine this. To make it more intuitive, let's set the `begin` and `end` of the `Tween` to be `Offset(0, 0)` (visible) and `Offset(0, 1)` (hidden), and then adjust the controller's forward/reverse logic accordingly. Let's see the complete, polished code.
3. The Complete, Polished Code and Detailed Explanation
Combining all the concepts we've discussed, here is the complete, ready-to-run code. For better intuition and a more natural effect, we've switched to using SizeTransition.
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';
void main() {
runApp(const MyApp());
}
class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Scroll Aware Bottom Bar',
theme: ThemeData(
primarySwatch: Colors.indigo,
scaffoldBackgroundColor: Colors.grey[200],
),
debugShowCheckedModeBanner: false,
home: const HomePage(),
);
}
}
class HomePage extends StatefulWidget {
const HomePage({Key? key}) : super(key: key);
@override
State<HomePage> createState() => _HomePageState();
}
class _HomePageState extends State<HomePage> with TickerProviderStateMixin {
int _selectedIndex = 0;
// Animation controller for the bottom bar
late final AnimationController _hideBottomBarAnimationController;
// A direct state variable to manage visibility
bool _isBottomBarVisible = true;
@override
void initState() {
super.initState();
_hideBottomBarAnimationController = AnimationController(
vsync: this,
duration: const Duration(milliseconds: 200),
// Initial value: 1.0 (fully visible)
value: 1.0,
);
}
@override
void dispose() {
_hideBottomBarAnimationController.dispose();
super.dispose();
}
// Scroll notification handler function
bool _handleScrollNotification(ScrollNotification notification) {
// We only care about user-driven scrolls
if (notification is UserScrollNotification) {
final UserScrollNotification userScroll = notification;
switch (userScroll.direction) {
case ScrollDirection.forward:
// Scrolling up: show the bar
if (!_isBottomBarVisible) {
setState(() {
_isBottomBarVisible = true;
_hideBottomBarAnimationController.forward();
});
}
break;
case ScrollDirection.reverse:
// Scrolling down: hide the bar
if (_isBottomBarVisible) {
setState(() {
_isBottomBarVisible = false;
_hideBottomBarAnimationController.reverse();
});
}
break;
case ScrollDirection.idle:
// Scroll has stopped: do nothing
break;
}
}
return false; // Return false to allow other listeners to receive the notification
}
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: const Text('Perfect Scroll-Aware Bar'),
),
body: NotificationListener<ScrollNotification>(
onNotification: _handleScrollNotification,
child: ListView.builder(
// A controller can be attached for future use (e.g., edge cases)
// controller: _scrollController,
itemCount: 100,
itemBuilder: (context, index) {
return Card(
margin: const EdgeInsets.symmetric(horizontal: 16, vertical: 8),
child: ListTile(
leading: CircleAvatar(child: Text('$index')),
title: Text('List Item $index'),
subtitle: const Text('Scroll up and down to see the magic!'),
),
);
},
),
),
// Use SizeTransition to animate the height
bottomNavigationBar: SizeTransition(
sizeFactor: _hideBottomBarAnimationController,
axisAlignment: -1.0, // Aligns the child to the bottom as it shrinks
child: BottomNavigationBar(
items: const <BottomNavigationBarItem>[
BottomNavigationBarItem(
icon: Icon(Icons.home),
label: 'Home',
),
BottomNavigationBarItem(
icon: Icon(Icons.search),
label: 'Search',
),
BottomNavigationBarItem(
icon: Icon(Icons.person),
label: 'Profile',
),
],
currentIndex: _selectedIndex,
onTap: (index) {
setState(() {
_selectedIndex = index;
});
},
selectedItemColor: Colors.indigo,
unselectedItemColor: Colors.grey,
),
),
);
}
}
While we used SlideTransition in the previous example, using SizeTransition often provides a more common and natural-looking effect. SizeTransition animates the height (or width) of its child based on the sizeFactor value (from 0.0 to 1.0). By directly connecting our animation controller to the sizeFactor, the bar will have its full height when the controller's value is 1.0 and a height of 0 when it's 0.0, creating a natural disappearing effect. The property axisAlignment: -1.0 is crucial here; it ensures that as the height decreases, the widget shrinks towards its bottom edge, making it look as if it's sliding down and away.
4. Advanced Topics: Edge Cases and Best Practices
The basic functionality is now complete. However, in a real production environment, various edge cases can arise. Let's explore a few advanced techniques to increase the robustness of our feature.
4.1. Handling Reaching the Scroll Edge
If a user "flings" the scroll very fast and hits the top or bottom of the list, the last scroll direction might have been reverse, leaving the bar hidden. Generally, it's better for the user experience if the navigation bar is always visible when the user is at the very top of the list.
To solve this, we can use a ScrollController in conjunction with our NotificationListener. Attach a controller to the ListView and check the scroll position within the notification callback or a separate listener.
// Add a ScrollController to _HomePageState
final ScrollController _scrollController = ScrollController();
// In initState, add a listener (or check within the NotificationListener)
@override
void initState() {
super.initState();
// ... existing code
_scrollController.addListener(_scrollListener);
}
void _scrollListener() {
// When the scroll position is at the top edge
if (_scrollController.position.atEdge && _scrollController.position.pixels == 0) {
if (!_isBottomBarVisible) {
setState(() {
_isBottomBarVisible = true;
_hideBottomBarAnimationController.forward();
});
}
}
}
// Attach the controller to the ListView
// ...
child: ListView.builder(
controller: _scrollController,
// ...
The code above uses a listener on the ScrollController to continuously monitor the scroll position. If position.atEdge is true and position.pixels is 0, it means we've reached the very top of the scroll view. At this point, we forcibly show the BottomNavigationBar. Combining NotificationListener and ScrollController.addListener allows for more sophisticated control.
4.2. Integrating with a State Management Library (e.g., Provider)
As your app grows, separating UI from business logic becomes critical. Using a state management library like Provider or Riverpod helps structure your code more cleanly. Let's refactor the BottomNavigationBar's visibility state into a ChangeNotifier.
4.2.1. Create a BottomBarVisibilityNotifier
import 'package:flutter/material.dart';
class BottomBarVisibilityNotifier with ChangeNotifier {
bool _isVisible = true;
bool get isVisible => _isVisible;
void show() {
if (!_isVisible) {
_isVisible = true;
notifyListeners();
}
}
void hide() {
if (_isVisible) {
_isVisible = false;
notifyListeners();
}
}
}
4.2.2. Configure Provider and Connect to the UI
Set up a ChangeNotifierProvider in your `main.dart` and use a Consumer or `context.watch` in the UI to subscribe to state changes.
// main.dart
void main() {
runApp(
ChangeNotifierProvider(
create: (_) => BottomBarVisibilityNotifier(),
child: const MyApp(),
),
);
}
// HomePage.dart
// Inside the _handleScrollNotification function, call the Notifier instead of setState
// ...
if (userScroll.direction == ScrollDirection.forward) {
context.read<BottomBarVisibilityNotifier>().show();
} else if (userScroll.direction == ScrollDirection.reverse) {
context.read<BottomBarVisibilityNotifier>().hide();
}
// ...
// Inside the build method
@override
Widget build(BuildContext context) {
// This is a naive implementation; a better way is needed to trigger the animation.
// A Listener or Consumer is better suited.
// final isVisible = context.watch<BottomBarVisibilityNotifier>().isVisible;
// ...
// A superior approach is to have the Notifier itself manage the AnimationController.
}
An even more advanced and cleaner architecture is for the BottomBarVisibilityNotifier to own and manage the AnimationController itself. This way, the UI widgets simply subscribe to the Notifier's state, and the animation logic is fully encapsulated within the Notifier, maximizing reusability and separation of concerns.
4.3. Compatibility with CustomScrollView and Sliver Widgets
The greatest advantage of our NotificationListener approach is its independence from any specific scroll widget. The same code will work flawlessly on a more complex screen that uses CustomScrollView with SliverAppBar, SliverList, and other slivers.
// The body can be replaced with a CustomScrollView and it will still work
body: NotificationListener<ScrollNotification>(
onNotification: _handleScrollNotification,
child: CustomScrollView(
slivers: [
const SliverAppBar(
title: Text('Complex Scroll'),
floating: true,
pinned: false,
),
SliverList(
delegate: SliverChildBuilderDelegate(
(context, index) => Card(
// ...
),
childCount: 100,
),
),
],
),
),
Because the NotificationListener can capture scroll notifications bubbling up from a CustomScrollView just as easily as from a ListView, our hide/show functionality remains consistent. This is what makes the NotificationListener approach more flexible and powerful than relying solely on a ScrollController.
Conclusion: The Details That Elevate User Experience
We have taken a deep dive into how to dynamically hide and show a BottomNavigationBar in Flutter based on the scroll direction. We've gone beyond a simple implementation to cover a flexible architecture using NotificationListener, smooth animations with AnimationController and SizeTransition, and even handling edge cases like reaching the end of a scroll view.
This kind of dynamic UI is not just a "nice-to-have" feature; it is a core UX element that allows users to immerse themselves more deeply in the app's content and makes the most efficient use of limited mobile screen space. We encourage you to apply the techniques you've learned today to your own projects to build apps that feel more professional and delightful to use.
Here are the key takeaways:
- Scroll Detection: Use
NotificationListener<UserScrollNotification>to capture the user's explicit scroll intent. - State Management: Manage the bar's visibility state with a simple
boolvariable or a more robustChangeNotifier. - Animation: Control an
AnimationControllerbased on the state, and useSizeTransitionorSlideTransitionto smoothly update the UI. - Edge Case Handling: Use a
ScrollControlleras a supplementary tool to handle special cases like reaching the scroll edges, thereby perfecting the implementation.
You should now be able to confidently implement a dynamic BottomNavigationBar that integrates perfectly with any scroll view in Flutter. We recommend you run the code yourself, experiment with different animation durations and curves, and find the style that best fits your app.
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