What is Flutter?

Flutter is an open-source UI toolkit created by Google for building natively compiled applications for mobile, web, and desktop from a single codebase. Think of it as a way to write code once and run it everywhere—on iOS, Android, web browsers, Windows, macOS, and Linux.

Imagine you want to build an app. Traditionally, you'd need to:

• Write code in Swift/Objective-C for iOS
• Write separate code in Kotlin/Java for Android
• Write yet another version for web

With Flutter, you write your code once in the Dart programming language, and it runs on all platforms. It's like having a universal translator for apps.

Core Concepts

1. Everything is a Widget

In Flutter, your entire UI is built from widgets. A widget is simply a description of part of your user interface. Buttons, text, layouts, animations—they're all widgets.

Text('Hello World')  // A text widget
Container()          // A box widget
Row()               // A horizontal layout widget


Widgets combine like building blocks. A simple app might look like:

App
 └─ Screen
     └─ Column
         ├─ Text
         ├─ Image
         └─ Button

2. Two Types of Widgets

Stateless Widgets: Unchanging widgets. Once built, they don't change. Like a static label or icon.

Stateful Widgets: Dynamic widgets that can change over time. Like a checkbox that toggles or a form that updates.

3. Hot Reload

One of Flutter's most loved features. When you change your code, you can see the results instantly (in less than a second) without restarting your app. This makes development incredibly fast.

How Flutter Works:

Architecture Layers

Flutter has a layered architecture with three main layers:

Framework Layer (Dart)

• Material and Cupertino widget libraries (pre-built UI components)
• Widgets layer (the building blocks)
• Rendering layer (layout and painting)
• Foundation library (basic utilities)

Engine Layer (C/C++)

• Skia graphics engine (does the actual drawing)
• Dart runtime
• Text layout and rendering
• Platform channels (communication with native code)

Platform Layer (Platform-Specific)

• Android/iOS/Web/Desktop embedders
• Native platform APIs

The Rendering Process

1. Widget Tree: You describe your UI using widgets
2. Element Tree: Flutter creates elements that manage the widget lifecycle
3. Render Tree: Render objects perform layout and painting
4. Layer Tree: Composited into layers for the graphics engine
5. Skia: Renders pixels to the screen

This process happens incredibly fast, achieving 60fps (or 120fps on capable devices).

Why Flutter is Fast

Compiled to Native Code: Unlike some frameworks that use web views or interpreters, Flutter compiles directly to ARM or x86 native code. Your app runs as fast as native apps written in Swift or Kotlin.

Custom Rendering Engine: Flutter doesn't use platform widgets. Instead, it draws everything itself using Skia. This means:

• Consistent look across platforms
• No performance penalties from bridging to native components
• Complete control over every pixel

Efficient Updates: Flutter's reconciliation algorithm efficiently updates only what changed, minimizing expensive rebuild operations.

The Dart Language

Flutter uses Dart, a language also created by Google. Key features:

• Object-oriented: Everything is an object
• Strongly typed: Catches errors before runtime
• Just-in-Time (JIT) compilation: For fast development and hot reload
• Ahead-of-Time (AOT) compilation: For fast production apps
• Garbage collected: Automatic memory management
• Async support: Built-in handling of asynchronous operations

Example Dart code:

void main() {
  print('Hello, Flutter!');
}

State Management

As apps grow complex, managing state (data that changes) becomes crucial. Flutter offers several approaches:

• setState(): Built-in, simple, for local state
• InheritedWidget/Provider: For sharing state across widgets
• Riverpod: Modern, type-safe state management
• Bloc: Business logic component pattern
• GetX/MobX: Reactive state management

Platform Integration

When you need platform-specific features (camera, GPS, sensors), Flutter provides:

Platform Channels: A way to communicate between Dart code and native code (Java/Kotlin for Android, Swift/Objective-C for iOS)

Plugins: Pre-built packages that handle platform-specific functionality. Thousands exist for common needs like databases, maps, authentication, etc.

Widget Composition vs Inheritance

Unlike some UI frameworks that use inheritance heavily, Flutter favors composition. Instead of extending widgets to customize them, you wrap and combine them:

Container(
  padding: EdgeInsets.all(16),
  color: Colors.blue,
  child: Text('Composed widget'),
)

This makes code more flexible and reusable.

The Build Method

Every widget has a build() method that returns a widget tree:

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Text('This is built when needed');
  }
}

Flutter calls build() when the widget needs to be rendered, and intelligently caches and reuses what it can.

Material and Cupertino

Flutter provides two design systems out of the box:

• Material Design: Google's design language (Android-style)
• Cupertino: Apple's iOS design language

You can use both, mix them, or create your own custom designs.

The Ecosystem

• pub.dev: Package repository with thousands of plugins
• Firebase: Deep integration with Google's backend services
• DevTools: Powerful debugging and profiling tools
• Flutter Inspector: Visual debugging of widget trees

When to Use Flutter

Great for:

• Startups needing rapid MVP development
• Apps requiring consistent UI across platforms
• Teams with limited native developers
• Apps with complex, custom UIs
• Projects prioritizing time-to-market

Consider alternatives when:

• Building platform-specific apps that heavily use native features
• Your team is already expert in native development
• App size is absolutely critical (Flutter apps are larger than minimal native apps)

Learning Curve

Flutter has a moderate learning curve. If you know programming, you can build simple apps in days. Mastering it—understanding state management, performance optimization, and advanced patterns—takes weeks to months.

The framework is declarative, meaning you describe what the UI should look like, and Flutter handles how to make it happen. This is different from imperative programming where you manually update the UI.