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Core Concepts


In Flutter, everything you see on the screen and can interact with, is called a Widget1 2. Whether they keep any state or not, widgets are to be considered immutable - when the state a widget depends on depends on changes, the widget does not merely change, but it gets replaced with a new instance. This is a concept that has been popularized by frameworks such as React.

Stateful vs. Stateless Widgets

Flutter distinguishes between stateless and stateful widgets. For those, familiar with React, the distinction is similar to that between class and functional React components3.

Stateless widgets

A stateless widget is the rough equivalent of a one-way street. It is given some final state at instantiation time, and throughoutits lifetime, it only makes use of those. A stateless widget cannot hold non-fina fields, in fact if you try to add one to a class that extends from StatelessWidget, you will see the following warning:

This class (or a class which this class inherits from) is marked as '@immutable', but one or more of its instance fields are not final.

If you dive deeper into the Flutter SDK code, you will find the following piece of documentation regarding @immutable:

A class is immutable if all of the instance fields of the class, whether defined directly or inherited, are final.

TODO: Add more detials from Widget - State - Context - InheritedWidget



Common Issues

Widgets appear behind navigation elements (e.g. behind nav bars)

A common issue when setting up a page Scaffold is to see content appearing behind the main navigation element (e.g. behind the nav bar). To avoid this, the immediate child widget of a Scaffold instance needs to be wrapped in a so called SafeArea6 widget:

class SearchPage1 extends StatelessWidget {
  Widget build(BuildContext context) {
    return new CupertinoPageScaffold(
        navigationBar: CupertinoNavigationBar(
          middle: Text('Saved Searches'),
        child: SafeArea(
          child: Column(
          // the rest of the widget continues

Row and Column

TODO: add more detials from Flutter — Row/Column Cheat Sheet

Advanced Topics


Slivers4 are not a very deeply discussed topic in Flutter, mainly because they sit underneath more commonly used Widgets such as ListView, and GridView. This is a problem, because it leads to lots of panic when it comes to working with slivers.

A sliver is just a portion of a scrollable area. Under the covers, all of the scrollable views you use, like ListView and GridView, are actually implemented using Slivers.5

State Management

TODO: elaborate on this List of state management approaches

When it comes to state management, a good advice for people just starting out with Flutter is: use ScopedModel for as long as you can. In a large majority of cases, a scoped model would be more than sufficient. When it does not scale, you can have a look at BLoC or at Redux for those coming from React.7 If you are new to Flutter and don't have a strong reason to choose any other approach, this is the first thing you should try, before moving on to some more compliated approaches.8