Understand Simple Flutter Basics: What Is BuildContext?

BuildContext is a handle to the location of a widget in the widget tree.

Flutter is a mobile app development framework created by Google. It allows developers to create beautiful and smooth-running apps for Android and iOS devices using a single codebase. One of the key concepts in Flutter is the idea of a “widget.” Widgets are small pieces of code that define how an app looks and behaves. They can be simple, like a button or a text field, or they can be more complex, like a list or a tab bar.

In Flutter, BuildContext is a special object that is passed to the “build” method of a widget. The build method is a function that is called every time a widget needs to be drawn on the screen. The BuildContext object contains information about the location of the widget in the app’s hierarchy, as well as other useful data.

Widget, widget, widget

In Flutter, everything is a widget. This includes small components like forms and menus, as well as buttons, text, padding, and margins. All the widgets in a Flutter app are organized in a Widget Tree, where parent and child widgets are connected. The BuildContext helps us locate a specific widget in the tree and understand its relationship with other widgets. Each widget has its own BuildContext, which is created by the build method and passed as an argument. The BuildContext of a parent widget is the parent of the BuildContext of its children. We use BuildContext for a few reasons: to access the app’s theme and localizations, to modify the parent widget or show dialogs/snackbars, and to build responsive widgets that work on different screens and devices.

State vs BuildContext

The relationship between state and BuildContext is that the state of a widget is often stored in its ancestor widgets. When the state of an ancestor widget changes, the BuildContext of its descendant widgets also changes, which triggers a rebuild of the widget tree.

We need BuildContext to change the look and feel of an app because it allows us to access and modify the widget tree and its ancestor widgets. This enables us to update the appearance and behavior of the app in response to user input or other events.

The BuildContext of a parent widget is the parent of the BuildContext of its direct children widgets. This means that a child widget can locate its parent widget by using the BuildContext. For example, the following code can be used to locate the Scaffold widget from the FlatButton widget in the widget tree structure: Scaffold > Center > Row > FlatButton:


In addition to locating parent and child widgets, BuildContext can also be used to pass messages from a parent widget to a child widget in the widget tree. This can be achieved using InheritedWidgets.

Every Flutter widget has an @override build() method with the argument of BuildContext.

class CartItemWidget extends StatelessWidget {

  Widget build(BuildContext context) {.....

Simple explanation is that the BuildContextin Flutter is:

  1. A BuildContext only belongs to one widget.
  2. BuildContext objects are passed to WidgetBuilder functions

A BuildContext only belongs to one widget.

A BuildContext is specific to one widget and is only accessible to that widget or its parent’s BuildContext. It is used in WidgetBuilder functions, like StatelessWidget.build and can be accessed with State.context. Some static functions, such as showDialog and Theme.of, also require a BuildContext as an argument to perform actions for the calling widget or to get data for a specific context.

Each widget has its own BuildContext, which becomes the parent of the widget returned by StatelessWidget.build or State.build. The build context of a widget’s build method is not the same as the build context of the returned widget. This can cause problems, for example, if a build method for a widget Q includes a Theme in its returned widget tree and tries to use Theme.of with its own context, it will not find the Theme object. Instead, it will find the nearest ancestor Theme to widget Q. To access the build context of a returned widget tree, use a Builder widget. The build context passed to Builder.builder will be the Builder’s own context.

Widget, buildContext and Theme Data

In Flutter, the BuildContext is a handle to the location of a widget within the widget tree. It is passed to the build method of a widget when it is inserted into the tree, and can be used to access the widget’s parent, ancestors, and children.

The BuildContext is also used to access the widget’s Theme data, which is used to style the widget. The BuildContext allows a widget to access the theme data that is most relevant to it, even if that theme data is defined higher up in the widget tree.

Additionally, the BuildContext can be used to access the widget’s State object, which is used to manage the internal state of the widget. This is useful for widgets that need to update their appearance or behavior based on changes to their state.

Things to Look out for:

BuildContext can cause issues. For example, if a build method tries to access a Theme using its own context, it will find the ancestor Theme object instead. Use a Builder widget with the build context passed to the builder callback to access the BuildContext for a part of the returned tree.

Referencing a widget that is not part of the current build tree can occur when using BuildContext, causing errors and unpredictable behavior.

It can be tough to understand the context in which a widget is being built when using BuildContext, making it hard to debug problems and make changes to the widget tree.

In Flutter, everything is a widget, including buttons, text, padding, and margins. These widgets are organized in a widget tree, where each widget has a parent-child relationship with the others. The BuildContext of a widget is a handle to its location in the widget tree. It is passed as an argument to the build method, which creates and draws the widget.

To Learn more about the topic, please refer to the original flutter docs – BuildContext in Flutter