What is the relation between stateful and stateless widgets in Flutter?

Question

A StatelessWidget will never rebuild by itself (but can from external events). A StatefulWidget can. That is the golden rule.

BUT any kind of widget can be repainted any times.

Stateless only means that all of its properties are immutable and that the only way to change them is to create a new instance of that widget. It doesn't e.g. lock the widget tree.

But you shouldn't care about what's the type of your children. It doesn't have any impact on you.

From the documentation at flutter.io:

...The important thing to note here is at the core both Stateless and Stateful widgets behave the same. They rebuild every frame, the difference is the StatefulWidget has a State object which stores state data across frames and restores it.

If you are in doubt, then always remember this rule: If a widget changes (the user interacts with it, for example) it’s stateful. However, if a child is reacting to change, the containing parent can still be a Stateless widget if the parent doesn’t react to change.

As mention in flutter docs

What’s the point?

Some widgets are stateful, and some are stateless. If a widget changes—the user interacts with it, for example—it’s stateful. A widget’s state consists of values that can change, like a slider’s current value or whether a checkbox is checked. A widget’s state is stored in a State object, separating the widget’s state from its appearance. When the widget’s state changes, the state object calls setState(), telling the framework to redraw the widget.

A stateless widget has no internal state to manage. Icon, IconButton, and Text are examples of stateless widgets, which subclass StatelessWidget.

A stateful widget is dynamic. The user can interact with a stateful widget (by typing into a form, or moving a slider, for example), or it changes over time (perhaps a data feed causes the UI to update). Checkbox, Radio, Slider, InkWell, Form, and TextField are examples of stateful widgets, which subclass StatefulWidget.

https://flutter.io/tutorials/interactive/#stateful-stateless

I can think of a very simple analogy. You have some piece of furniture with books, decorations, and a TV. The furniture is stateless, it does nothing doesn't move. In the TV, on the other side, you can turn it on, off, change channel, play a movie if it has some DVD attached, etc. The TV has a internal state which affects the way it behaves. In the furniture you have no state. The presence of the TV in the furniture is not adding a state to it. Hope this helps.

State is information that (1) can be read synchronously when the widget is built and (2) might change during the lifetime of the widget. It is the responsibility of the widget implementer to ensure that the State is promptly notified when such state changes, using State.setState.

StatefulWidget:

A stateful widget is a widget that describes part of the user interface by building a constellation of other widgets that describe the user interface more concretely. The building process continues recursively until the description of the user interface is fully concrete (e.g., consists entirely of RenderObjectWidgets, which describe concrete RenderObjects).

Stateful widget are useful when the part of the user interface you are describing can change dynamically, e.g. due to having an internal clock-driven state, or depending on some system state. For compositions that depend only on the configuration information in the object itself and the BuildContext in which the widget is inflated, consider using StatelessWidget.

StatefulWidget instances themselves are immutable and store their mutable state either in separate State objects that are created by the createState method, or in objects to which that State subscribes, for example Stream or ChangeNotifier objects, to which references are stored in final fields on the StatefulWidget itself.

StatelessWidget:

A stateless widget is a widget that describes part of the user interface by building a constellation of other widgets that describe the user interface more concretely. The building process continues recursively until the description of the user interface is fully concrete (e.g., consists entirely of RenderObjectWidgets, which describe concrete RenderObjects).

Stateless widget are useful when the part of the user interface you are describing does not depend on anything other than the configuration information in the object itself and the BuildContext in which the widget is inflated. For compositions that can change dynamically, e.g. due to having an internal clock-driven state, or depending on some system state, consider using StatefulWidget.

Stateless Widgets are static widgets. You just need to pass few properties before initializing Stateless Widgets. They do not depend on any data change or any behavior change. For Example. Text, Icon, RaisedButton are Stateless Widgets.

Stateful Widgets are dynamic widgets, they can be updated during runtime based on user action or data change. If a Widget can change its state during run time it will be stateful widget.

Edit 15/11/2018

Stateless Widgets can re-render if the input/external data changed (external data being data that is passed through the constructor). Because Stateless Widgets do not have a state, they will be rendered once and will not update themselves, but will only be updated when external data changes.

Whereas Stateful Widgets have an internal state and can re-render if the input data changes or if Widget's state changes.

Both stateless and stateful widgets have different lifecycle.

When writing an app, you’ll commonly author new widgets that are subclasses of either StatelessWidget or StatefulWidget

Here is some Differences Between StatelessWidget and StatefulWidget Widgets:

Stateless Widget:

1. A widget that has an immutable state.
2. Stateless Widgets are static widgets.
3. They do not depend on any data change or any behavior change.
4. Stateless Widgets do not have a state, they will be rendered once and will not update themselves, but will only be updated when external data changes.
5. For Example: Text, Icon, RaisedButton are Stateless Widgets.

Stateful Widget:

1. A widget that has a mutable state.
2. Stateful Widgets are dynamic widgets.
3. They can be updated during runtime based on user action or data change.
4. Stateful Widgets have an internal state and can re-render if the input data changes or if the Widget’s state changes.
5. For Example: Checkbox, Radio Button, Slider are Stateful Widgets