I sometimes see stateful widget with the state which doesn't have any state fields. What is the reason of using such stateful widget? Is not better in that case to convert that stateful widget into stateless widget?
State objects (created by StatefulWidget) have their own lifecycle. They can override initState, didChangeDependencies, didUpdateWidget, dispose, etc to perform some tasks at specific cases. See lifecycle description in State class docs
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In the Flutter job interviews, sometimes I am asked how we can rebuild the StatelessWidget and the answer to this is to call the markNeedsBuild() method of the widget's Element or BuildContext.
So the question is, are there any cases when we need to rebuild a StatelessWidget and it's better to use a StatelessWidget instead of the StatefulWidget?
I found cases where people wanted to rebuild the StatelessWidget, such as How to invoke a rebuild of a stateless widget?, but in all these cases it was better to use StatefulWidget or state management instead.
The whole purpose of the StatelessWidget is not to rebuild.
If you require rebuild you have to use StatefullWidget.
I guess the interviewers are trying to trick you xD
Summary
Stateless Widget:
Stateless Widgets are static widgets.
They do not depend on any data change or any behavior change.
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.
For Example: Text, Icon, RaisedButton are Stateless Widgets.
Stateful Widget:
Stateful Widgets are dynamic widgets.
They can be updated during runtime based on user action or data change.
Stateful Widgets have an internal state and can re-render if the input data changes or if Widget’s state changes.
For Example: Checkbox, Radio Button, Slider are Stateful Widgets
I have one ParentWidget having two child widget.
I want to pass some callback action from FirstChild() to SecondChild() Widget without rebuild ParentWidget()like setState((){}).
I want to do because ParentWidget() has many widget. And this callback action continuously happen.
And continuously setState((){}) is not viable option for ParentWidget()
For example
setState((){}) call from FirstChild() than I want to rebuild SecondChild() without rebuild ParentWidget()
StatefulWidget is only useful in very simple business logic cases. I highly recommend that you have a look at Riverpod, where you can access a "state" from any widget, regardless of who its parent widget is. And because of how Riverpod is designed, it will not trigger any unnecessary rebuilds.
Sticking with StatefulWidget, you will need to use something like InheritedWidget as a parent of both widgets, and then have them both access it.
You can get help with state management solutions like Provider or Riverpod.
I may write an example code if you use Riverpod, but generally speaking, in such case, you need to place the state outside the Parent widget into some external variable. Then you can access and share it between both Children without rebuilding Parent.
How does Flutter rebuild/repaint work in case of:
Creating a Stateless widget inside a Stateful widget.
Creating a Stateful widget inside a Stateless widget.
Does a Stateless widget inside a Stateful widget render each time the Stateful widget State changes?
Can a Stateful widget change inside a Stateless widget? How does it affect the Stateless widget?
First here is the difference between stateless and stateful from the flutter documentation
A stateless widget never changes. Icon, IconButton, and Text are examples of stateless widgets. Stateless widgets subclass StatelessWidget.
A stateful widget is dynamic: for example, it can change its
appearance in response to events triggered by user interactions or
when it receives data.
For the first scenario "Creating a Stateless widget inside a Stateful widget."
when we update the state of the Stateful widget it will re-run its build function, when a stateless widget is part of that build function, flutter will check if its input data has changed or not, if not it will return the same instance of the widget, if it's changed then it will create another instance from the stateless widget and discard the previous one.
here is an illustration of the above scenario from this article
For the second scenario "Creating a Stateful widget inside a Stateless widget."
let's try to illustrate the widget tree first to visualize what's going on.
Stateless (1)
/ \
/ \
(2) Stateful Stateless (3)
if we update the state of the stateful widget (2), both (1) and (3) won't get affected, but our build function in the stateful widget will be called again.
and if the input coming for the parent (1) changes, then (1) and (2) will be rebuilt again and flutter will check if (3) has input data changes it will be also rebuilt, if not it will return the same instance.
hope this answers your question and makes you better understand how flutter renders its UI
Note: this is my current understanding of the way flutter handles rebuilding stateful and stateless widgets, feel free to correct me if anything is not correct.
I'm Searching a lot about the Difference between Inherited Widget and Bloc State management
I found that Inherited Widget is immutable but Bloc don't
I know the mutable and Immutable concept well but I just want to ask
Why inherited widget is Immutable and what difference between it and Bloc ?
Bloc and inheritedWidget are very different things.
Bloc is component that takes events as an input, react to this event and yields state through stream. Widgets then can listen to this stream of events and rebuild when new pierce of data is available. Bloc Is independent of widgets that listen to it, or insert events.
Inherited Widget just provides piece of data down to all his children. You have access to this data through buildContext. Also you can notify inherited Widget that Its data changed, which will result in rebuilding all widgets under it.
I am learning Dart/flutter and trying to understand how Widgets system works. But I can't understand what difference between stateless and stateful widgets? For example I have button. What type it have?
Let's imagine two cases.
I send text to button and it's display it.
I send text to button and it's change color.
What will be if I will create not proper Widget type?
There are 3 kind of widgets, not just 2.
Stateful widget
Stateless widget
Inherited widget
A stateless widget is like a constant. It is immutable. If you want to change what is displayed by a stateless widget, you'll have to create a new one.
Stateful widgets are the opposite. They are alive and can interact with the user. Stateful widgets have access to a method named setState, which basically says to the framework "Hello, I want to display something else. Can you redraw me please ?".
Finally, Inherited widget is a mixt of both worlds. It is immutable and stateless. But another widget (whatever it is) can subscribe to that inherited widget.
Which means that when you replace your inherited widget by a new one, all the widgets that has subscribed to the old one will be redrawn.
In the end, a stateful widget will usually be used like a Controller.
A stateless widget will be used like a View.
And the inherited widget will be your configuration file or your Model.
According to flutter.io:
Stateless widget
Stateless widgets are immutable, meaning that their properties can’t
change — all values are final.
Here is doc.
Stateful widget
Stateful widgets maintain state that might change during the lifetime
of the widget. Implementing a stateful widget requires at least two
classes: 1) a StatefulWidget class that creates an instance of 2) a
State class. The StatefulWidget class is, itself, immutable, but the
State class persists over the lifetime of the widget.
As a example if you want to change text in text widget when a button press, you have to use StatefulWidget and it will let you to change the state of a variable.
But in StatelessWidget you cannot do that because it doesn't keep state.
Read more from doc.
This tutorial will help anyone who trying to understand these two.
Check out the Flutter Interactivity Tutorial.
If your widget's build method depends entirely on its immutable constructor arguments, you should use a StatelessWidget because they're simpler. If you want to store some persistent private data that you expect to mutate over time, use a StatefulWidget and store the data on the State.