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.
Related
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'm quite new to flutter and I'm trying to figure out the best way to build my widgets in the aspect of good performance while I have three choices:
1- Stateful Wigdet.
2- Stateless Widget.
3- Widget function(){} with a StatefulBuilder if needed.
Thanks.
A StatefulWidget has State, which means that you can change stuff inside the widget without needing to destroy it and create a new version. They work slower than StatelessWidget but are better for checkboxes or screens which need to be frequently updated.
A StatelessWidget is locked, and you cannot change values once it is initiated. In order to change things inside the widget, you need to re-call the build function to build an updated version. They are quite fast but are best used for static screens.
Widget function(){} widgets are custom-made. Essentially, you can take any existing widget and modify it, adding childs and other widgets to it. However, the widget type normally inherits from a StatefulWidget or StatelessWidget. These can cover basically everything.
Is it possible to make a Text Widget Stateful so that the Text can be changed when i press a Button without having my Class to be Stateful?
example:
class Rangliste extends StatelessWidget{.... Text(Test), Text(Test2)....}
Test and Test2 are Strings.
So when i press a Button the values inside the Text Widgets should change. Is there any Way to achieve this without changing my Class to Stateful? Or if not can somebody explain how i can change the Class to Stateful without messing up the rest of my Code
No, you can't, a stateless widget is an immutable widget that cannot react to state changes and rerender, you will have to use some form of state management.
If your button and your text and your data are all within the same widget, then it is better to make your widget Stateful and use setState(() {}) to change your text and rerender.
You can use the Flutter plugin in order to convert your widget from a Stateless widget to a StatefulWidget, the plugin will handle everything.
I am a newbie in the world of flutter, and I recently learned (or I think I did) about stateful and stateless widgets which is kind of the base for flutter widgets.
We use stateless widgets for things that are not redrawn on the display, (like text, button etc.) but stateful widgets can redraw themselves.
So my question is why do we need stateless widgets if stateful widgets can be used to draw the same kind of widgets that a stateless widget can?
Or is there any specific reasons to use stateless over stateful widgets in flutter? Or can we use stateful widgets all the time rather than stateless widgets which can draw content only once?
Thanks, and sorry if this is a stupid question.
EDIT
Well the question is not the difference between stateless and stateful.
I know the difference but what is the impact of using only stateful widgets since by using it we could also implement most of the things a stateless widget can do then why do we need stateless widgets?what's the importance of it in a flutter environment were most of the apps will be re-drawn time-to-time?
From their documentation:
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. (= use when you don't need to "update the UI here").
Stateful widgets are more resource consuming and you always need to think about performance.
Here is more about this.
Push the state to the leaves. For example, if your page has a ticking
clock, rather than putting the state at the top of the page and
rebuilding the entire page each time the clock ticks, create a
dedicated clock widget that only updates itself.
Even more on this :)
I hope this answers your question.
Yes, StatefulWidget can rebuild. That happens typically when using Inheritedwidgets.
StatelessWidget exists to split a big widget tree into smaller reusable widgets.
You might think "but I can use StatefulWidget or functions for this". Which is true, but not exactly:
StatefulWidget comes with a huge boilerplate, which you do not need in that situation. So this just adds noise and makes your code less readable.
Functions cannot rebuild independently, nor to they have access to key and override ==. So they can be less performant or introduce bugs.
Every time we use a Stateful widget, a state object is created. If we use all Stateful widgets, there will be a lot of unnecessary state objects which will consume a lot of memory. So where we don't need to change the state, we should use the simpler one - the Stateless widget.
Another reason to use Stateless widgets over Stateful widgets is Stateful widget comes with a huge boilerplate and according to Flutter API Documentation using a bunch of nested Stateful widgets, passing data through all those build methods and constructors can get cumbersome.
This is what I understand ...
When you use a stateful widget and redraw it, all other widgets inside will be redrawn as well. So we try to use stateless widgets so as not to redraw the other widgets within them, but you know we generally need to change the data on the screen and it should happen inside a "single widget" and this widget should be the one stateful widget to use as little computing power as possible.
Now ... I guess you're thinking: "but what if i just use stateful widgets and don't redraw them?" well, as you know, when you use a stateful widget you have two classes: widget and state. I have gotten that when you declare a state you tell the phone to keep and save this state in memory whether the widget is redrawn or not, so you use the phone memory for no reason because you don't need to redraw its widget.
We should think about always using stateless widgets because they are lighter than stateful widgets and we should always make our stateful widgets the smallest in our application to redraw as few widgets as possible in the widget tree of the application.
I hope I helped a little.
Theoretically I think you can use stateful widgets every where, but your overall program would be heavier and would have a lot of redundancies.
That said, I don't think there is any advantage of using stateless widgets over stateful widgets.
In the Flutter docs they say that a Widget is a mutable configuration (description) for an Element, which I have no problem understanding.
But then we have the stateless widget, which extends the Widget class, and must implement a build method. The two classes look like they do the same job, can any one explain the difference ?
Thanks.
Widget is just an interface. You're never gonna use it directly, and this class does absolutely nothing.
It exists just for the compiler to know that your program is type-safe.
every thing with flutter made with widgets there is no widget only it is Stateless widgets and statefull widgets
and the stateless is static and the other is dynamic
stateless for fixed layouts if you need any dynamic programming like(validation, requests, listeners for buttons and others..)
you need to use Statefull widgets
for more information see here..
difference between stateless and statefull widgets