Re: How can setState() be called in a Stateless Widget?

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Steven McDowall

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Mar 4, 2019, 6:21:04 AM3/4/19

to Kamil Geagea, Flutter Dev

I don't see where setState() is being called in a Stateless widget at all ? I see setState being used in _ProductManagerState which is certainly a STATE widget ..

class _ProductManagerState extends State<ProductManager> {

Maybe read this?

https://flutterdoc.com/stateful-or-stateless-widgets-42a132e529ed

On Mar 3, 2019, at 11:09 PM, Kamil Geagea <KamilGe...@gmail.com> wrote:

Hello, I am following this online course where we just separated the RaisedButton from the Stateful Widget to a Stateless Widget created in the State object.
I don't understand how setState() is called in a StatelessWidget...
When we create the element ProductControl we pass the function _addProduct() that calls setState(). And when the button is pressed, setState() is called in ProductControl which is a Stateless Widget...
Can someone explain me how does it work please? Thank you

Here is some code:
//StatefulWidget
class ProductManager extends StatefulWidget {
final String startingProduct;

ProductManager({this.startingProduct = 'Sweets Tester'});

@override
State<StatefulWidget> createState() {
return _ProductManagerState();
}
}

class _ProductManagerState extends State<ProductManager> {
List<String> _products = [];

@override
void initState() {

super.initState();

_products.add(widget.startingProduct);
}

void _addProduct(String product) {
setState((){
_products.add(product);
});
}

@override
Widget build(BuildContext context) {
return Column(
children: <Widget> [
Container(
margin: EdgeInsets.all(10.0),
child: ProductControl(_addProduct),
),
Products(_products),
]
);
}
}

// ProductControl() in another file

class ProductControl extends StatelessWidget {
final Function addProduct;

ProductControl(this.addProduct);

@override
Widget build(BuildContext context) {
return RaisedButton(
onPressed: () {
addProduct('Sweets');
},
child: Text('Add Product'),
);
}
}

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Pedro Massango

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Mar 4, 2019, 6:35:58 AM3/4/19

to Steven McDowall, Kamil Geagea, Flutter Dev

setState() can't be called in a statelessWidget

Ian B

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Mar 4, 2019, 6:37:34 AM3/4/19

to Kamil Geagea, Flutter Dev

I think the issue you are probably seeing, is that the call is instigated from a stateless widget, but that function was passed in via a stateful widget.

So when setState is called, it's actually referencing the _ProductManagerState.

It may be helpful to look at topics like closures in methods, as it's related.

Ian Hickson

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Mar 6, 2019, 5:31:00 PM3/6/19

to Kamil Geagea, Flutter Dev

The difference between a StatefulWidget and a StatelessWidget is that a StatefulWidget has state and a StatelessWidget does not. You can setState when you have state, and when you do not have state, you cannot setState.

--

Ian Hickson

Mykhailo Yarushevskyi

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Mar 8, 2019, 4:31:50 PM3/8/19

to Flutter Dev

In your case, your ProductManager (StateFulWidget) via constructor of ProductControl (StatelessWidget) shares access to an instance of a class Function '_addProduct', and you call this method from the instance of the ProductManager when clicked a RaisedButton 'Add Product'. Since this method executing in the ProductManager (StateFulWidget) context, therefore management of the state (a renewval ProductManager as StateFulWidget) via setState() method work well.

понеділок, 4 березня 2019 р. 06:09:25 UTC+2 користувач Kamil Geagea написав:

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