Setstate Flutter

Question

Good afternoon.

I’m starting with flutter (I came from Reactnative), watching some courses... and I got the question: What does setState really control?

In Reactnative he looks at everything we set in the "state" object through the setState() method. But in the flutter I have this code snippet:

setState(() {
   bloc.calculate();
});

And it updates on the screen the data that is stored in the class instance. Soon everything I create, new Bloc(), new Map... within the state class of my Komponent will be "observed"?

Answer 1

In the document of setState() we can find the following:

• Notify the framework that the Internal state of this Object has changed.
• Calling setState notifies the framework that the Internal state of this Object has changed in a way that Might Impact the user interface in this subtree, which causes the framework to Schedule a build for this State Object.

So we can understand that the setState() when called, will redraw your widget or better, will call your method build again, but will not be "listening" to the changes... If there is any other change to the object, to be redesigned again, you need to call the setState() once again.

Example

In the example below, when calling setState(), you will be calling again build, so the data on the screen will be redesigned with the new values set.

class Cliente{
  String nome = "";
  String sobrenome = "";
}

class CadastroCliente extends StatefulWidget {
  const CadastroCliente({ Key key }) : super(key: key);

  @override
  _CadastroClienteState createState() => _CadastroClienteState();
}

class _CadastroClienteState extends State<CadastroCliente> {

  Cliente cliente = Cliente();
  String apelido = "";

  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget> [
        Column(
          children: <Widget>[
            Text(cliente.nome),
            Text(cliente.sobrenome),
            Text(apelido),
            RaisedButton( /* Botão 1 */
              child: Icon(Icons.refresh),
              onPressed: (){
                setState((){
                  cliente.nome = "Matheus";
                });
                cliente.sobrenome = "Ribeiro";
                apelido = "Matt";
              }
            ),
          ]
        )
      ]
    );
  }
}

~~ It’s just an example for understanding, there may be some errors when trying to run :D ~~

I can not say for sure why, but it is advisable you set the values within the

setState((){ 
   cliente.nome = "Matheus";
   cliente.sobrenome = "Ribeiro";
   apelido = "Matt";
});

for being a synchronous method, it is good to do what you need within yourself.

And yes, all the objects that are in _CadastroClienteState shall be tested for new values at the time when the build be called again.

Observing

If you are using the Bloc standard, you will not use the setState(), of a study on, for it is interesting you use StreamBuilder within a StatelessWidget.