Call method of a widget from another widget

I need to call the setState() method on click of a button

You may have a few options (or alternatives) to achieve the end result (all with different tradeoffs):

• Elevate the state (i.e. count) to an Inherited Widget above the Button and Display Widget. This may be the easiest or most appropriate.
• Leverage some kind of Action-based communication such as Flutter Redux (where you dispatch an action, which affects the display widget via a StoreConnector and rebuilds). This can be seen as just another way to 'elevate' state. However, this requires a whole new dependency and a lot of overhead given your simple example, but I wanted to point it out.
• You can create some kind of Stream and StreamController that the Display widget subscribes/listens to. However, this may be overkill and I'm not sure how appropriate representing button clicks over a stream would be.

There may be other solutions that I'm not thinking of; however, remember that the goal of reactive UI is to keep state simple.

So if you have multiple leaf widgets that care about a piece of state (want to know it, want to change it), it might mean that this belongs to a higher level component (e.g a App-level piece of state, or maybe some other common ancestor - but both could use Inherited Widget to prevent passing the piece of state all around)

You should use your Scaffold in the State instead of using in StatelessWidget. Here is the working solution.

class MyApp extends StatelessWidget { @override build(BuildContext context) { return MaterialApp( title: "My app title", home: MyStateFullWidget()); } } class MyStateFullWidget extends StatefulWidget { @override State<StatefulWidget> createState() { return MyStateFullWidgetState(); } } class MyStateFullWidgetState extends State<MyStateFullWidget> { int count = 0; @override Widget build(BuildContext context) { return Scaffold( appBar: AppBar( title: Text("My App"), backgroundColor: Colors.amber, ), body: Container( child: Center( child:Text("Count: $count"), ), ), persistentFooterButtons: <Widget>[ FlatButton( onPressed: update, child: Text("Click Here"), color: Colors.amber, textColor: Colors.white, ), ], ); } void update() { setState(() { count++; }); } }