How do I disable a Button in Flutter?

I think you may want to introduce some helper functions to build your button as well as a Stateful widget along with some property to key off of.

• Use a StatefulWidget/State and create a variable to hold your condition (e.g. isButtonDisabled)
• Set this to true initially (if that's what you desire)
• When rendering the button, don't directly set the onPressed value to either null or some function onPressed: () {}
• Instead, conditionally set it using a ternary or a helper function (example below)
• Check the isButtonDisabled as part of this conditional and return either null or some function.
• When the button is pressed (or whenever you want to disable the button) use setState(() => isButtonDisabled = true) to flip the conditional variable.
• Flutter will call the build() method again with the new state and the button will be rendered with a null press handler and be disabled.

Here's is some more context using the Flutter counter project.

class MyHomePage extends StatefulWidget {  @override  _MyHomePageState createState() => new _MyHomePageState();}class _MyHomePageState extends State<MyHomePage> {  int _counter = 0;  bool _isButtonDisabled;  @override  void initState() {    _isButtonDisabled = false;  }  void _incrementCounter() {    setState(() {      _isButtonDisabled = true;      _counter++;    });  }  @override  Widget build(BuildContext context) {    return new Scaffold(      appBar: new AppBar(        title: new Text("The App"),      ),      body: new Center(        child: new Column(          mainAxisAlignment: MainAxisAlignment.center,          children: <Widget>[            new Text(              'You have pushed the button this many times:',            ),            new Text(              '$_counter',              style: Theme.of(context).textTheme.display1,            ),            _buildCounterButton(),          ],        ),      ),    );  }  Widget _buildCounterButton() {    return new RaisedButton(      child: new Text(        _isButtonDisabled ? "Hold on..." : "Increment"      ),      onPressed: _isButtonDisabled ? null : _incrementCounter,    );  }}

In this example I am using an inline ternary to conditionally set the Text and onPressed, but it may be more appropriate for you to extract this into a function (you can use this same method to change the text of the button as well):

Widget _buildCounterButton() {    return new RaisedButton(      child: new Text(        _isButtonDisabled ? "Hold on..." : "Increment"      ),      onPressed: _counterButtonPress(),    );  }  Function _counterButtonPress() {    if (_isButtonDisabled) {      return null;    } else {      return () {        // do anything else you may want to here        _incrementCounter();      };    }  }

According to the docs:

"If the onPressed callback is null, then the button will be disabled and by default will resemble a flat button in the disabledColor."

https://docs.flutter.io/flutter/material/RaisedButton-class.html

So, you might do something like this:

    RaisedButton(      onPressed: calculateWhetherDisabledReturnsBool() ? null : () => whatToDoOnPressed,      child: Text('Button text')    );

The simple answer is onPressed : null gives a disabled button.