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.

So, you might do something like this:

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