Retrieve the dimensions of a rendered widget

Question

Suppose a Widget has a varying size based on certain properties of the underlying data structure.

May I render a Widget offscreen just to measure its width and height?

How would I do that?

Update - 2020-09-25

I need the dimensions of one rendered widget to assign correct dimension to compute and assign dimensions to the items in a widget tree to get rendered.

Thus I need to retrieve dimensions before I use them in the widget tree.

Answer 1

You can make a widget (either visible or not, use the Opacity widget to hide it if necessary), give it a GlobalKey, and then use that key to get its render object, then get the size from it.

For example, I added 3 lines of code, to the default "flutter counter example" project. Now clicking on the floating action button will print out the size of the column.

Sample output: Size(297.0, 741.0)

Code:

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key, required this.title}) : super(key: key);

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final GlobalKey _key = GlobalKey(); // <--- 1. add this line
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
    print(_key.currentContext?.size);  // <--- 2. add this line
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          key: _key,  // <--- 3. add this line
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            const Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ),
    );
  }
}