medium_06_layout_constraints.dart

import 'package:flutter/material.dart';

GlobalKey _keyMyApp = GlobalKey();
GlobalKey _keyMaterialApp = GlobalKey();
GlobalKey _keyHomePage = GlobalKey();
GlobalKey _keyScaffold = GlobalKey();
GlobalKey _keyAppbar = GlobalKey();
GlobalKey _keyCenter = GlobalKey();
GlobalKey _keyFAB = GlobalKey();
GlobalKey _keyText = GlobalKey();

void printConstraint(String name, BoxConstraints c) {
  print(
    'CONSTRAINT of $name: min(w=${c.minWidth.toInt()},h=${c.minHeight.toInt()}) max(w=${c.maxWidth.toInt()},h=${c.maxHeight.toInt()})',
  );
}

void printSizes() {
  printSize('MyApp', _keyMyApp);
  printSize('MaterialApp', _keyMaterialApp);
  printSize('HomePage', _keyHomePage);
  printSize('Scaffold', _keyScaffold);
  printSize('Appbar', _keyAppbar);
  printSize('Center', _keyCenter);
  printSize('Text', _keyText);
  printSize('FAB', _keyFAB);
}

void printSize(String name, GlobalKey key) {
  final RenderBox renderBox = key.currentContext.findRenderObject();
  final size = renderBox.size;
  print("SIZE of $name: w=${size.width.toInt()},h=${size.height.toInt()}");
}

void printPositions() {
  printPosition('MyApp', _keyMyApp);
  printPosition('MaterialApp', _keyMaterialApp);
  printPosition('HomePage', _keyHomePage);
  printPosition('Scaffold', _keyScaffold);
  printPosition('Appbar', _keyAppbar);
  printPosition('Center', _keyCenter);
  printPosition('Text', _keyText);
  printPosition('FAB', _keyFAB);
}

void printPosition(String name, GlobalKey key) {
  final RenderBox renderBox = key.currentContext.findRenderObject();
  final position = renderBox.localToGlobal(Offset.zero);
  print("POSITION of $name: $position ");
}

void main() {
  runApp(LayoutBuilder(
    builder: (context, constraints) {
      printConstraint('MyApp', constraints);
      return MyApp();
    },
  ));
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return LayoutBuilder(
      key: _keyMyApp,
      builder: (context, constraints) {
        printConstraint('MaterialApp', constraints);
        return MaterialApp(
          key: _keyMaterialApp,
          title: 'Flutter Demo',
          theme: ThemeData(
            primarySwatch: Colors.blue,
            visualDensity: VisualDensity.adaptivePlatformDensity,
          ),
          home: LayoutBuilder(
            builder: (context, constraints) {
              printConstraint('HomePage', constraints);
              return HomePage(
                key: _keyHomePage,
                title: 'Flutter Demo Home Page',
              );
            },
          ),
        );
      },
    );
  }
}

class HomePage extends StatefulWidget {
  HomePage({Key key, this.title}) : super(key: key);
  final String title;

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

class _HomePageState extends State<HomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  void initState() {
    WidgetsBinding.instance.addPostFrameCallback(_afterLayout);
    super.initState();
  }

  void _afterLayout(_) {
    printSizes();
    printPositions();
  }

  @override
  Widget build(BuildContext context) {
    return LayoutBuilder(builder: (context, constraints) {
      printConstraint('Scaffold', constraints);
      return Scaffold(
        backgroundColor: Colors.purple,
        key: _keyScaffold,
        appBar: AppBar(
          key: _keyAppbar,
          title: Text(widget.title),
        ),
        body: LayoutBuilder(
          builder: (context, constraints) {
            printConstraint('Center', constraints);
            return Center(
              key: _keyCenter,
              child: LayoutBuilder(builder: (context, constraints) {
                printConstraint('Text', constraints);
                return Text(
                  'You have pushed the button this many times:',
                  key: _keyText,
                  style: TextStyle(color: Colors.white),
                );
              }),
            );
          },
        ),
        floatingActionButton: LayoutBuilder(
          builder: (context, constraints) {
            printConstraint('FAB', constraints);
            return FloatingActionButton(
              key: _keyFAB,
              onPressed: printSizes,
              tooltip: 'Increment',
              child: Icon(Icons.add),
            );
          },
        ),
      );
    });
  }
}