musicus_mobile/lib/app.dart

import 'dart:async';

import 'package:flutter/material.dart';

import 'backend.dart';
import 'screens/home.dart';
import 'widgets/player_bar.dart';

class App extends StatefulWidget {
  @override
  _AppState createState() => _AppState();
}

class _AppState extends State<App> with SingleTickerProviderStateMixin {
  final nestedNavigator = GlobalKey<NavigatorState>();

  AnimationController playerBarAnimation;
  Backend backend;
  StreamSubscription<bool> playerActiveSubscription;

  @override
  void initState() {
    super.initState();

    playerBarAnimation = AnimationController(
      vsync: this,
      duration: Duration(milliseconds: 300),
    );
  }

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();

    backend = Backend.of(context);
    playerBarAnimation.value = backend.playerActive.value ? 1.0 : 0.0;

    if (playerActiveSubscription != null) {
      playerActiveSubscription.cancel();
    }

    playerActiveSubscription = backend.playerActive.listen((active) =>
        active ? playerBarAnimation.forward() : playerBarAnimation.reverse());
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Musicus',
      theme: ThemeData(
        brightness: Brightness.dark,
        accentColor: Colors.amber,
        textSelectionColor: Colors.grey[600],
        cursorColor: Colors.amber,
        textSelectionHandleColor: Colors.amber,
        toggleableActiveColor: Colors.amber,
        // Added for sliders. Not everything seems to obey this.
        colorScheme: ColorScheme.dark(
          primary: Colors.amber,
        ),
        fontFamily: 'Libertinus Sans',
      ),
      // The nested Navigator is for every screen from which the player bar at
      // the bottom should be accessible. The WillPopScope widget intercepts
      // taps on the system back button and redirects them to the nested
      // navigator.
      home: WillPopScope(
        onWillPop: () async => !(await nestedNavigator.currentState.maybePop()),
        child: Scaffold(
          body: Navigator(
            key: nestedNavigator,
            onGenerateRoute: (settings) => settings.name == '/'
                ? MaterialPageRoute(
                    builder: (context) => HomeScreen(),
                  )
                : null,
            initialRoute: '/',
          ),
          bottomNavigationBar: SizeTransition(
            sizeFactor: CurvedAnimation(
              curve: Curves.easeOut,
              parent: playerBarAnimation,
            ),
            axisAlignment: -1.0,
            child: PlayerBar(),
          ),
        ),
      ),
    );
  }

  @override
  void dispose() {
    super.dispose();
    playerActiveSubscription.cancel();
  }
}