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samples/null_safety/null_safe_app/lib/goodapp.dart

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2.3 KiB

// Copyright 2020, the Flutter project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.
import 'package:flutter/material.dart';
import 'services.dart';
class GoodMyApp extends StatelessWidget {
const GoodMyApp({Key? key}) : super(key: key);
@override
Widget build(BuildContext context) {
final localizedAppName = Config.getAppName();
final temperatures = WeatherService.getTemperatures();
List<Widget> tempWidgets;
if (temperatures == null) {
tempWidgets = [const Text('Temperature: Failed getting forecast :-(')];
} else {
// Null safety uses flow analysis. The code checked for null in the branch
// above, so in this branch it known that temperatures cannot be null.
// Notice how we access temperatures without getting an analysis error.
tempWidgets = [
const Text('Temperature next 3 days:'),
for (final t in temperatures)
// We first use the conditional member access operator to only call
// round() and toString() if t isn't null. We then test if that
// succeeded, and if not use a null conditional operator to supply a
// value handling the error case.
Text(t?.round().toString() ?? 'no forecast'),
];
}
return MaterialApp(
home: Scaffold(
// Here we again use the null conditional operator.
appBar: AppBar(title: Text(localizedAppName ?? 'Weather')),
body: Padding(
padding: const EdgeInsets.all(8.0),
child: Column(
crossAxisAlignment: CrossAxisAlignment.start,
// Notice how we're *not* getting an error saying that `tempWidgets`
// is null here despite tempWidgets being declared in line 9 without
// an initial value. This is because of null safety's definite
// assignment feature: We know that in all branches of the code
// (both the if- and else- branches above), the variable was
// assigned a value, and thus isn't null. Try removing the assigment
// from the if-branch and notice the immediate error.
children: tempWidgets,
),
),
),
);
}
}