{"id":9091,"date":"2022-06-19T18:38:03","date_gmt":"2022-06-19T16:38:03","guid":{"rendered":"https:\/\/via-internet.de\/blog\/?p=9091"},"modified":"2022-06-21T09:29:46","modified_gmt":"2022-06-21T07:29:46","slug":"dart-learning-dart","status":"publish","type":"post","link":"https:\/\/via-internet.de\/blog\/2022\/06\/19\/dart-learning-dart\/","title":{"rendered":"Dart | Learning Dart"},"content":{"rendered":"\n

Introduction<\/h2>\n\n\n\n

Creating a simple class<\/h3>\n\n\n\n
class Bicycle {\n  int cadence;\n  int _speed = 0;\n  int get speed => _speed;\n  int gear;\n\n  Bicycle(this.cadence, this.gear);\n\n  void applyBrake(int decrement) {\n    _speed -= decrement;\n  }\n\n  void speedUp(int increment) {\n    _speed += increment;\n  }\n\n  @override\n  String toString() => 'Bicycle: $_speed mph';\n}\n\nvoid main() {\n  var bike = Bicycle(2, 1);\n  print(bike);\n}<\/pre>\n\n\n\n
Using optional parameters<\/h3>\n\n\n\n
import 'dart:math';\n\nclass Rectangle {\n  int width = 0;\n  int height = 0;\n  Point origin = Point(0, 0);\n\n  Rectangle({this.origin = const Point(0, 0), this.width = 0, this.height = 0});\n\n  @override\n  String toString() =>\n      'Origin: (${origin.x}, ${origin.y}), width: $width, height: $height';\n}\n\nmain() {\n  print(Rectangle(origin: const Point(10, 20), width: 100, height: 200));\n  print(Rectangle(origin: const Point(10, 10)));\n  print(Rectangle(width: 200));\n  print(Rectangle());\n}\n<\/pre>\n\n\n\n
Create a factory<\/h3>\n\n\n\n
import 'dart:math';\n\nabstract class Shape {\n  \/\/ Option 2: Create a factory constructor\n\n  factory Shape(String type) {\n    if (type == 'circle') return Circle(2);\n    if (type == 'square') return Square(2);\n    throw 'Can\\'t create $type.';\n  }\n\n  num get area;\n}\n\nclass Circle implements Shape {\n  final num radius;\n  Circle(this.radius);\n\n  @override\n  num get area => pi * pow(radius, 2);\n}\n\nclass Square implements Shape {\n  final num side;\n  Square(this.side);\n\n  @override\n  num get area => pow(side, 2);\n}\n\n\/\/ Option 1: Create a top-level function\nShape shapeFactory(String type) {\n  if (type == 'circle') return Circle(2);\n  if (type == 'square') return Square(2);\n  throw 'Can\\'t create $type.';\n}\n\nmain() {\n  \/\/ final circle = Circle(2);\n  \/\/ final square = Square(2);\n\n  \/\/ Option 1: Create a top-level function\n  \/\/ final circle = shapeFactory('circle');\n  \/\/ final square = shapeFactory('square');\n\n  \/\/ Option 2: Create a factory constructor\n  final circle = Shape('circle');\n  final square = Shape('square');\n  \n  print(circle.area);\n  print(square.area);\n}\n<\/pre>\n\n\n\n
Implement an interface<\/h3>\n\n\n\n
class CircleMock implements Circle {\n  num area = 0;\n  num radius = 0;\n}<\/pre>\n\n\n\n
Use Dart for functional programming<\/h3>\n\n\n\n
String scream(int length) => \"A${'a' * length}h!\";\n\nmain() {\n  final values = [1, 2, 3, 5, 10, 50];\n  for (var length in values) {\n    print(scream(length));\n  }\n  \n  \/\/ Functional\n  values.map(scream).forEach(print);\n}<\/pre>\n\n\n\n
Language samples<\/h2>\n\n\n\n
The following is copied from the Dart Website.<\/a><\/p>\n\n\n\n
<\/a>Hello World<\/h3>\n\n\n\n
Every app has a main()<\/code> function. To display text on the console, you can use the top-level print()<\/code> function:<\/p>\n\n\n\n
void main() {\n  print('Hello, World!');\n}<\/pre>\n\n\n\n
<\/a>Variables<\/h3>\n\n\n\n
Even in type-safe Dart code, most variables don\u2019t need explicit types, thanks to type inference:<\/p>\n\n\n\n
var name = 'Voyager I';\nvar year = 1977;\nvar antennaDiameter = 3.7;\nvar flybyObjects = ['Jupiter', 'Saturn', 'Uranus', 'Neptune'];\nvar image = {\n  'tags': ['saturn'],\n  'url': '\/\/path\/to\/saturn.jpg'\n};<\/pre>\n\n\n\n
Read more<\/a> about variables in Dart, including default values, the final<\/code> and const<\/code> keywords, and static types.<\/p>\n\n\n\n
<\/a>Control flow statements<\/h3>\n\n\n\n
Dart supports the usual control flow statements:<\/p>\n\n\n\n
if (year >= 2001) {\n  print('21st century');\n} else if (year >= 1901) {\n  print('20th century');\n}\n\nfor (final object in flybyObjects) {\n  print(object);\n}\n\nfor (int month = 1; month <= 12; month++) {\n  print(month);\n}\n\nwhile (year > 2016) {\n  year += 1;\n}<\/pre>\n\n\n\n
Read more<\/a> about control flow statements in Dart, including break<\/code> and continue<\/code>, switch<\/code> and case<\/code>, and assert<\/code>.<\/p>\n\n\n\n
<\/a>Functions<\/h3>\n\n\n\n
We recommend<\/a> specifying the types of each function\u2019s arguments and return value:<\/p>\n\n\n\n
int fibonacci(int n) {\n  if (n == 0 || n == 1) return n;\n  return fibonacci(n - 1) + fibonacci(n - 2);\n}\n\nvar result = fibonacci(20);<\/pre>\n\n\n\n
A shorthand =><\/code> (arrow<\/em>) syntax is handy for functions that contain a single statement. This syntax is especially useful when passing anonymous functions as arguments:<\/p>\n\n\n\n
flybyObjects.where((name) => name.contains('turn')).forEach(print);<\/pre>\n\n\n\n
Besides showing an anonymous function (the argument to where()<\/code>), this code shows that you can use a function as an argument: the top-level print()<\/code> function is an argument to forEach()<\/code>.<\/p>\n\n\n\n
Read more<\/a> about functions in Dart, including optional parameters, default parameter values, and lexical scope.<\/p>\n\n\n\n
<\/a>Comments<\/h3>\n\n\n\n
Dart comments usually start with \/\/<\/code>.<\/p>\n\n\n\n
\/\/ This is a normal, one-line comment.\n\n\/\/\/ This is a documentation comment, used to document libraries,\n\/\/\/ classes, and their members. Tools like IDEs and dartdoc treat\n\/\/\/ doc comments specially.\n\n\/* Comments like these are also supported. *\/\n<\/pre>\n\n\n\n
Read more<\/a> about comments in Dart, including how the documentation tooling works.<\/p>\n\n\n\n
<\/a>Imports<\/h3>\n\n\n\n
To access APIs defined in other libraries, use import<\/code>.<\/p>\n\n\n\n
\/\/ Importing core libraries\nimport 'dart:math';\n\n\/\/ Importing libraries from external packages\nimport 'package:test\/test.dart';\n\n\/\/ Importing files\nimport 'path\/to\/my_other_file.dart';<\/pre>\n\n\n\n
Read more<\/a> about libraries and visibility in Dart, including library prefixes, show<\/code> and hide<\/code>, and lazy loading through the deferred<\/code> keyword.<\/p>\n\n\n\n
<\/a>Classes<\/h3>\n\n\n\n
Here\u2019s an example of a class with three properties, two constructors, and a method. One of the properties can\u2019t be set directly, so it\u2019s defined using a getter method (instead of a variable).<\/p>\n\n\n\n
class Spacecraft {\n  String name;\n  DateTime? launchDate;\n\n  \/\/ Read-only non-final property\n  int? get launchYear => launchDate?.year;\n\n  \/\/ Constructor, with syntactic sugar for assignment to members.\n  Spacecraft(this.name, this.launchDate) {\n    \/\/ Initialization code goes here.\n  }\n\n  \/\/ Named constructor that forwards to the default one.\n  Spacecraft.unlaunched(String name) : this(name, null);\n\n  \/\/ Method.\n  void describe() {\n    print('Spacecraft: $name');\n    \/\/ Type promotion doesn't work on getters.\n    var launchDate = this.launchDate;\n    if (launchDate != null) {\n      int years = DateTime.now().difference(launchDate).inDays ~\/ 365;\n      print('Launched: $launchYear ($years years ago)');\n    } else {\n      print('Unlaunched');\n    }\n  }\n}<\/pre>\n\n\n\n
You might use the Spacecraft<\/code> class like this:<\/p>\n\n\n\n
var voyager = Spacecraft('Voyager I', DateTime(1977, 9, 5));\nvoyager.describe();\n\nvar voyager3 = Spacecraft.unlaunched('Voyager III');\nvoyager3.describe();<\/pre>\n\n\n\n
Read more<\/a> about classes in Dart, including initializer lists, optional new<\/code> and const<\/code>, redirecting constructors, factory<\/code> constructors, getters, setters, and much more.<\/p>\n\n\n\n
<\/a>Enums<\/h3>\n\n\n\n
Enums are a way of enumerating a predefined set of values or instances in a way which ensures that there cannot be any other instances of that type.<\/p>\n\n\n\n
Here is an example of a simple enum<\/code> that defines a simple list of predefined planet types:<\/p>\n\n\n\n
enum PlanetType { terrestrial, gas, ice }<\/pre>\n\n\n\n
Here is an example of an enhanced enum declaration of a class describing planets, with a defined set of constant instances, namely the planets of our own solar system.<\/p>\n\n\n\n
\/\/\/ Enum that enumerates the different planets in our solar system\n\/\/\/ and some of their properties.\nenum Planet {\n  mercury(planetType: PlanetType.terrestrial, moons: 0, hasRings: false),\n  venus(planetType: PlanetType.terrestrial, moons: 0, hasRings: false),\n  \/\/ \u00b7\u00b7\u00b7\n  uranus(planetType: PlanetType.ice, moons: 27, hasRings: true),\n  neptune(planetType: PlanetType.ice, moons: 14, hasRings: true);\n\n  \/\/\/ A constant generating constructor\n  const Planet(\n      {required this.planetType, required this.moons, required this.hasRings});\n\n  \/\/\/ All instance variables are final\n  final PlanetType planetType;\n  final int moons;\n  final bool hasRings;\n\n  \/\/\/ Enhanced enums support getters and other methods\n  bool get isGiant =>\n      planetType == PlanetType.gas || planetType == PlanetType.ice;\n}<\/pre>\n\n\n\n
You might use the Planet<\/code> enum like this:<\/p>\n\n\n\n
final yourPlanet = Planet.earth;\n\nif (!yourPlanet.isGiant) {\n  print('Your planet is not a \"giant planet\".');\n}<\/pre>\n\n\n\n
Read more<\/a> about enums in Dart, including enhanced enum requirements, automatically introduced properties, accessing enumerated value names, switch statement support, and much more.<\/p>\n\n\n\n
<\/a>Inheritance<\/h3>\n\n\n\n
Dart has single inheritance.<\/p>\n\n\n\n
class Orbiter extends Spacecraft {\n  double altitude;\n\n  Orbiter(super.name, DateTime super.launchDate, this.altitude);\n}<\/pre>\n\n\n\n
Read more<\/a> about extending classes, the optional @override<\/code> annotation, and more.<\/p>\n\n\n\n
<\/a>Mixins<\/h3>\n\n\n\n
Mixins are a way of reusing code in multiple class hierarchies. The following is a mixin declaration:<\/p>\n\n\n\n
mixin Piloted {\n  int astronauts = 1;\n\n  void describeCrew() {\n    print('Number of astronauts: $astronauts');\n  }\n}<\/pre>\n\n\n\n
To add a mixin\u2019s capabilities to a class, just extend the class with the mixin.<\/p>\n\n\n\n
class PilotedCraft extends Spacecraft with Piloted {\n  \/\/ \u00b7\u00b7\u00b7\n}<\/pre>\n\n\n\n
PilotedCraft<\/code> now has the astronauts<\/code> field as well as the describeCrew()<\/code> method.<\/p>\n\n\n\n
Read more<\/a> about mixins.<\/p>\n\n\n\n
<\/a>Interfaces and abstract classes<\/h3>\n\n\n\n
Dart has no interface<\/code> keyword. Instead, all classes implicitly define an interface. Therefore, you can implement any class.<\/p>\n\n\n\n
class MockSpaceship implements Spacecraft {\n  \/\/ \u00b7\u00b7\u00b7\n}<\/pre>\n\n\n\n
Read more<\/a> about implicit interfaces.<\/p>\n\n\n\n
You can create an abstract class to be extended (or implemented) by a concrete class. Abstract classes can contain abstract methods (with empty bodies).<\/p>\n\n\n\n
abstract class Describable {\n  void describe();\n\n  void describeWithEmphasis() {\n    print('=========');\n    describe();\n    print('=========');\n  }\n}<\/pre>\n\n\n\n
Any class extending Describable<\/code> has the describeWithEmphasis()<\/code> method, which calls the extender\u2019s implementation of describe()<\/code>.<\/p>\n\n\n\n
Read more<\/a> about abstract classes and methods.<\/p>\n\n\n\n
<\/a>Async<\/h3>\n\n\n\n
Avoid callback hell and make your code much more readable by using async<\/code> and await<\/code>.<\/p>\n\n\n\n
const oneSecond = Duration(seconds: 1);\n\/\/ \u00b7\u00b7\u00b7\nFuture<void> printWithDelay(String message) async {\n  await Future.delayed(oneSecond);\n  print(message);\n}<\/pre>\n\n\n\n
The method above is equivalent to:<\/p>\n\n\n\n
Future<void> printWithDelay(String message) {\n  return Future.delayed(oneSecond).then((_) {\n    print(message);\n  });\n}<\/pre>\n\n\n\n
As the next example shows, async<\/code> and await<\/code> help make asynchronous code easy to read.<\/p>\n\n\n\n
Future<void> createDescriptions(Iterable<String> objects) async {\n  for (final object in objects) {\n    try {\n      var file = File('$object.txt');\n      if (await file.exists()) {\n        var modified = await file.lastModified();\n        print(\n            'File for $object already exists. It was modified on $modified.');\n        continue;\n      }\n      await file.create();\n      await file.writeAsString('Start describing $object in this file.');\n    } on IOException catch (e) {\n      print('Cannot create description for $object: $e');\n    }\n  }\n}<\/pre>\n\n\n\n
You can also use async*<\/code>, which gives you a nice, readable way to build streams.<\/p>\n\n\n\n
Stream<String> report(Spacecraft craft, Iterable<String> objects) async* {\n  for (final object in objects) {\n    await Future.delayed(oneSecond);\n    yield '${craft.name} flies by $object';\n  }\n}<\/pre>\n\n\n\n
Read more<\/a> about asynchrony support, including async<\/code> functions, Future<\/code>, Stream<\/code>, and the asynchronous loop (await for<\/code>).<\/p>\n\n\n\n
<\/a>Exceptions<\/h3>\n\n\n\n
To raise an exception, use throw<\/code>:<\/p>\n\n\n\n
if (astronauts == 0) {\n  throw StateError('No astronauts.');\n}<\/pre>\n\n\n\n
To catch an exception, use a try<\/code> statement with on<\/code> or catch<\/code> (or both):<\/p>\n\n\n\n
try {\n  for (final object in flybyObjects) {\n    var description = await File('$object.txt').readAsString();\n    print(description);\n  }\n} on IOException catch (e) {\n  print('Could not describe object: $e');\n} finally {\n  flybyObjects.clear();\n}<\/pre>\n\n\n\n
Note that the code above is asynchronous; try<\/code> works for both synchronous code and code in an async<\/code> function.<\/p>\n\n\n\n
Read more<\/a> about exceptions, including stack traces, rethrow<\/code>, and the difference between Error<\/code> and Exception<\/code>.<\/p>\n\n\n\n
<\/a>Other topics<\/h2>\n\n\n\n
Many more code samples are in the language tour<\/a> and the library tour<\/a>. Also see the Dart API reference,<\/a> which often contains examples.<\/p>\n\n\n\n
<\/p>\n\n\n\n
Learn more<\/h2>\n\n\n\n
Articles<\/strong><\/p>\n\n\n\n