Dart 学习笔记

https://dart.dev/guides/language/language-tour

A basic Dart program

// Define a function.
void printInteger(int aNumber) {
  print('The number is $aNumber.'); // Print to console.
}

// This is where the app starts executing.
void main() {
  var number = 42; // Declare and initialize a variable.
  printInteger(number); // Call a function.
}

• string literal - 'xxx' or "xxx"
• string interpolation - $variable or ${variable}

Important concepts

• Everything you can place in a variable is an object, and every object is an instance of a class.
• all objects inherit from the Object class.
• Dart is strongly typed
• You can make a variable nullable by putting a question mark (?) at the end of its type. For example, a variable of type int? might be an integer, or it might be null.
• If you enable null safety, variables can’t contain null unless you say they can.
• null is an object
• Dart supports generic types, like List<int>
• If an identifier starts with an underscore (_), it’s private to its library.

Variables

var name = 'Bob'; // 类型推断
Object name = 'Bob'; // 相当于弱类型
String name = 'Bob'; // 指明类型

• Production code ignores the assert() call.

late String description;

void main() {
  description = 'Feijoada!';
  print(description);
}

• A final variable can be set only once.
• A const variable is a compile-time constant. (Const variables are implicitly final.)
• Instance variables can be final but not const.
• You can also use const to create constant values.
• Although a final object cannot be modified, its fields can be changed. In comparison, a const object and its fields cannot be changed: they’re immutable.

Built-in types

• Numbers (int, double)
• Strings (String)
• Booleans (bool)
• Lists (List, also known as arrays)
• Sets (Set)
• Maps (Map)
• Runes (Runes; often replaced by the characters API)
• Symbols (Symbol)
• The value null (Null)

---

• Object: The superclass of all Dart classes except Null.
• Enum: The superclass of all enums.
• Future and Stream: Used in asynchrony support.
• Iterable: Used in for-in loops and in synchronous generator functions.
• Never: Indicates that an expression can never successfully finish evaluating. Most often used for functions that always throw an exception.
• dynamic: Indicates that you want to disable static checking. Usually you should use Object or Object? instead.
• void: Indicates that a value is never used. Often used as a return type.

To get the string corresponding to an object, Dart calls the object’s toString() method.

var s1 = '''
You can create
multi-line strings like this one.
''';

var s2 = """This is also a
multi-line string.""";

var s = r'In a raw string, not even \n gets special treatment.';

spread operator (...):

var list = [1, 2, 3];
var list2 = [0, ...list];
assert(list2.length == 4);

var listOfInts = [1, 2, 3];
var listOfStrings = ['#0', for (var i in listOfInts) '#$i'];
assert(listOfStrings[1] == '#1');

Set<String> names = {}; // This works, too.

Functions

Dart is a true object-oriented language, so even functions are objects and have a type, Function.

When defining a function, use {param1, param2, ...} to specify named parameters.

void enableFlags({bool bold = false, bool hidden = false}) {...}

The main() function returns void and has an optional List<String> parameter for arguments.

// Run the app like this: dart args.dart 1 test
void main(List<String> arguments) {
  print(arguments);

  assert(arguments.length == 2);
  assert(int.parse(arguments[0]) == 1);
  assert(arguments[1] == 'test');
}

args lib

函数作为变量：

var loudify = (msg) => '!!! ${msg.toUpperCase()} !!!';
assert(loudify('hello') == '!!! HELLO !!!');

匿名函数：

const list = ['apples', 'bananas', 'oranges'];
list.forEach((item) {
  print('${list.indexOf(item)}: $item');
});

All functions return a value. If no return value is specified, the statement return null; is implicitly appended to the function body.

Operators

assert(5 / 2 == 2.5); // Result is a double
assert(5 ~/ 2 == 2); // Result is an int

• Type test operators
  • as Typecast
  • is True if the object has the specified type
  • is! True if the object doesn’t have the specified type

condition ? expr1 : expr2 expr1 ?? expr2

Cascade notation

var paint = Paint()
  ..color = Colors.black
  ..strokeCap = StrokeCap.round
  ..strokeWidth = 5.0;

querySelector('#confirm') // Get an object.
  ?..text = 'Confirm' // Use its members.
  ..classes.add('important')
  ..onClick.listen((e) => window.alert('Confirmed!'))
  ..scrollIntoView();

var message = StringBuffer('Dart is fun');
for (var i = 0; i < 5; i++) {
  message.write('!');
}

var collection = [1, 2, 3];
collection.forEach(print); // 1 2 3

switch-case 需要有 break。

Classes

Use a type test operator rather than runtimeType to test an object’s type. In production environments, the test object is Type is more stable than the test object.runtimeType == Type.

构造函数：

class Point {
  double x = 0;
  double y = 0;

  Point(double x, double y) {
    // See initializing formal parameters for a better way
    // to initialize instance variables.
    this.x = x;
    this.y = y;
  }
}

Use this only when there is a name conflict. Otherwise, Dart style omits the this.

class Rectangle {
  double left, top, width, height;

  Rectangle(this.left, this.top, this.width, this.height);

  // Define two calculated properties: right and bottom.
  double get right => left + width;
  set right(double value) => left = value - width;
  double get bottom => top + height;
  set bottom(double value) => top = value - height;
}

void main() {
  var rect = Rectangle(3, 4, 20, 15);
  assert(rect.left == 3);
  rect.right = 12;
  assert(rect.left == -8);
}

Adding features to a class: mixins

class Musician extends Performer with Musical {
  // ···
}

class Maestro extends Person with Musical, Aggressive, Demented {
  Maestro(String maestroName) {
    name = maestroName;
    canConduct = true;
  }
}

Libraries

import 'dart:html';
import 'package:test/test.dart';
import 'package:lib1/lib1.dart';
import 'package:lib2/lib2.dart' as lib2;
import 'package:lib1/lib1.dart' show foo;
import 'package:lib2/lib2.dart' hide foo;

Typedefs

typedef IntList = List<int>;
IntList il = [1, 2, 3];