官网教程 https://www.dartlang.org/guides/language/language-tour

dart是一个单线程的语言,没有多线程

Final and const

If you never intend to change a variable, use final or const, either instead of var or in addition to a type. A final variable can be set only once; a const variable is a compile-time constant. (Const variables are implicitly final.) A final top-level or class variable is initialized the first time it’s used.

Here’s an example of creating and setting a final variable:

final name = 'Bob'; // Without a type annotation
// name = 'Alice';  // Uncommenting this causes an error
final String nickname = 'Bobby';

Use const for variables that you want to be compile-time constants. If the const variable is at the class level, mark it static const. Where you declare the variable, set the value to a compile-time constant such as a number or string literal, a const variable, or the result of an arithmetic operation on constant numbers:

const bar = 1000000; // Unit of pressure (dynes/cm2)
const double atm = 1.01325 * bar; // Standard atmosphere

The const keyword isn’t just for declaring constant variables. You can also use it to create constant values, as well as to declare constructors that create constant values. Any variable can have a constant value.

// Note: [] creates an empty list.
// const [] creates an empty, immutable list (EIL).
var foo = const []; // foo is currently an EIL.
final bar = const []; // bar will always be an EIL.
const baz = const []; // baz is a compile-time constant EIL.

// You can change the value of a non-final, non-const variable,
// even if it used to have a const value.
foo = [];

// You can't change the value of a final or const variable.
// bar = []; // Unhandled exception.
// baz = []; // Unhandled exception.

For more information on using const to create constant values, see Lists, Maps, and Classes.

 -----------------------------------------------------------

Default parameter values

Your function can use = to define default values for both named and positional parameters. The default values must be compile-time constants. If no default value is provided, the default value is null.

void doStuff(
    {List<int> list = const [1, 2, 3],
    Map<String, String> gifts = const {
      'first': 'paper',
      'second': 'cotton',
      'third': 'leather'
    }}) {
  print('list:  $list');
  print('gifts: $gifts');
}

 ------------------------------------------------------------

The main() function

Every app must have a top-level main() function, which serves as the entrypoint to the app. 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');
}

 ------------------------------------------------------------

Operators

if null 　　　　　　　　　　　　??

cascade　　　　　　　　　　　 ..

Arithmetic operators

/ 　　　　　　　　　　　　　　Divide
~/ 　　　　　　　　　　　　　　　Divide, returning an integer result

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

Type test operators

as 　　　　　　　　　　　　Typecast
is 　　　　　　　　　　　　True if the object has the specified type
is! 　　　　　　　　　　　　False if the object has the specified type

Assignment operators

As you’ve already seen, you can assign values using the = operator. To assign only if the assigned-to variable is null, use the ??= operator.

// Assign value to a
a = value;
// Assign value to b if b is null; otherwise, b stays the same
b ??= value;

Conditional expressions

condition ? expr1 : expr2　　If condition is true, evaluates expr1 (and returns its value); otherwise, evaluates and returns the value of expr2.

expr1 ?? expr2　　　　　　　　If expr1 is non-null, returns its value; otherwise, evaluates and returns the value of expr2.

Cascade notation (..)

Cascades (..) allow you to make a sequence of operations on the same object. In addition to function calls, you can also access fields on that same object. This often saves you the step of creating a temporary variable and allows you to write more fluid code.

Consider the following code:

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

The first method call, querySelector(), returns a selector object. The code that follows the cascade notation operates on this selector object, ignoring any subsequent values that might be returned.

The previous example is equivalent to:

var button = querySelector('#confirm');
button.text = 'Confirm';
button.classes.add('important');
button.onClick.listen((e) => window.alert('Confirmed!'));

You can also nest your cascades. For example:

final addressBook = (new AddressBookBuilder()
      ..name = 'jenny'
      ..email = '[email protected]'
      ..phone = (new PhoneNumberBuilder()
            ..number = '415-555-0100'
            ..label = 'home')
          .build())
    .build();

Be careful to construct your cascade on a function that returns an actual object. For example, the following code fails:

var sb = new StringBuffer();
sb.write('foo')
  ..write('bar'); // Error: method 'write' isn't defined for 'void'.

The sb.write() call returns void, and you can’t construct a cascade on void.

Other operators

?. 　　　　　　　　　　Conditional member access Like ., but the leftmost operand can be null; example: foo?.bar selects property bar from expression foo unless foo is null (in which case the value of foo?.bar is null)

------------------------------------------------------------

Exceptions

Your Dart code can throw and catch exceptions. Exceptions are errors indicating that something unexpected happened. If the exception isn’t caught, the isolate that raised the exception is suspended, and typically the isolate and its program are terminated.

In contrast to Java, all of Dart’s exceptions are unchecked exceptions. Methods do not declare which exceptions they might throw, and you are not required to catch any exceptions.

Dart provides Exception and Error types, as well as numerous predefined subtypes. You can, of course, define your own exceptions. However, Dart programs can throw any non-null object—not just Exception and Error objects—as an exception.

Throw

Here’s an example of throwing, or raising, an exception:

throw new FormatException('Expected at least 1 section');

You can also throw arbitrary objects:

throw 'Out of llamas!';

Because throwing an exception is an expression, you can throw exceptions in => statements, as well as anywhere else that allows expressions:

void distanceTo(Point other) =>
    throw new UnimplementedError();

------------------------------------------------------------

Classes

Dart is an object-oriented language with classes and mixin-based inheritance. Every object is an instance of a class, and all classes descend from Object. Mixin-based inheritance means that although every class (except for Object) has exactly one superclass, a class body can be reused in multiple class hierarchies.

To create an object, you can use the new keyword with a constructor for a class. Constructor names can be either ClassName or ClassName.identifier. For example:

var jsonData = jsonDecode('{"x":1, "y":2}');

// Create a Point using Point().
var p1 = new Point(2, 2);

// Create a Point using Point.fromJson().
var p2 = new Point.fromJson(jsonData);

Dart 2 note: You can omit the new before the constructor. Example: p1 = Point(2, 2)..

Some classes provide constant constructors. To create a compile-time constant using a constant constructor, use const instead of new:

var p = const ImmutablePoint(2, 2);

Dart 2 note: You can omit the const before the constructor. Example: p = ImmutablePoint(2, 2).

Constructing two identical compile-time constants results in a single, canonical instance:

var a = const ImmutablePoint(1, 1);
var b = const ImmutablePoint(1, 1);

assert(identical(a, b)); // They are the same instance!

To get an object’s type at runtime, you can use Object’s runtimeType property, which returns a Type object.

print('The type of a is ${a.runtimeType}');

Instance variables

Here’s how you declare instance variables:

class Point {
  num x; // Declare instance variable x, initially null.
  num y; // Declare y, initially null.
  num z = 0; // Declare z, initially 0.
}

All uninitialized instance variables have the value null.

All instance variables generate an implicit getter method. Non-final instance variables also generate an implicit setter method. For details, see Getters and setters.

class Point {
  num x;
  num y;
}

void main() {
  var point = new Point();
  point.x = 4; // Use the setter method for x.
  assert(point.x == 4); // Use the getter method for x.
  assert(point.y == null); // Values default to null.
}

If you initialize an instance variable where it is declared (instead of in a constructor or method), the value is set when the instance is created, which is before the constructor and its initializer list execute.

Constructors

Declare a constructor by creating a function with the same name as its class (plus, optionally, an additional identifier as described in Named constructors). The most common form of constructor, the generative constructor, creates a new instance of a class:

class Point {
  num x, y;

  Point(num x, num y) {
    // There's a better way to do this, stay tuned.
    this.x = x;
    this.y = y;
  }
}

The this keyword refers to the current instance.

The pattern of assigning a constructor argument to an instance variable is so common, Dart has syntactic sugar to make it easy:

class Point {
  num x, y;

  // Syntactic sugar for setting x and y
  // before the constructor body runs.
  Point(this.x, this.y);
}

Default constructors

If you don’t declare a constructor, a default constructor is provided for you. The default constructor has no arguments and invokes the no-argument constructor in the superclass.

Named constructors

Use a named constructor to implement multiple constructors for a class or to provide extra clarity:

class Point {
  num x, y;

  Point(this.x, this.y);

  // Named constructor
  Point.origin() {
    x = 0;
    y = 0;
  }
}

Remember that constructors are not inherited, which means that a superclass’s named constructor is not inherited by a subclass. If you want a subclass to be created with a named constructor defined in the superclass, you must implement that constructor in the subclass.

Invoking a non-default superclass constructor

By default, a constructor in a subclass calls the superclass’s unnamed, no-argument constructor. The superclass’s constructor is called at the beginning of the constructor body. If an initializer list is also being used, it executes before the superclass is called. In summary, the order of execution is as follows:

1. initializer list
2. superclass’s no-arg constructor
3. main class’s no-arg constructor

If the superclass doesn’t have an unnamed, no-argument constructor, then you must manually call one of the constructors in the superclass. Specify the superclass constructor after a colon (:), just before the constructor body (if any).

class Employee extends Person {
  Employee() : super.fromJson(getDefaultData());
  // ···
}

// Initializer list sets instance variables before
// the constructor body runs.
Point.fromJson(Map<String, num> json)
    : x = json['x'],
      y = json['y'] {
  print('In Point.fromJson(): ($x, $y)');
}

Point.withAssert(this.x, this.y) : assert(x >= 0) {
  print('In Point.withAssert(): ($x, $y)');
}

class Point {
  num x, y;

  // The main constructor for this class.
  Point(this.x, this.y);

  // Delegates to the main constructor.
  Point.alongXAxis(num x) : this(x, 0);
}

class ImmutablePoint {
  static final ImmutablePoint origin =
      const ImmutablePoint(0, 0);

  final num x, y;

  const ImmutablePoint(this.x, this.y);
}

class Logger {
  final String name;
  bool mute = false;

  // _cache is library-private, thanks to
  // the _ in front of its name.
  static final Map<String, Logger> _cache =
      <String, Logger>{};

  factory Logger(String name) {
    if (_cache.containsKey(name)) {
      return _cache[name];
    } else {
      final logger = new Logger._internal(name);
      _cache[name] = logger;
      return logger;
    }
  }

  Logger._internal(this.name);

  void log(String msg) {
    if (!mute) print(msg);
  }
}

var logger = new Logger('UI');
logger.log('Button clicked');

Methods are functions that provide behavior for an object.

Getters and setters

Getters and setters are special methods that provide read and write access to an object’s properties. Recall that each instance variable has an implicit getter, plus a setter if appropriate. You can create additional properties by implementing getters and setters, using the get and set keywords:

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

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

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

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

With getters and setters, you can start with instance variables, later wrapping them with methods, all without changing client code.

Abstract methods

Instance, getter, and setter methods can be abstract, defining an interface but leaving its implementation up to other classes. Abstract methods can only exist in abstract classes.

To make a method abstract, use a semicolon (;) instead of a method body:

abstract class Doer {
  // Define instance variables and methods...

  void doSomething(); // Define an abstract method.
}

class EffectiveDoer extends Doer {
  void doSomething() {
    // Provide an implementation, so the method is not abstract here...
  }
}

Calling an abstract method results in a runtime error.

Overridable operators

You can override the operators shown in the following table. For example, if you define a Vector class, you might define a + method to add two vectors.

Abstract classes

Use the abstract modifier to define an abstract class—a class that can’t be instantiated. Abstract classes are useful for defining interfaces, often with some implementation. If you want your abstract class to appear to be instantiable, define a factory constructor.

Implicit interfaces

Every class implicitly defines an interface containing all the instance members of the class and of any interfaces it implements. If you want to create a class A that supports class B’s API without inheriting B’s implementation, class A should implement the B interface.

A class implements one or more interfaces by declaring them in an implements clause and then providing the APIs required by the interfaces. For example:

// A person. The implicit interface contains greet().
class Person {
  // In the interface, but visible only in this library.
  final _name;

  // Not in the interface, since this is a constructor.
  Person(this._name);

  // In the interface.
  String greet(String who) => 'Hello, $who. I am $_name.';
}

// An implementation of the Person interface.
class Impostor implements Person {
  get _name => '';

  String greet(String who) => 'Hi $who. Do you know who I am?';
}

String greetBob(Person person) => person.greet('Bob');

void main() {
  print(greetBob(new Person('Kathy')));
  print(greetBob(new Impostor()));
}

Here’s an example of specifying that a class implements multiple interfaces:

class Point implements Comparable, Location {
  // ···
}

Extending a class

Use extends to create a subclass, and super to refer to the superclass:

class Television {
  void turnOn() {
    _illuminateDisplay();
    _activateIrSensor();
  }
  // ···
}

class SmartTelevision extends Television {
  void turnOn() {
    super