Dart语言

Variables

var name = 'Bob';
dynamic name = 'Bob';
String name = 'Bob';

Default value

int lineCount;
assert(lineCount == null);

Final and const

final name = 'Bob'; // Without a type annotation
final String nickname = 'Bobby';
name = 'Alice'; // Error: a final variable can only be set once.

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

var foo = const [];
final bar = const [];
const baz = []; // Equivalent to `const []`
foo = [1, 2, 3]; // Was const []
baz = [42]; // Error: Constant variables can't be assigned a value.

Built-in types

• numbers
  int

var x = 1;
var hex = 0xDEADBEEF;
var y = 1.1;
var exponents = 1.42e5;

double

double z = 1; // Equivalent to double z = 1.0.

convert

// String -> int
var one = int.parse('1');
assert(one == 1);

// String -> double
var onePointOne = double.parse('1.1');
assert(onePointOne == 1.1);

// int -> String
String oneAsString = 1.toString();
assert(oneAsString == '1');

// double -> String
String piAsString = 3.14159.toStringAsFixed(2);
assert(piAsString == '3.14');

assert((3 << 1) == 6); // 0011 << 1 == 0110
assert((3 >> 1) == 1); // 0011 >> 1 == 0001
assert((3 | 4) == 7); // 0011 | 0100 == 0111

const msPerSecond = 1000;
const secondsUntilRetry = 5;
const msUntilRetry = secondsUntilRetry * msPerSecond;

• strings

var s1 = 'Single quotes work well for string literals.';
var s2 = "Double quotes work just as well.";
var s3 = 'It\'s easy to escape the string delimiter.';
var s4 = "It's even easier to use the other delimiter.";

var s = 'string interpolation';

assert('Dart has $s, which is very handy.' ==
    'Dart has string interpolation, ' +
        'which is very handy.');
assert('That deserves all caps. ' +
        '${s.toUpperCase()} is very handy!' ==
    'That deserves all caps. ' +
        'STRING INTERPOLATION is very handy!');

var s1 = 'String '
    'concatenation'
    " works even over line breaks.";
assert(s1 ==
    'String concatenation works even over '
        'line breaks.');

var s2 = 'The + operator ' + 'works, as well.';
assert(s2 == 'The + operator works, as well.');

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.';

// These work in a const string.
const aConstNum = 0;
const aConstBool = true;
const aConstString = 'a constant string';

// These do NOT work in a const string.
var aNum = 0;
var aBool = true;
var aString = 'a string';
const aConstList = [1, 2, 3];

const validConstString = '$aConstNum $aConstBool $aConstString';
// const invalidConstString = '$aNum $aBool $aString $aConstList';

• booleans

// Check for an empty string.
var fullName = '';
assert(fullName.isEmpty);

// Check for zero.
var hitPoints = 0;
assert(hitPoints <= 0);

// Check for null.
var unicorn;
assert(unicorn == null);

// Check for NaN.
var iMeantToDoThis = 0 / 0;
assert(iMeantToDoThis.isNaN);

• lists (also known as arrays)

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

list[1] = 1;
assert(list[1] == 1);

var constantList = const [1, 2, 3];

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

var list;
var list2 = [0, ...?list];
assert(list2.length == 1);

var nav = [
  'Home',
  'Furniture',
  'Plants',
  if (promoActive) 'Outlet'
];

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

• sets

var halogens = {'fluorine', 'chlorine', 'bromine', 'iodine', 'astatine'};

var names = <String>{};
// Set<String> names = {}; // This works, too.
// var names = {}; // Creates a map, not a set.

var elements = <String>{};
elements.add('fluorine');
elements.addAll(halogens);

var elements = <String>{};
elements.add('fluorine');
elements.addAll(halogens);
assert(elements.length == 5);

final constantSet = const {
  'fluorine',
  'chlorine',
  'bromine',
  'iodine',
  'astatine',
};
// constantSet.add('helium'); // Uncommenting this causes an error.

• maps

var gifts = {
  // Key:    Value
  'first': 'partridge',
  'second': 'turtledoves',
  'fifth': 'golden rings'
};

var nobleGases = {
  2: 'helium',
  10: 'neon',
  18: 'argon',
};

var gifts = Map();
gifts['first'] = 'partridge';
gifts['second'] = 'turtledoves';
gifts['fifth'] = 'golden rings';

var nobleGases = Map();
nobleGases[2] = 'helium';
nobleGases[10] = 'neon';
nobleGases[18] = 'argon';

var gifts = {'first': 'partridge'};
gifts['fourth'] = 'calling birds'; // Add a key-value pair

var gifts = {'first': 'partridge'};
assert(gifts['first'] == 'partridge');

var gifts = {'first': 'partridge'};
assert(gifts['fifth'] == null);

var gifts = {'first': 'partridge'};
gifts['fourth'] = 'calling birds';
assert(gifts.length == 2);

final constantMap = const {
  2: 'helium',
  10: 'neon',
  18: 'argon',
};

// constantMap[2] = 'Helium'; // Uncommenting this causes an error.

• runes (for expressing Unicode characters in a string)

main() {
  var clapping = '\u{1f44f}';
  print(clapping);
  print(clapping.codeUnits);
  print(clapping.runes.toList());

  Runes input = new Runes(
      '\u2665  \u{1f605}  \u{1f60e}  \u{1f47b}  \u{1f596}  \u{1f44d}');
  print(new String.fromCharCodes(input));
}

• symbols

#radix
#bar

Functions

bool isNoble(int atomicNumber) {
  return _nobleGases[atomicNumber] != null;
}

isNoble(atomicNumber) {
  return _nobleGases[atomicNumber] != null;
}

bool isNoble(int atomicNumber) => _nobleGases[atomicNumber] != null;

Optional parameters

enableFlags(bold: true, hidden: false);

/// Sets the [bold] and [hidden] flags ...
void enableFlags({bool bold, bool hidden}) {...}

const Scrollbar({Key key, @required Widget child})

String say(String from, String msg, [String device]) {
  var result = '$from says $msg';
  if (device != null) {
    result = '$result with a $device';
  }
  return result;
}

assert(say('Bob', 'Howdy') == 'Bob says Howdy');

assert(say('Bob', 'Howdy', 'smoke signal') ==
    'Bob says Howdy with a smoke signal');

Default parameter values

/// Sets the [bold] and [hidden] flags ...
void enableFlags({bool bold = false, bool hidden = false}) {...}

// bold will be true; hidden will be false.
enableFlags(bold: true);

String say(String from, String msg,
    [String device = 'carrier pigeon', String mood]) {
  var result = '$from says $msg';
  if (device != null) {
    result = '$result with a $device';
  }
  if (mood != null) {
    result = '$result (in a $mood mood)';
  }
  return result;
}

assert(say('Bob', 'Howdy') ==
    'Bob says Howdy with a carrier pigeon');

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

void main() {
  querySelector('#sample_text_id')
    ..text = 'Click me!'
    ..onClick.listen(reverseText);
}

// 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');
}

Functions as first-class objects

void printElement(int element) {
  print(element);
}

var list = [1, 2, 3];

// Pass printElement as a parameter.
list.forEach(printElement);

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

Anonymous functions

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

list.forEach(
    (item) => print('${list.indexOf(item)}: $item'));

Lexical scope

bool topLevel = true;

void main() {
  var insideMain = true;

  void myFunction() {
    var insideFunction = true;

    void nestedFunction() {
      var insideNestedFunction = true;

      assert(topLevel);
      assert(insideMain);
      assert(insideFunction);
      assert(insideNestedFunction);
    }
  }
}

Lexical closures

/// Returns a function that adds [addBy] to the
/// function's argument.
Function makeAdder(num addBy) {
  return (num i) => addBy + i;
}

void main() {
  // Create a function that adds 2.
  var add2 = makeAdder(2);

  // Create a function that adds 4.
  var add4 = makeAdder(4);

  assert(add2(3) == 5);
  assert(add4(3) == 7);
}

Testing functions for equality

void foo() {} // A top-level function

class A {
  static void bar() {} // A static method
  void baz() {} // An instance method
}

void main() {
  var x;

  // Comparing top-level functions.
  x = foo;
  assert(foo == x);

  // Comparing static methods.
  x = A.bar;
  assert(A.bar == x);

  // Comparing instance methods.
  var v = A(); // Instance #1 of A
  var w = A(); // Instance #2 of A
  var y = w;
  x = w.baz;

  // These closures refer to the same instance (#2),
  // so they're equal.
  assert(y.baz == x);

  // These closures refer to different instances,
  // so they're unequal.
  assert(v.baz != w.baz);
}

Return values

foo() {}

assert(foo() == null);

Operators

操作符

a++
a + b
a = b
a == b
c ? a : b
a is T

// Parentheses improve readability.
if ((n % i == 0) && (d % i == 0)) ...

// Harder to read, but equivalent.
if (n % i == 0 && d % i == 0) ...

assert(2 + 3 == 5);
assert(2 - 3 == -1);
assert(2 * 3 == 6);
assert(5 / 2 == 2.5); // Result is a double
assert(5 ~/ 2 == 2); // Result is an int
assert(5 % 2 == 1); // Remainder

assert('5/2 = ${5 ~/ 2} r ${5 % 2}' == '5/2 = 2 r 1');

var a, b;

a = 0;
b = ++a; // Increment a before b gets its value.
assert(a == b); // 1 == 1

a = 0;
b = a++; // Increment a AFTER b gets its value.
assert(a != b); // 1 != 0

a = 0;
b = --a; // Decrement a before b gets its value.
assert(a == b); // -1 == -1

a = 0;
b = a--; // Decrement a AFTER b gets its value.
assert(a != b); // -1 != 0


assert(2 == 2);
assert(2 != 3);
assert(3 > 2);
assert(2 < 3);
assert(3 >= 3);
assert(2 <= 3);

if (emp is Person) {
  // Type check
  emp.firstName = 'Bob';
}

(emp as Person).firstName = 'Bob';

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

var a = 2; // Assign using =
a *= 3; // Assign and multiply: a = a * 3
assert(a == 6);

if (!done && (col == 0 || col == 3)) {
  // ...Do something...
}

inal value = 0x22;
final bitmask = 0x0f;

assert((value & bitmask) == 0x02); // AND
assert((value & ~bitmask) == 0x20); // AND NOT
assert((value | bitmask) == 0x2f); // OR
assert((value ^ bitmask) == 0x2d); // XOR
assert((value << 4) == 0x220); // Shift left
assert((value >> 4) == 0x02); // Shift right

var visibility = isPublic ? 'public' : 'private';

String playerName(String name) => name ?? 'Guest';

// Slightly longer version uses ?: operator.
String playerName(String name) => name != null ? name : 'Guest';

// Very long version uses if-else statement.
String playerName(String name) {
  if (name != null) {
    return name;
  } else {
    return 'Guest';
  }
}

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

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

final addressBook = (AddressBookBuilder()
      ..name = 'jenny'
      ..email = 'jenny@example.com'
      ..phone = (PhoneNumberBuilder()
            ..number = '415-555-0100'
            ..label = 'home')
          .build())
    .build();

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

if else

if (isRaining()) {
  you.bringRainCoat();
} else if (isSnowing()) {
  you.wearJacket();
} else {
  car.putTopDown();
}

for

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

var callbacks = [];
for (var i = 0; i < 2; i++) {
  callbacks.add(() => print(i));
}
callbacks.forEach((c) => c());

candidates.forEach((candidate) => candidate.interview());

var collection = [0, 1, 2];
for (var x in collection) {
  print(x); // 0 1 2
}

while

while (!isDone()) {
  doSomething();
}

do {
  printLine();
} while (!atEndOfPage());

Break and continue

while (true) {
  if (shutDownRequested()) break;
  processIncomingRequests();
}

for (int i = 0; i < candidates.length; i++) {
  var candidate = candidates[i];
  if (candidate.yearsExperience < 5) {
    continue;
  }
  candidate.interview();
}

candidates
    .where((c) => c.yearsExperience >= 5)
    .forEach((c) => c.interview());

Switch and case

var command = 'OPEN';
switch (command) {
  case 'CLOSED':
    executeClosed();
    break;
  case 'PENDING':
    executePending();
    break;
  case 'APPROVED':
    executeApproved();
    break;
  case 'DENIED':
    executeDenied();
    break;
  case 'OPEN':
    executeOpen();
    break;
  default:
    executeUnknown();
}

var command = 'OPEN';
switch (command) {
  case 'OPEN':
    executeOpen();
    // ERROR: Missing break

  case 'CLOSED':
    executeClosed();
    break;
}

var command = 'CLOSED';
switch (command) {
  case 'CLOSED': // Empty case falls through.
  case 'NOW_CLOSED':
    // Runs for both CLOSED and NOW_CLOSED.
    executeNowClosed();
    break;
}

var command = 'CLOSED';
switch (command) {
  case 'CLOSED':
    executeClosed();
    continue nowClosed;
  // Continues executing at the nowClosed label.

  nowClosed:
  case 'NOW_CLOSED':
    // Runs for both CLOSED and NOW_CLOSED.
    executeNowClosed();
    break;
}

Assert

// Make sure the variable has a non-null value.
assert(text != null);

// Make sure the value is less than 100.
assert(number < 100);

// Make sure this is an https URL.
assert(urlString.startsWith('https'));

assert(urlString.startsWith('https'),
    'URL ($urlString) should start with "https".');

Exceptions

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

throw 'Out of llamas!';

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

try {
  breedMoreLlamas();
} on OutOfLlamasException {
  buyMoreLlamas();
}

try {
  breedMoreLlamas();
} on OutOfLlamasException {
  // A specific exception
  buyMoreLlamas();
} on Exception catch (e) {
  // Anything else that is an exception
  print('Unknown exception: $e');
} catch (e) {
  // No specified type, handles all
  print('Something really unknown: $e');
}

try {
  // ···
} on Exception catch (e) {
  print('Exception details:\n $e');
} catch (e, s) {
  print('Exception details:\n $e');
  print('Stack trace:\n $s');
}

void misbehave() {
  try {
    dynamic foo = true;
    print(foo++); // Runtime error
  } catch (e) {
    print('misbehave() partially handled ${e.runtimeType}.');
    rethrow; // Allow callers to see the exception.
  }
}

void main() {
  try {
    misbehave();
  } catch (e) {
    print('main() finished handling ${e.runtimeType}.');
  }
}

try {
  breedMoreLlamas();
} finally {
  // Always clean up, even if an exception is thrown.
  cleanLlamaStalls();
}

try {
  breedMoreLlamas();
} catch (e) {
  print('Error: $e'); // Handle the exception first.
} finally {
  cleanLlamaStalls(); // Then clean up.
}

Classes

var p = Point(2, 2);

// Set the value of the instance variable y.
p.y = 3;

// Get the value of y.
assert(p.y == 3);

// Invoke distanceTo() on p.
num distance = p.distanceTo(Point(4, 4));

// If p is non-null, set its y value to 4.
p?.y = 4;

var p1 = Point(2, 2);
var p2 = Point.fromJson({'x': 1, 'y': 2});

var p1 = new Point(2, 2);
var p2 = new Point.fromJson({'x': 1, 'y': 2});

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

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

// Lots of const keywords here.
const pointAndLine = const {
  'point': const [const ImmutablePoint(0, 0)],
  'line': const [const ImmutablePoint(1, 10), const ImmutablePoint(-2, 11)],
};

// Only one const, which establishes the constant context.
const pointAndLine = {
  'point': [ImmutablePoint(0, 0)],
  'line': [ImmutablePoint(1, 10), ImmutablePoint(-2, 11)],
};

var a = const ImmutablePoint(1, 1); // Creates a constant
var b = ImmutablePoint(1, 1); // Does NOT create a constant

assert(!identical(a, b)); // NOT the same instance!

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

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

class Point {
  num x;
  num y;
}

void main() {
  var point = 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.
}

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;
  }
}

class Point {
  num x, y;

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

class Point {
  num x, y;

  Point(this.x, this.y);

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

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 = Logger._internal(name);
      _cache[name] = logger;
      return logger;
    }
  }

  Logger._internal(this.name);

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

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


import 'dart:math';

class Point {
  num x, y;

  Point(this.x, this.y);

  num distanceTo(Point other) {
    var dx = x - other.x;
    var dy = y - other.y;
    return sqrt(dx * dx + dy * dy);
  }
}


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

Abstract methods

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...
  }
}

Abstract classes

// This class is declared abstract and thus
// can't be instantiated.
abstract class AbstractContainer {
  // Define constructors, fields, methods...

  void updateChildren(); // Abstract method.
}

Implicit interfaces

// 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(Person('Kathy')));
  print(greetBob(Impostor()));
}

Extending a class

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

class SmartTelevision extends Television {
  void turnOn() {
    super.turnOn();
    _bootNetworkInterface();
    _initializeMemory();
    _upgradeApps();
  }
  // ···
}

Overriding members

class SmartTelevision extends Television {
  @override
  void turnOn() {...}
  // ···
}

Overridable operators

class Vector {
  final int x, y;

  Vector(this.x, this.y);

  Vector operator +(Vector v) => Vector(x + v.x, y + v.y);
  Vector operator -(Vector v) => Vector(x - v.x, y - v.y);

  // Operator == and hashCode not shown. For details, see note below.
  // ···
}

void main() {
  final v = Vector(2, 3);
  final w = Vector(2, 2);

  assert(v + w == Vector(4, 5));
  assert(v - w == Vector(0, 1));
}

noSuchMethod()

class A {
  // Unless you override noSuchMethod, using a
  // non-existent member results in a NoSuchMethodError.
  @override
  void noSuchMethod(Invocation invocation) {
    print('You tried to use a non-existent member: ' +
        '${invocation.memberName}');
  }
}

Enumerated types

enum Color { red, green, blue }

assert(Color.red.index == 0);
assert(Color.green.index == 1);
assert(Color.blue.index == 2);

List<Color> colors = Color.values;
assert(colors[2] == Color.blue);

var aColor = Color.blue;

switch (aColor) {
  case Color.red:
    print('Red as roses!');
    break;
  case Color.green:
    print('Green as grass!');
    break;
  default: // Without this, you see a WARNING.
    print(aColor); // 'Color.blue'
}

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;
  }
}

mixin Musical {
  bool canPlayPiano = false;
  bool canCompose = false;
  bool canConduct = false;

  void entertainMe() {
    if (canPlayPiano) {
      print('Playing piano');
    } else if (canConduct) {
      print('Waving hands');
    } else {
      print('Humming to self');
    }
  }
}

mixin MusicalPerformer on Musician {
  // ···
}

Class variables and methods

class Queue {
  static const initialCapacity = 16;
  // ···
}

void main() {
  assert(Queue.initialCapacity == 16);
}

import 'dart:math';

class Point {
  num x, y;
  Point(this.x, this.y);

  static num distanceBetween(Point a, Point b) {
    var dx = a.x - b.x;
    var dy = a.y - b.y;
    return sqrt(dx * dx + dy * dy);
  }
}

void main() {
  var a = Point(2, 2);
  var b = Point(4, 4);
  var distance = Point.distanceBetween(a, b);
  assert(2.8 < distance && distance < 2.9);
  print(distance);
}

Generics

var names = List<String>();
names.addAll(['Seth', 'Kathy', 'Lars']);
names.add(42); // Error

abstract class ObjectCache {
  Object getByKey(String key);
  void setByKey(String key, Object value);
}

abstract class StringCache {
  String getByKey(String key);
  void setByKey(String key, String value);
}

abstract class Cache<T> {
  T getByKey(String key);
  void setByKey(String key, T value);
}

Using collection literals

var names = <String>['Seth', 'Kathy', 'Lars'];
var uniqueNames = <String>{'Seth', 'Kathy', 'Lars'};
var pages = <String, String>{
  'index.html': 'Homepage',
  'robots.txt': 'Hints for web robots',
  'humans.txt': 'We are people, not machines'
};

Using parameterized types with constructors

var nameSet = Set<String>.from(names);
var views = Map<int, View>();

Generic collections and the types they contain

var names = List<String>();
names.addAll(['Seth', 'Kathy', 'Lars']);
print(names is List<String>); // true

Restricting the parameterized type

class Foo<T extends SomeBaseClass> {
  // Implementation goes here...
  String toString() => "Instance of 'Foo<$T>'";
}

class Extender extends SomeBaseClass {...}

var someBaseClassFoo = Foo<SomeBaseClass>();
var extenderFoo = Foo<Extender>();

var foo = Foo();
print(foo); // Instance of 'Foo<SomeBaseClass>'

var foo = Foo<Object>(); // Error

Using generic methods

T first<T>(List<T> ts) {
  // Do some initial work or error checking, then...
  T tmp = ts[0];
  // Do some additional checking or processing...
  return tmp;
}

Libraries and visibility

import 'dart:html';
import 'package:test/test.dart';

import 'package:lib1/lib1.dart';
import 'package:lib2/lib2.dart' as lib2;

// Uses Element from lib1.
Element element1 = Element();

// Uses Element from lib2.
lib2.Element element2 = lib2.Element();

// Import only foo.
import 'package:lib1/lib1.dart' show foo;

// Import all names EXCEPT foo.
import 'package:lib2/lib2.dart' hide foo;

Lazily loading a library

import 'package:greetings/hello.dart' deferred as hello;

Future greet() async {
  await hello.loadLibrary();
  hello.printGreeting();
}

Asynchrony support

Handling Futures

await lookUpVersion();

Future checkVersion() async {
  var version = await lookUpVersion();
  // Do something with version
}

try {
  version = await lookUpVersion();
} catch (e) {
  // React to inability to look up the version
}

var entrypoint = await findEntrypoint();
var exitCode = await runExecutable(entrypoint, args);
await flushThenExit(exitCode);

Future main() async {
  checkVersion();
  print('In main: version is ${await lookUpVersion()}');
}

Declaring async functions

String lookUpVersion() => '1.0.0';
Future<String> lookUpVersion() async => '1.0.0';

Handling Streams

await for (varOrType identifier in expression) {
  // Executes each time the stream emits a value.
}

Future main() async {
  // ...
  await for (var request in requestServer) {
    handleRequest(request);
  }
  // ...
}

Generators

Iterable<int> naturalsTo(int n) sync* {
  int k = 0;
  while (k < n) yield k++;
}

Stream<int> asynchronousNaturalsTo(int n) async* {
  int k = 0;
  while (k < n) yield k++;
}

Iterable<int> naturalsDownFrom(int n) sync* {
  if (n > 0) {
    yield n;
    yield* naturalsDownFrom(n - 1);
  }
}

Callable classes

class WannabeFunction {
  call(String a, String b, String c) => '$a $b $c!';
}

main() {
  var wf = new WannabeFunction();
  var out = wf("Hi","there,","gang");
  print('$out');
}

Isolates

Typedefs

class SortedCollection {
  Function compare;

  SortedCollection(int f(Object a, Object b)) {
    compare = f;
  }
}

// Initial, broken implementation.
int sort(Object a, Object b) => 0;

void main() {
  SortedCollection coll = SortedCollection(sort);

  // All we know is that compare is a function,
  // but what type of function?
  assert(coll.compare is Function);
}

typedef Compare = int Function(Object a, Object b);

class SortedCollection {
  Compare compare;

  SortedCollection(this.compare);
}

// Initial, broken implementation.
int sort(Object a, Object b) => 0;

void main() {
  SortedCollection coll = SortedCollection(sort);
  assert(coll.compare is Function);
  assert(coll.compare is Compare);
}

typedef Compare<T> = int Function(T a, T b);

int sort(int a, int b) => a - b;

void main() {
  assert(sort is Compare<int>); // True!
}

Metadata

class Television {
  /// _Deprecated: Use [turnOn] instead._
  @deprecated
  void activate() {
    turnOn();
  }

  /// Turns the TV's power on.
  void turnOn() {...}
}

library todo;

class Todo {
  final String who;
  final String what;

  const Todo(this.who, this.what);
}

import 'todo.dart';

@Todo('seth', 'make this do something')
void doSomething() {
  print('do something');
}

Comments

void main() {
  // TODO: refactor into an AbstractLlamaGreetingFactory?
  print('Welcome to my Llama farm!');
}

void main() {
  /*
   * This is a lot of work. Consider raising chickens.

  Llama larry = Llama();
  larry.feed();
  larry.exercise();
  larry.clean();
   */
}

Documentation comments

/// A domesticated South American camelid (Lama glama).
///
/// Andean cultures have used llamas as meat and pack
/// animals since pre-Hispanic times.
class Llama {
  String name;

  /// Feeds your llama [Food].
  ///
  /// The typical llama eats one bale of hay per week.
  void feed(Food food) {
    // ...
  }

  /// Exercises your llama with an [activity] for
  /// [timeLimit] minutes.
  void exercise(Activity activity, int timeLimit) {
    // ...
  }
}