---
language: dart
filename: learndart.dart
contributors:
    - ["Joao Pedrosa", "https://github.com/jpedrosa/"]
translators:
    - ["Guokai Han", "https://github.com/hanguokai/"]
---

Dart 是编程语言王国的新人。
它借鉴了许多其他主流语言，并且不会偏离它的兄弟语言 JavaScript 太多。
就像 JavaScript，Dart 的目标是提供良好的浏览器集成。

Dart 最有争议的特性必然是它的可选类型。

```javascript
import "dart:collection";
import "dart:math" as DM;

// Welcome to Learn Dart in 15 minutes. http://www.dartlang.org/
// This is an executable tutorial. You can run it with Dart or on
// the Try Dart! site if you copy/paste it there. http://try.dartlang.org/

// Function declaration and method declaration look the same. Function
// declarations can be nested. The declaration takes the form of
// name() {} or name() => singleLineExpression;
// The fat arrow function declaration has an implicit return for the result of
// the expression.
example1() {
  example1nested1() {
    example1nested2() => print("Example1 nested 1 nested 2");
    example1nested2();
  }
  example1nested1();
}

// Anonymous functions don't include a name.
example2() {
  example2nested1(fn) {
    fn();
  }
  example2nested1(() => print("Example2 nested 1"));
}

// When a function parameter is declared, the declaration can include the
// number of parameters the function takes by specifying the names of the
// parameters it takes.
example3() {
  example3nested1(fn(informSomething)) {
    fn("Example3 nested 1");
  }
  example3planB(fn) { // Or don't declare number of parameters.
    fn("Example3 plan B");
  }
  example3nested1((s) => print(s));
  example3planB((s) => print(s));
}

// Functions have closure access to outer variables.
var example4Something = "Example4 nested 1";
example4() {
  example4nested1(fn(informSomething)) {
    fn(example4Something);
  }
  example4nested1((s) => print(s));
}

// Class declaration with a sayIt method, which also has closure access
// to the outer variable as though it were a function as seen before.
var example5method = "Example5 sayIt";
class Example5Class {
  sayIt() {
    print(example5method);
  }
}
example5() {
  // Create an anonymous instance of the Example5Class and call the sayIt
  // method on it.
  new Example5Class().sayIt();
}

// Class declaration takes the form of class name { [classBody] }.
// Where classBody can include instance methods and variables, but also
// class methods and variables.
class Example6Class {
  var example6InstanceVariable = "Example6 instance variable"; 
  sayIt() {
    print(example6InstanceVariable);
  }
}
example6() {
  new Example6Class().sayIt();
}

// Class methods and variables are declared with "static" terms.
class Example7Class {
  static var example7ClassVariable = "Example7 class variable"; 
  static sayItFromClass() {
    print(example7ClassVariable);
  }
  sayItFromInstance() {
    print(example7ClassVariable);
  }
}
example7() {
  Example7Class.sayItFromClass();
  new Example7Class().sayItFromInstance();
}

// Literals are great, but there's a restriction for what literals can be
// outside of function/method bodies. Literals on the outer scope of class
// or outside of class have to be constant. Strings and numbers are constant
// by default. But arrays and maps are not. They can be made constant by
// declaring them "const".
var example8A = const ["Example8 const array"],
  example8M = const {"someKey": "Example8 const map"}; 
example8() {
  print(example8A[0]);
  print(example8M["someKey"]);
}

// Loops in Dart take the form of standard for () {} or while () {} loops,
// slightly more modern for (.. in ..) {}, or functional callbacks with many
// supported features, starting with forEach.
var example9A = const ["a", "b"];
example9() {
  for (var i = 0; i < example9A.length; i++) {
    print("Example9 for loop '${example9A[i]}'");
  }
  var i = 0;
  while (i < example9A.length) {
    print("Example9 while loop '${example9A[i]}'");
    i++;
  }
  for (var e in example9A) {
    print("Example9 for-in loop '${e}'");
  }
  example9A.forEach((e) => print("Example9 forEach loop '${e}'"));
}

// To loop over the characters of a string or to extract a substring.
var example10S = "ab";
example10() {
  for (var i = 0; i < example10S.length; i++) {
    print("Example10 String character loop '${example10S[i]}'");
  }
  for (var i = 0; i < example10S.length; i++) {
    print("Example10 substring loop '${example10S.substring(i, i + 1)}'");
  }
}

// Int and double are the two supported number formats.
example11() {
  var i = 1 + 320, d = 3.2 + 0.01;
  print("Example11 int ${i}");
  print("Example11 double ${d}");
}

// DateTime provides date/time arithmetic.
example12() {
  var now = new DateTime.now();
  print("Example12 now '${now}'");
  now = now.add(new Duration(days: 1));
  print("Example12 tomorrow '${now}'");
}

// Regular expressions are supported.
example13() {
  var s1 = "some string", s2 = "some", re = new RegExp("^s.+?g\$");
  match(s) {
    if (re.hasMatch(s)) {
      print("Example13 regexp matches '${s}'");
    } else {
      print("Example13 regexp doesn't match '${s}'");
    }
  }
  match(s1);
  match(s2);
}

// Boolean expressions need to resolve to either true or false, as no
// implicit conversions are supported.
example14() {
  var v = true;
  if (v) {
    print("Example14 value is true");
  }
  v = null;
  try {
    if (v) {
      // Never runs
    } else {
      // Never runs
    }
  } catch (e) {
    print("Example14 null value causes an exception: '${e}'");
  }
}

// try/catch/finally and throw are used for exception handling.
// throw takes any object as parameter;
example15() {
  try {
    try {
      throw "Some unexpected error.";
    } catch (e) {
      print("Example15 an exception: '${e}'");
      throw e; // Re-throw
    }
  } catch (e) {
    print("Example15 catch exception being re-thrown: '${e}'");
  } finally {
    print("Example15 Still run finally");
  }
}

// To be efficient when creating a long string dynamically, use
// StringBuffer. Or you could join a string array.
example16() {
  var sb = new StringBuffer(), a = ["a", "b", "c", "d"], e;
  for (e in a) { sb.write(e); }
  print("Example16 dynamic string created with "
    "StringBuffer '${sb.toString()}'");
  print("Example16 join string array '${a.join()}'");
}

// Strings can be concatenated by just having string literals next to
// one another with no further operator needed.
example17() {
  print("Example17 "
      "concatenate "
      "strings "
      "just like that");
}

// Strings have single-quote or double-quote for delimiters with no
// actual difference between the two. The given flexibility can be good
// to avoid the need to escape content that matches the delimiter being
// used. For example, double-quotes of HTML attributes if the string
// contains HTML content.
example18() {
  print('Example18 <a href="etc">'
      "Don't can't I'm Etc"
      '</a>');
}

// Strings with triple single-quotes or triple double-quotes span
// multiple lines and include line delimiters.
example19() {
  print('''Example19 <a href="etc"> 
Example19 Don't can't I'm Etc
Example19 </a>''');
}

// Strings have the nice interpolation feature with the $ character.
// With $ { [expression] }, the return of the expression is interpolated.
// $ followed by a variable name interpolates the content of that variable.
// $ can be escaped like so \$ to just add it to the string instead.
example20() {
  var s1 = "'\${s}'", s2 = "'\$s'";
  print("Example20 \$ interpolation ${s1} or $s2 works.");
}

// Optional types allow for the annotation of APIs and come to the aid of
// IDEs so the IDEs can better refactor, auto-complete and check for
// errors. So far we haven't declared any types and the programs have
// worked just fine. In fact, types are disregarded during runtime.
// Types can even be wrong and the program will still be given the
// benefit of the doubt and be run as though the types didn't matter.
// There's a runtime parameter that checks for type errors which is
// the checked mode, which is said to be useful during development time,
// but which is also slower because of the extra checking and is thus
// avoided during deployment runtime.
class Example21 {
  List<String> _names;
  Example21() {
    _names = ["a", "b"]; 
  }
  List<String> get names => _names;
  set names(List<String> list) {
    _names = list;
  }
  int get length => _names.length;
  void add(String name) {
    _names.add(name);
  }
}
void example21() {
  Example21 o = new Example21();
  o.add("c");
  print("Example21 names '${o.names}' and length '${o.length}'");
  o.names = ["d", "e"];
  print("Example21 names '${o.names}' and length '${o.length}'");
}

// Class inheritance takes the form of class name extends AnotherClassName {}.
class Example22A {
  var _name = "Some Name!";
  get name => _name;
}
class Example22B extends Example22A {}
example22() {
  var o = new Example22B();
  print("Example22 class inheritance '${o.name}'");
}

// Class mixin is also available, and takes the form of
// class name extends SomeClass with AnotherClassName {}.
// It's necessary to extend some class to be able to mixin another one.
// The template class of mixin cannot at the moment have a constructor.
// Mixin is mostly used to share methods with distant classes, so the
// single inheritance doesn't get in the way of reusable code.
// Mixins follow the "with" statement during the class declaration.
class Example23A {}
class Example23Utils {
  addTwo(n1, n2) {
    return n1 + n2;
  }
}
class Example23B extends Example23A with Example23Utils {
  addThree(n1, n2, n3) {
    return addTwo(n1, n2) + n3;
  }
}
example23() {
  var o = new Example23B(), r1 = o.addThree(1, 2, 3),
    r2 = o.addTwo(1, 2);
  print("Example23 addThree(1, 2, 3) results in '${r1}'");
  print("Example23 addTwo(1, 2) results in '${r2}'");
}

// The Class constructor method uses the same name of the class and
// takes the form of SomeClass() : super() {}, where the ": super()"
// part is optional and it's used to delegate constant parameters to the
// super-parent's constructor.
class Example24A {
  var _value;
  Example24A({value: "someValue"}) {
    _value = value;
  }
  get value => _value;
}
class Example24B extends Example24A {
  Example24B({value: "someOtherValue"}) : super(value: value);
}
example24() {
  var o1 = new Example24B(),
    o2 = new Example24B(value: "evenMore");
  print("Example24 calling super during constructor '${o1.value}'");
  print("Example24 calling super during constructor '${o2.value}'");
}

// There's a shortcut to set constructor parameters in case of simpler classes.
// Just use the this.parameterName prefix and it will set the parameter on
// an instance variable of same name.
class Example25 {
  var value, anotherValue;
  Example25({this.value, this.anotherValue});
}
example25() {
  var o = new Example25(value: "a", anotherValue: "b");
  print("Example25 shortcut for constructor '${o.value}' and "
    "'${o.anotherValue}'");
}

// Named parameters are available when declared between {}.
// Parameter order can be optional when declared between {}.
// Parameters can be made optional when declared between [].
example26() {
  var _name, _surname, _email;
  setConfig1({name, surname}) {
    _name = name;
    _surname = surname;
  }
  setConfig2(name, [surname, email]) {
    _name = name;
    _surname = surname;
    _email = email;
  }
  setConfig1(surname: "Doe", name: "John");
  print("Example26 name '${_name}', surname '${_surname}', "
    "email '${_email}'");
  setConfig2("Mary", "Jane");
  print("Example26 name '${_name}', surname '${_surname}', "
  "email '${_email}'");
}

// Variables declared with final can only be set once.
// In case of classes, final instance variables can be set via constant
// constructor parameter.
class Example27 {
  final color1, color2;
  // A little flexibility to set final instance variables with syntax
  // that follows the :
  Example27({this.color1, color2}) : color2 = color2;
}
example27() {
  final color = "orange", o = new Example27(color1: "lilac", color2: "white");
  print("Example27 color is '${color}'");
  print("Example27 color is '${o.color1}' and '${o.color2}'");
}

// To import a library, use import "libraryPath" or if it's a core library,
// import "dart:libraryName". There's also the "pub" package management with
// its own convention of import "package:packageName".
// See import "dart:collection"; at the top. Imports must come before
// other code declarations. IterableBase comes from dart:collection.
class Example28 extends IterableBase {
  var names;
  Example28() {
    names = ["a", "b"];
  }
  get iterator => names.iterator;
}
example28() {
  var o = new Example28();
  o.forEach((name) => print("Example28 '${name}'"));
}

// For control flow we have:
// * standard switch with must break statements
// * if-else if-else and ternary ..?..:.. operator
// * closures and anonymous functions
// * break, continue and return statements
example29() {
  var v = true ? 30 : 60;
  switch (v) {
    case 30:
      print("Example29 switch statement");
      break;
  }
  if (v < 30) {
  } else if (v > 30) {
  } else {
    print("Example29 if-else statement");
  }
  callItForMe(fn()) {
    return fn();
  }
  rand() {
    v = new DM.Random().nextInt(50);
    return v;
  }
  while (true) {
    print("Example29 callItForMe(rand) '${callItForMe(rand)}'");
    if (v != 30) {
      break;
    } else {
      continue;
    }
    // Never gets here.
  }
}

// Parse int, convert double to int, or just keep int when dividing numbers
// by using the ~/ operation. Let's play a guess game too.
example30() {
  var gn, tooHigh = false,
    n, n2 = (2.0).toInt(), top = int.parse("123") ~/ n2, bottom = 0;
  top = top ~/ 6;
  gn = new DM.Random().nextInt(top + 1); // +1 because nextInt top is exclusive
  print("Example30 Guess a number between 0 and ${top}");
  guessNumber(i) {
    if (n == gn) {
      print("Example30 Guessed right! The number is ${gn}");
    } else {
      tooHigh = n > gn;
      print("Example30 Number ${n} is too "
        "${tooHigh ? 'high' : 'low'}. Try again");
    }
    return n == gn;
  }
  n = (top - bottom) ~/ 2;
  while (!guessNumber(n)) {
    if (tooHigh) {
      top = n - 1;
    } else {
      bottom = n + 1;
    }
    n = bottom + ((top - bottom) ~/ 2);
  }
}

// Programs have only one entry point in the main function.
// Nothing is expected to be executed on the outer scope before a program
// starts running with what's in its main function.
// This helps with faster loading and even lazily loading of just what
// the program needs to startup with.
main() {
  print("Learn Dart in 15 minutes!");
  [example1, example2, example3, example4, example5, example6, example7,
    example8, example9, example10, example11, example12, example13, example14,
    example15, example16, example17, example18, example19, example20,
    example21, example22, example23, example24, example25, example26,
    example27, example28, example29, example30
    ].forEach((ef) => ef());
}

```

## 延伸阅读

Dart 有一个全面的网站。它涵盖了 API 参考、入门向导、文章以及更多，
包括一个有用的 Dart 在线试用。
http://www.dartlang.org/
http://try.dartlang.org/