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+---
+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/
+
+
+