| language | filename | contributors | |||
|---|---|---|---|---|---|
dart |
learndart.dart |
|
Dart is a newcomer into the realm of programming languages. It borrows a lot from other mainstream languages, having as a goal not to deviate too much from its JavaScript sibling. Like JavaScript, Dart aims for great browser integration.
Dart's most controversial feature must be its Optional Typing.
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());
}Dart has a comprehenshive web-site. It covers API reference, tutorials, articles and more, including a useful Try Dart online. http://www.dartlang.org/ http://try.dartlang.org/