Dart is an open source, structured programming language for creating complex, browser-based web applications.
first dart demo
main() {
var d = "Dart";
String w = "World";
print("Hello $w! I love $d"); // Hello World! I love Dart
}
string interpolation
void main() {
var h = "Hello";
final w = "World";
print('$h $w'); // Hello World
print(r'$h $w'); // $h $w
var helloWorld = "Hello " "World";
print(helloWorld); // Hello World
print("${helloWorld.toUpperCase()}"); // HELLO WORLD
print("The answer is ${5 + 10}"); // The answer is 15
var multiline = """
<div id='greeting'>
"Hello World"
</div>""";
print(multiline);
var o = Object();
print(o.toString()); // Instance of 'Object'
print("$o"); // // Instance of 'Object'
}
Multiline strings ignore first line break following """ and also can contain both single and double quotes.
dart class in use
class Greeter {
var greeting; // public property
var _name; // private property
sayHello() => "$greeting ${this.name}";
get name => _name; // getter method for _name
set name(value) => _name = value; // setter method for _name
}
main() {
var greeter = Greeter();
greeter.greeting = "Hello ";
greeter.name = "World";
print(greeter.sayHello()); // Hello World
}
Implied interface definitions
Dart has interfaces just like Java and C#, but in Dart, you use the class structure to define an interface. This works on the basis that all classes define an implicit interface on their public members.
class Welcomer {
printGreeting() => print("Hello ${name}");
var name;
}
class Greeter implements Welcomer {
printGreeting() => print("Greetings ${name}");
var name;
}
void sayHello(Welcomer welcomer) {
welcomer.printGreeting();
}
main() {
var welcomer = Welcomer();
welcomer.name = "Tom";
sayHello(welcomer); // Hello Tom
var greeter = Greeter();
greeter.name = "Tom";
sayHello(greeter); // Greetings Tom
}
Factory constructors to provide default implementations
abstract class IGreetable {
String sayHello(String name);
factory IGreetable() => Greeter();
}
class Greeter implements IGreetable {
sayHello(name) => "Hello $name";
}
void main() {
IGreetable myGreetable = IGreetable();
var message = myGreetable.sayHello("Dart");
print(message); // Hello Dart
}
Libraries and scope
library "my_library"; // Declares that file is a library
import "../lib/my_other_library.dart"; // Imports another library from a different folder
part "greeter.dart"; // Includes other source files (containing Greeter class)
part "leaver.dart";
greetFunc() { // Defines function in top- level library scope
var g = new Greeter(); // Uses class from greeter.dart file
sayHello(g); // Calls function in top-level scope of my_other_library
}
To avoid naming conflicts, you can use as keywords.
import "../lib/my_other_library.dart" as other;
Functions as first-class objects
String sayHello(name) => "Hello $name"; // Declares function using function shorthand
main() {
var myFunc = sayHello; // Assigns function into variable
print(myFunc("World")); // Calls function stored in variable
var mySumFunc = (a, b) => a + b; // Defines anonymous function
var c = mySumFunc(1, 2); // Calls anonymous function
print(c); //
}
Mixins
Mixins are a way of reusing code in multiple class hierarchies. The following class can act as a mixin:
class Piloted {
int astronauts = 1;
void describeCrew() {
print('Number of astronauts: $astronauts');
}
}
class PilotedCraft with Piloted {} // now has the astronauts field as well as the describeCrew() method.
main(List<String> args) {
var plotedCraft = PilotedCraft();
plotedCraft.astronauts = 23;
plotedCraft.describeCrew(); // Number of astronauts: 23
}
To specify that only certain types can use the mixin.
mixin MusicalPerformer on Musician {
// ···
}
spread operator (...) and the null-aware spread operator (...?)(Dart 2.3)
you can use the spread operator (...) to insert all the elements of a list into another list:
var list = [1, 2, 3];
var list2 = [0, ...list];
assert(list2.length == 4);
If the expression to the right of the spread operator might be null, you can avoid exceptions by using a null-aware spread operator (...?):
var list;
var list2 = [0, ...?list];
assert(list2.length == 1);
collection if and collection for(Dart 2.3)
using collection if to create a list with three or four items in it:
var nav = [
'Home',
'Furniture',
'Plants',
if (promoActive) 'Outlet'
];
using collection for to manipulate the items of a list before adding them to another list:
var listOfInts = [1, 2, 3];
var listOfStrings = [
'#0',
for (var i in listOfInts) '#$i'
];
assert(listOfStrings[1] == '#1');
Optional parameters
Optional parameters can be either named or positional, but not both.
Named parameters
When calling a function, you can specify named parameters using paramName: value. For example:
// define method which parameters with {}
void enableFlags({bool bold, bool hidden}) {...}
// call method
enableFlags(bold: true, hidden: false);
Although named parameters are a kind of optional parameter, you can annotate them with @required to indicate that the parameter is mandatory.
const Scrollbar({Key key, @required Widget child})
Positional parameters
Wrapping a set of function parameters in [] marks them as optional positional parameters:
String say(String from, String msg, [String device]) {
var result = '$from says $msg';
if (device != null) {
result = '$result with a $device';
}
return result;
}
assert(say('Bob', 'Howdy') == 'Bob says Howdy');
assert(say('Bob', 'Howdy', 'smoke signal') =='Bob says Howdy with a smoke signal');
Default parameter values
Your function can use = to define default values for both named and positional parameters. The default values must be compile-time constants. If no default value is provided, the default value is null.
String say(String from, String msg, [String device = 'carrier pigeon', String mood]) {
var result = '$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');
division use / or ~/
assert(5 / 2 == 2.5); // Result is a double
assert(5 ~/ 2 == 2); // Result is an int
assert(5 % 2 == 1); // Remainder
Type test operators
The as, is, and is! operators are handy for checking types at runtime.
if (emp is Person) {
// Type check
emp.firstName = 'Bob';
}
Conditional expressions
Dart has two operators that let you concisely evaluate expressions that might otherwise require if-else statements:
- condition ? expr1 : expr2:
If condition is true, evaluates expr1 (and returns its value); otherwise, evaluates and returns the value of expr2.
var visibility = isPublic ? 'public' : 'private';
- expr1 ?? expr2
If expr1 is non-null, returns its value; otherwise, evaluates and returns the value of expr2.
// If the boolean expression tests for null, consider using ??.
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';
}
}
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.
// first way
querySelector('#confirm') // Get an object.
..text = 'Confirm' // Use its members.
..classes.add('important')
..onClick.listen((e) => window.alert('Confirmed!'));
// second way
var button = querySelector('#confirm');
button.text = 'Confirm';
button.classes.add('important');
button.onClick.listen((e) => window.alert('Confirmed!'));
give an example:
class Person {
String name;
int age;
Person({this.name, this.age});
}
main(List<String> args) {
Person person = Person()
..age = 26
..name = "huhx";
print('${person.age} name ${person.name}'); // 26 name huhx
person
..age = 27
..name = "gohuhx";
print('${person.age} name ${person.name}'); // 27 name gohuhx
}
operator ?.
import 'dart:math';
main(List<String> args) {
var p = Point(2, 2);
print(p.y); // 2
var p2;
print(p2); // null
print(p2?.y); // null
}
Getting an object’s type
To get an object’s type at runtime, you can use Object’s runtimeType property, which returns a Type object.
import 'dart:math';
main(List<String> args) {
var p = Point(2, 2);
print(p.runtimeType); // Point<int>
var s = "string";
print(s.runtimeType); // String
}
Constructors
class Point {
double x, y;
Point(double x, double y) {
// There's a better way to do this, stay tuned.
this.x = x;
this.y = y;
}
}
// better way to construct
class Point {
double x, y;
// Syntactic sugar for setting x and y before the constructor body runs.
Point(this.x, this.y);
}
Extension
Extension methods, introduced in Dart 2.7, are a way to add functionality to existing libraries.
extension NumberParsing on String {
int parseInt() {
return int.parse(this);
}
}
main(List<String> args) {
print('42'.parseInt()); // 42
}