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|
---
language: dart
filename: learndart.dart
contributors:
- ["Joao Pedrosa", "https://github.com/jpedrosa/"]
- ["Vince Ramces Oliveros", "https://github.com/ram231"]
---
**Dart** is a single threaded, general purpose programming languages.
It borrows a lot from other mainstream languages.
It supports Streams, Futures(known as Promises in JavaScript), Generics, First-class functions(closures) and static type checking.
Dart can run in any platform such as Web, CLI, Desktop, Mobile and IoT devices.
Dart's most controversial feature is its ~~Optional Typing~~ Static Type safety and [Sound Type checks](https://dart.dev/guides/language/sound-dart).
```dart
import "dart:collection";
import "dart:math" as math;
/// Welcome to Learn Dart in 15 minutes. http://dart.dev/
/// This is an executable tutorial. You can run it with Dart or on
/// the Try Dart! site if you copy/paste it there. http://dartpad.dev/
/// You can also run Flutter in DartPad by click the `< > New Pad ` and choose Flutter
/// In Dart, Everything is an Object.
/// Every declaration of an object is an instance of Null and
/// Null is also an object.
/// 3 Types of comments in dart
// Single line comment
/**
* Multi-line comment
* Can comment more than 2 lines
*/
/// Code doc comment
/// It uses markdown syntax to generate code docs when making an API.
/// Code doc comment is the recommended choice when documenting your APIs, classes and methods.
/// 4 types of variable declaration.
/// Constants are variables that are immutable cannot be change or altered.
/// `const` in dart should practice SCREAMING_SNAKE_CASE name declaration.
const CONSTANT_VALUE = "I CANNOT CHANGE";
CONSTANT_VALUE = "DID I?"; //Error
/// Final is another variable declaration that cannot be change once it has been instantiated. Commonly used in classes and functions
/// `final` can be declared in pascalCase.
final finalValue = "value cannot be change once instantiated";
finalValue = "Seems not"; //Error
/// `var` is another variable declaration that is mutable and can change its value. Dart will infer types and will not change its data type
var mutableValue = "Variable string";
mutableValue = "this is valid";
mutableValue = false; // Error.
/// `dynamic` is another variable declaration in which the type is not evaluated by the dart static type checking.
/// It can change its value and data type.
/// Some dartisans uses dynamic cautiously as it cannot keep track of its data type. so use it at your own risk
dynamic dynamicValue = "I'm a string";
dynamicValue = false; // false
/// Functions can be declared in a global space
/// 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 can be an implicit or
/// explicit return for the result of the expression.
/// Dart will execute a function called `main()` anywhere in the dart project.
///
example1() {
nested1() {
nested2() => print("Example1 nested 1 nested 2");
nested2();
}
nested1();
}
/// Anonymous functions don't include a name but can take number of arguments
example2() {
//// Explicit return type.
nested1(Function<void> fn) {
fn();
}
nested1(() => print("Example2 nested 1"));
}
/// When a function parameter is declared, the declaration can include the
/// number of parameters the function takes by explicitly specifying the names of the
/// parameters it takes.
example3() {
planA(fn(String informSomething)) {
fn("Example3 plan A");
}
planB(fn) {
// Or don't declare number of parameters.
fn("Example3 plan B");
}
planA((s) => print(s));
planB((s) => print(s));
}
/// Functions have closure access to outer variables.
/// Dart will infer types when the variable has a value of something.
/// In this example dart knows that this variable is a String.
var example4Something = "Example4 nested 1";
example4() {
nested1(fn(informSomething)) {
fn(example4Something);
}
nested1((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.
/// the `new` keyword is optional in Dart.
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 instanceVariable = "Example6 instance variable";
sayIt() {
print(instanceVariable);
}
}
example6() {
Example6Class().sayIt();
}
/// Class methods and variables are declared with "static" terms.
class Example7Class {
static var classVariable = "Example7 class variable";
static sayItFromClass() {
print(classVariable);
}
sayItFromInstance() {
print(classVariable);
}
}
example7() {
Example7Class.sayItFromClass();
new Example7Class().sayItFromInstance();
}
/// Dart supports Generics.
/// Generics refers to the technique of writing the code for a class
/// without specifying the data type(s) that the class works on.
/// Source: https://stackoverflow.com/questions/4560890/what-are-generics-in-c
/// Type `T` refers to any type that has been instantiated
/// you can call whatever you want
/// Programmers uses the convention in the following
/// T - Type(used for class and primitype types)
/// E - Element(used for List, Set, or Iterable)
/// K,V - Key Value(used for Map)
class GenericExample<T>{
void printType(){
print("$T")
}
// methods can also have generics
genericMethod<M>(){
print("class:$T, method: $M");
}
}
/// List are similar to arrays but list is a child of Iterable<E>
/// Therefore Maps, List, LinkedList are all child of Iterable<E> to be able to loop using the keyword `for`
/// Important things to remember:
/// () - Iterable<E>
/// [] - List<E>
/// {} - Map<K,V>
/// List are great, but there's a restriction for what List can be
/// outside of function/method bodies. List on the outer scope of class
/// or outside of class have to be constant. Strings and numbers are constant
/// by default. But arrays and maps are not. They can be made constant by
/// declaring them "const". Kind of similar to Javascript's Object.freeze()
const example8List = ["Example8 const array"],
const example8Map = {"someKey": "Example8 const map"};
/// Declare List or Maps as Objects.
List<String> explicitList = new List<String>();
Map<String,dynamic> explicitMaps = new Map<String,dynamic>();
explicitList.add("SomeArray");
example8() {
print(example8Array[0]);
print(example8Map["someKey"]);
print(explicitList[0]);
}
/// Assigning a list from one variable to another will not be the same result.
/// Because dart is pass-reference-by-value.
/// So when you assign an existing list to a new variable.
/// Instead of List, it becomes an Iterable
var iterableExplicitList = explicitList;
print(iterableExplicitList) // ("SomeArray"); "[]" becomes "()"
var newExplicitLists = explicitList.toList() // Converts Iterable<E> to List<E>
/// Loops in Dart take the form of standard for () {} or while () {} loops,
/// slightly more modern for (.. in ..) {}, or functional callbacks with many
/// supported features, starting with forEach,map and where.
var example9Array = const ["a", "b"];
example9() {
for (final i = 0; i < example9Array.length; i++) {
print("Example9 for loop '${example9Array[i]}'");
}
var i = 0;
while (i < example9Array.length) {
print("Example9 while loop '${example9Array[i]}'");
i++;
}
for (final e in example9Array) {
print("Example9 for-in loop '${e}'");
}
example9Array.forEach((e) => print("Example9 forEach loop '${e}'"));
}
/// To loop over the characters of a string or to extract a substring.
var example10String = "ab";
example10() {
for (var i = 0; i < example10String.length; i++) {
print("Example10 String character loop '${example10String[i]}'");
}
for (var i = 0; i < example10String.length; i++) {
print("Example10 substring loop '${example10String.substring(i, i + 1)}'");
}
}
/// `int`, `double` and `num` are the three supported number formats.
/// `num` can be either `int` or `double`.
/// `int` and `double` are children of type `num`
example11() {
var i = 1 + 320, d = 3.2 + 0.01;
num myNumDouble = 2.2;
num myNumInt = 2;
int myInt = 1;
double myDouble = 0; // Dart will add decimal prefix, becomes 0.0;
myNumDouble = myInt; // valid
myNumDouble = myDouble; //valid
myNumDouble = myNumInt; //valid
myNumInt = myInt; // valid
myNumInt = myDouble; // valid
myNumInt = myNumDouble; // valid
myInt = myNumDouble; //Error
myInt = myDouble; //Error
myInt = myNumInt; //valid
myDouble = myInt; //error
myDouble = myNumInt; //valid
myDouble = myNumDouble; //valid
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 support implicit conversions and dynamic type
example14() {
var a = true;
if (a) {
print("true, a is $a");
}
a = null;
if (a) {
print("true, a is $a");
} else {
print("false, a is $a"); /// runs here
}
/// dynamic typed null can be convert to bool
var b;/// b is dynamic type
b = "abc";
try {
if (b) {
print("true, b is $b");
} else {
print("false, b is $b");
}
} catch (e) {
print("error, b is $b"); /// this could be run but got error
}
b = null;
if (b) {
print("true, b is $b");
} else {
print("false, b is $b"); /// runs here
}
/// statically typed null can not be convert to bool
var c = "abc";
c = null;
/// complie failed
/// if (c) {
/// print("true, c is $c");
/// } else {
/// print("false, c is $c");
/// }
}
/// try/catch/finally and throw are used for exception handling.
/// throw takes any object as parameter;
example15() {
try {
try {
throw "Some unexpected error.";
} catch (e) {
print("Example15 an exception: '${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 List 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);
}
}
/// Optional Positional Parameter:
/// parameter will be disclosed with square bracket [ ] & square bracketed parameter are optional.
example31() {
findVolume31(int length, int breath, [int height]) {
print('length = $length, breath = $breath, height = $height');
}
findVolume31(10,20,30); //valid
findVolume31(10,20); //also valid
}
/// Optional Named Parameter:
/// parameter will be disclosed with curly bracket { }
/// curly bracketed parameter are optional.
/// have to use parameter name to assign a value which separated with colan :
/// in curly bracketed parameter order does not matter
/// these type parameter help us to avoid confusion while passing value for a function which has many parameter.
example32() {
findVolume32(int length, int breath, {int height}) {
print('length = $length, breath = $breath, height = $height');
}
findVolume32(10,20,height:30);//valid & we can see the parameter name is mentioned here.
findVolume32(10,20);//also valid
}
/// Optional Default Parameter:
/// same like optional named parameter in addition we can assign default value for this parameter.
/// which means no value is passed this default value will be taken.
example33() {
findVolume33(int length, int breath, {int height=10}) {
print('length = $length, breath = $breath, height = $height');
}
findVolume33(10,20,height:30);//valid
findVolume33(10,20);//valid
}
/// Dart has also added feature such as Null aware operators
var isBool = true;
var hasString = isBool ?? "default String";
var hasValue ??= "default Value";
/// 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 // Adding this comment stops the dart formatter from putting all items on a new line
].forEach((ef) => ef());
}
```
## Further Reading
Dart has a comprehensive web-site. It covers API reference, tutorials, articles and more, including a
useful Try Dart online.
[https://www.dartlang.org](https://www.dartlang.org)
[https://try.dartlang.org](https://try.dartlang.org)
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