A Dart cheat sheet with the most important concepts, functions, methods, and more. A complete quick reference for beginners.
// top-level function where app execution starts
void main(){
print("Hello World!"); // Print to console
}
Every app has a main() function
int x = 2; // explicitly typed
var p = 5; // type inferred - Generic var with type inference
dynamic z = 8; // variable can take on any type
z = "cool"; // cool
// if you never intend to change a variable use final or const. Something like this:
final email = "temid@gmail.com"; // Same as var but cannot be reassigned
final String email = "temid@gmail.com"; // you can't change the value
const qty = 5; // Compile-time constant
int age = 20; // integers, range -2^63 to 2^63 - 1
double height = 1.85; // floating-point numbers
// You can also declare a variable as a num
num x = 1; // x can have both int and double values
num += 2.5;
print(num); //Print: 3.5
String name = "Nicola";
bool isFavourite = true;
bool isLoaded = false;
// can use single or double qoutes for String type
var firstName = 'Nicola';
var lastName = "Tesla";
//can embed variables in string with $
String fullName = "$firstName $lastName";
// concatenate with +
var name = "Albert " + "Einstein";
String upperCase = '${firstName.toUpperCase()}';
print(upperCase); //Print: NICOLA
// This is a normal, one-line comment.
/// This is a documentation comment, used to document libraries,
/// classes, and their members. Tools like IDEs and dartdoc treat
/// doc comments specially.
/* Comments like these are also supported. */
// Importing core libraries
import 'dart:math';
// Importing libraries from external packages
import 'package:test/test.dart';
// Importing files
import 'path/to/my_other_file.dart';
print(2 + 3); //Print: 5
print(2 - 3); //Print: -1
print(2 * 3); //Print: 6
print(5 / 2); //Print: 2.5 - Result is a double
print(5 ~/ 2); //Print: 2 - Result is an int
print(5 % 2); //Print: 1 - Remainder
int a = 1, b;
// Increment
b = ++a; // preIncrement - Increment a before b gets its value.
b = a++; // postIncrement - Increment a AFTER b gets its value.
//Decrement
b = --a; // predecrement - Decrement a before b gets its value.
b = a--; // postdecrement - Decrement a AFTER b gets its value.
print(2 == 2); //Print: true - Equal
print(2 != 3); //Print: true - Not Equal
print(3 > 2); //Print: true - Grater than
print(2 < 3); //Print: true - Less than
print(3 >= 3); //Print: true - Greater than or equal to
print(2 <= 3); //Print: true - Less than or equal to
// !expr inverts the expression (changes false to true, and vice versa)
// || logical OR
// && logical AND
bool isOutOfStock = false;
int quantity = 3;
if (!isOutOfStock && (quantity == 2 || quantity == 3)) {
// ...Order the product...
}
if(age < 18){
print("Teen");
} else if( age > 18 && age <60){
print("Adult");
} else {
print("Old");
}
enum Pet {dog, cat}
Pet myPet = Pet.dog;
switch(myPet){
case Pet.dog:
print('My Pet is Dog.');
break;
case Pet.cat:
print('My Pet is Cat.');
break;
default:
print('I don\'t have a Pet');
}
// Prints: My Pet is Dog.
while (!dreamsAchieved) {
workHard();
}
while loop check condition before iteration of the loop
do {
workHard();
} while (!dreamsAchieved);
do-while loop verifies the condition after the execution of the statements inside the loop
for(int i=0; i< 10; i++){
print(i);
}
var numbers = [1,2,3];
// for-in loop for lists
for(var number in numbers){
print(number);
}
// ordered group of objects
var list = [1, 2, 3];
print(list.length); //Print: 3
print(list[1]); //Print: 2
// other ways of list declaration and initializations
List<String> cities = <String>["New York", "Mumbai", "Tokyo"];
// To create a list that’s a compile-time constant
const constantCities = const ["New York", "Mumbai", "Tokyo"];
// A set in Dart is an unordered collection of unique items.
var halogens = {'fluorine', 'chlorine', 'bromine', 'iodine', 'astatine'};
// to create an empty set
var names = <String>{};
Set<String> names = {}; // This works, too.
//var names = {}; // Creates a map, not a set.
// a map is an object that associates keys and values
var person = Map<String, String>();
// To initialize the map, do this:
person['firstName'] = 'Nicola';
person['lastName'] = 'Tesla';
print(person); //Print: {firstName: Nicola, lastName: Tesla}
print(person['lastName']); //Print: Tesla
var nobleGases = {
// Key: Value
2: 'helium',
10: 'neon',
18: 'argon',
};
// functions in dart are objects and have a type
int add(int a, int b){
return a+b;
}
// functions can be assigned to variables
int sum = add(2,3); // returns: 5
// can be passed as arguments to other functions
int totalSum = add(2, add(2,3)); // returns : 7
// functions that contain just one expression, you can use a shorthand syntax
bool isFav(Product product) => favProductsList.contains(product);
// small one line functions that dont have name
int add(a,b) => a+b;
// lambda functions mostly passed as parameter to other functions
const list = ['apples', 'bananas', 'oranges'];
list.forEach(
(item) => print('${list.indexOf(item)}: $item'));
//Prints: 0: apples 1: bananas 2: oranges
class Cat {
String name;
// method
void voice(){
print("Meow");
}
}
// instance of a class
// below myCat is Object of class Cat
void main(){
Cat myCat = Cat();
myCat.name = "Kitty";
myCat.voice(); // Prints: Meow
}
class Cat {
String name;
Cat(this.name);
}
void main(){
Cat myCat = Cat("Kitty");
print(myCat.name); // Prints: Kitty
}
// abstract class—a class that can’t be instantiated
// This class is declared abstract and thus can't be instantiated.
abstract class AbstractContainer {
// Define constructors, fields, methods...
void updateChildren(); // Abstract method.
}
// provide read and write access to an object’s properties
class Cat {
String name;
// getter
String get catName {
return name;
}
// setter
void set catName(String name){
this.name = name;
}
}
// A person. The implicit interface contains greet().
class Person {
// In the interface, but visible only in this library.
final String _name;
// Not in the interface, since this is a constructor.
Person(this._name);
// In the interface.
String greet(String who) => 'Hello, $who. I am $_name.';
}
// An implementation of the Person interface.
class Impostor implements Person {
String get _name => '';
String greet(String who) => 'Hi $who. Do you know who I am?';
}
String greetBob(Person person) => person.greet('Bob');
void main() {
print(greetBob(Person('Kathy'))); // Hello, Bob. I am Kathy.
print(greetBob(Impostor())); // Hi Bob. Do you know who I am?
}
class Phone {
void use(){
_call();
_sendMessage();
}
}
// Use extends to create a subclass
class SmartPhone extends Phone {
void use(){
// use super to refer to the superclass
super.use();
_takePhotos();
_playGames();
}
}
// throws or raises and exception
throw IntegerDivisionByZeroException();
// You can also throw arbitrary objects
throw "Product out of stock!";
try {
int c = 3/0;
print(c);
} on IntegerDivisionByZeroException {
// A specific exception
print('Can not divide integer by 0.')
} on Exception catch (e) {
// Anything else that is an exception
print('Unknown exception: $e');
} catch (e) {
// No specified type, handles all
print('Something really unknown: $e');
}
// To ensure that some code runs whether or not an exception is thrown
try {
cookFood();
} catch (e) {
print('Error: $e'); // Handle the exception first.
} finally {
cleanKitchen(); // Then clean up.
}
// functions which are asynchronous: they return after setting up a possibly time-consuming operation
// The async and await keywords support asynchronous programming
Future<String> login() {
String userName="Temidjoy";
return
Future.delayed(
Duration(seconds: 4), () => userName);
}
// Asynchronous
main() async {
print('Authenticating please wait...');
print(await login());
}
// Extensions allow you to add methods to existing
// classes without modifying them.
// Instead of defining a util class.
class StringUtil {
static bool isValidEmail(String str) {
final emailRegExp = RegExp(r"^[a-zA-Z0-9.]+@[a-zA-Z0-9]+\.[a-zA-Z]+");
return emailRegExp.hasMatch(str);
}
}
print(StringUtil.isValidEmail('someString')); //Print: false
// We can define an extension which will be applied
// on a certain type.
extension StringExtensions on String {
bool get isValidEmail {
final emailRegExp = RegExp(r"^[a-zA-Z0-9.]+@[a-zA-Z0-9]+\.[a-zA-Z]+");
return emailRegExp.hasMatch(this);
}
}
print('test@example.com'.isValidEmail); //Print: true
print('SomeString'.isValidEmail); //Print: false
// allows you to apply the same logic to a range of types.
extension NumGenericExtensions<T extends num> on T {
T addTwo() => this + 2 as T;
}
print(7.addTwo()); // Print: 9
extension ContextExtension on BuildContext {
double get screenHeight => MediaQuery.of(this).size.height;
double get screenWidth => MediaQuery.of(this).size.width;
}
// usage
@override
Widget build(BuildContext context) => MaterialApp(
home: Scaffold(
body: Container(
width: context.screenWidth * 0.5,
height: context.screenHeight * 0.3,
color: Colors.blue,
child: Text('Hello World!'),
),
),
);
int x; // The initial value of any object is null
// ?? null aware operator
x ??=6; // ??= assignment operator, which assigns a value of a variable only if that variable is currently null
print(x); //Print: 6
x ??=3;
print(x); // Print: 6 - result is still 6
print(null ?? 10); // Prints: 10. Display the value on the left if it's not null else return the value on the right
// condition ? exprIfTrue : exprIfFalse
bool isAvailable;
isAvailable ? orderproduct() : addToFavourite();
// to insert multiple values into a collection.
var list = [1, 2, 3];
var list2 = [0, ...list];
print(list2.length); //Print: 4
// allows you to make a sequence of operations on the same object
// rather than doing this
var user = User();
user.name = "Nicola";
user.email = "nicola@g.c";
user.age = 24;
// you can do this
var user = User()
..name = "Nicola"
..email = "nicola@g.c"
..age = 24;
userObject?.userName
//The code snippet above is equivalent to following:
(userObject != null) ? userObject.userName : null
//You can chain multiple uses of ?. together in a single expression
userObject?.userName?.toString()
// The preceeding code returns null and never calls toString() if either userObject or userObject.userName is null