Learning Dart

Variables

The following syntax illustrates how to declare a variable:

type variableName;

The following shows some common types in Dart:

int – represents whole numbers like -1, 0, 1, 2.
double – represents practical values like 0.5, and 9.98.
String – represents text such as "Good Morning!".
bool – represents Boolean values including true and false.

Therefore, you don’t need to specify the type of the httpStatusCode variable explicitly. Instead, you can use the var keyword:

var variableName;

By convention, the variable name use lowerCaseCamel, See the example.

Constant

Use the const keyword to define a constant. A constant is an identifier whose value doesn’t change. See the example.

Final Keyword

The final keyword, is similar like const keyword. Unlike the const keyword, you don’t need to initialize the finalVariable in the declaration immediately.

  final int money;
  money = 100; // ok

  const String country;
  country = 'Indonesia'; // error

Function

A function is a reusable unit of code that performs a task. In programming, you often need to perform the same task multiple times. And you don’t want to copy & paste the same code all over places in the program. To do this, you wrap the code in a function and use it instead of copying/pasting the same code. See the example.

void greet(String name) {
  print('Halo ${name}');
}


void main() {
  greet('Hanasa'); // Halo Hanasa
}

Named Paramter

Named parameters to make the parameters clear in function calls. Use {} to surround the named parameters. By default, named parameters are optional. Use the required keyword to make them required.

void connect(String host,
    {int port = 3306, required String user, required String password}) {
  print('Connecting to $host on $port using credential $user/$password');
}

Specify the parameter names when calling a function with named parameters.

void main() {
  connect('localhost', user: 'Hanasa', password: '1z23');
}

Dart functions are First-class Citizens

Functions are first-class citizens. This means that you can treat a function as a value of other types. So you can:

Assign a function to a variable.

int add(int x, int y) {
  return x + y;
}

void main() {
  var fn = add;
  var result = fn(10, 20);
  print(result); // 30
}

Pass a function to another function as an argument.

bool isOddNumber(int x) {
  return x % 2 != 0;
}

bool isEvenNumber(int x) {
  return x % 2 == 0;
}

void show(Function fn) {
  for (int i = 0; i < 10; i++) {
    if (fn(i)) {
      print(i);
    }
  }
}

void main() {
  print("Even numbers:");
  show(isEvenNumber);

  print("Odd numbers:");
  show(isOddNumber);
}

Result

Even numbers:
0
2
4
6
8
Odd numbers:
1
3
5
7
9

Return a function from a function.

add(int x, int y) {
  return x + y;
}

subtract(int x, int y) {
  return x - y;
}

Function calculation(String op) {
  if (op == '+') return add;
  if (op == '-') return subtract;
  return add;
}

void main() {
  var fn = calculation('+');
  print(fn(10, 20));

  fn = calculation('-');
  print(fn(30,20));
}

An anonymous function

An anonymous function is a function that doesn’t have a name. See the example.

Arrow function

If a function body has only one line, you can use an arrow function with the following syntax to make it more compact. Use arrow functions for the functions with one line to make the code more concise. See the example.

Class

An object consists of states and behaviors:

The state describes the values that an object has at a specified time.
The behaviors are actions that an object can do to manipulate its state.

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