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Dart Closures Tutorial with Examples

  1. What is Closure?
  2. Arrow Syntax
  3. Function Class
  4. Closure in parameter

1. What is Closure?

In the Dart programming language, a Closure is a special function.
  • Similar to a function, a Closure is a block of statements with parameters and can return a value or nothing.
  • Unlike a function, a Closure has no name. However, you can identify a Closure through a variable.
Syntax:
// Closure syntax
(data_type1 arg1, data_type2 arg2, data_typeN argN) {
   // Statements
}
// Declare a variable as an alias for Closure
var alias = (data_type1 arg1, data_type2 arg2, data_typeN argN) {
   // Statements
};
Example: We declare a Closure with 1 parameter, and call it directly.
closure_ex1.dart
void main() {  
  // Create a Closure!
  (String name) {
    print('Hello $name!');
  };
  // Create a Closure and call it directly!
  (String name)  {
    print('Bye $name!');
  }('Tom');
}
Output:
Bye Tom!
Example: Declare a variable as an identifier for a Closure:
closure_ex2.dart
void main() {  
  // Declare a variable as an alias for Closure
  var sum = (int a, int b) {
    return a + b;
  };
  // Call a closure.
  var result = sum(100, 200); // 300
  print('result = $result');
}

2. Arrow Syntax

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If the Closure's content consists of only one expression, you can write it more succinctly with the arrow syntax:
var alias1 =  (data_type1 arg1, data_type2 arg2, data_typeN argN) {
     print('Something');
}
// Same as:
var alias1 =  (data_type1 arg1, data_type2 arg2, data_typeN argN) => print('Something');

var alias2 =  (data_type1 arg1, data_type2 arg2, data_typeN argN) {
     return a_value;
}
// Same as:
var alias2 =  (data_type1 arg1, data_type2 arg2, data_typeN argN)  => a_value;
Example:
closure_arrow_ex1.dart
void main() {
  // Create a Closure
  var sum = (int a, int b) => a + b;

  var result = sum(100, 200); // Call a closure.
  print('result = $result');

  // Create a Closure
  var sayHello = (String name) => print('Hello $name');
  sayHello('Tom'); // Call a closure.
}
Output:
result = 300
Hello Tom

3. Function Class

In the Dart language, a Closure is an object of the Function class, so you can declare a Closure with the following syntax:
Syntax:
Function(type1 arg1, type2 arg2, typeN argN) func = (type1 arg1, type2 arg2, typeN argN) {
   // Statements
};
// Or:
return_type Function(type1 arg1, type2 arg2, typeN argN) func;

func = (type1 arg1, type2 arg2, typeN argN) {
   // Statements
};
Example:
closure_func_ex1.dart
void main() {
  // Function Type: (int, int) --> (int)
  Function(int a, int b) sum = (int a, int b) {
    return a + b;
  };
  // Function Type: (int, int) --> (int)
  int Function(int a, int b) minus;
  minus = (int a, int b) => a - b;

  var result1 = sum(100, 200); // 300
  print('result1 = $result1');

  var result2 = minus(100, 200); // -100
  print('result2 = $result2');
}

4. Closure in parameter

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Closure can appear as a parameter of another function, closure, method, or constructor.
// A Function to calculate Tax.
// It accepts 2 arguments:
//  1 - int amount
//  2 - Closure
void printTaxAmount(int amount, double Function(int amount) taxFormula) {
  var tax = taxFormula(amount);
  print('tax = $tax');
}

// Same as:
void printTaxAmount(int amount, double taxFormula(int amount)) {
  var tax = taxFormula(amount);
  print('tax = $tax');
}
Example:
closure_in_args_ex1.dart
// A Function to calculate Tax.
// It accepts 2 arguments:
//  1 - int amount
//  2 - Closure
void printTaxAmount(int amount, double Function(int amount) taxFormula) {
  var tax = taxFormula(amount);
  print('tax = $tax');
}
void main() {
  // Create a Closure (Tax calucation formula of US)
  var usTaxFormula = (int amount) {
    return 10 * amount / 100; // return a double type.
  };
  // Create a Closure (Tax calucation formula of Vietnam)
  var vnTaxFormula = (int amount) {
    return 5 * amount / 100; // return a double type.
  };
  const amount = 1000;
  printTaxAmount(amount, usTaxFormula); // 100.0
  printTaxAmount(amount, vnTaxFormula); // 50.0
}
Output:
tax = 100.0
tax = 50.0
List.generate Example:
The List.generate constructor of the List class has a Closure as a parameter. Let's see an example of how to use this constructor.
List.generate Constructor
external factory List.generate(int length, E generator(int index), {bool growable = true});
Example:
closure_in_args_ex3.dart
// List.generate Constructor:
// external factory List.generate(int length, E generator(int index), {bool growable = true});
void main() {
  // (int index) --> (String)
  var generator = (int index) {
    return 'Value ' + index.toString();
  };
  // Create a List with 5 elements:
  List<String> myList = List.generate(5, generator, growable: true);
  print(myList); // [Value 0, Value 1, Value 2, Value 3, Value 4]
}
Output:
[Value 0, Value 1, Value 2, Value 3, Value 4]
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