Concept of Callable Classes in Dart

Dart allows the user to create a callable class which allows the instance of the class to be called as a function. To allow an instance of your Dart class to be called like a function, implement the call() method. 

Syntax:

class class_name {
  ... // class content
  
  return_type call ( parameters ) {
    ... // call function content
  }
  
}

In the above syntax, we can see that to create a callable class we have to define a call method with a return type and parameters within it.

Example 1: Implementing a callable class in Dart.

Dart

// Creating Class GeeksForGeeks
class GeeksForGeeks {

  // Defining call method which create the class callable
  String call(String a, String b, String c) => 'Welcome to $a$b$c!';
}

// Main Function
void main() {
  // Creating instance of class
  var geek_input = GeeksForGeeks();

  // Calling the class through its instance
  var geek_output = geek_input('Geeks', 'For', 'Geeks');

  // Printing the output
  print(geek_output);
}

Output:

Welcome to GeeksForGeeks!

It must be noted that Dart doesn’t support multiple callable methods i.e. if we try to create more than one callable function for the same class it will display error.

Example 2: Implementing multiple callable functions in a class of Dart.

Dart

// Creating Class GeeksForGeeks
class GeeksForGeeks {
  // Defining call method which create the class callable
  String call(String a, String b, String c) => 'Welcome to $a$b$c!';

  // Defining another call method for the same class
  String call(String a) => 'Welcome to $a!';
}

// Main Function
void main() {
  // Creating instance of class
  var geek_input = GeeksForGeeks();

  // Calling the class through its instance
  var geek_output = geek_input('Geeks', 'For', 'Geeks');

  // Printing the output
  print(geek_output);
}

Output:

Error compiling to JavaScript:
main.dart:3:10:
Error: 'call' is already declared in this scope.
  String call(String a) => 'Welcome to $a!';
         ^^^^
main.dart:2:10:
Info: Previous declaration of 'call'.
  String call(String a, String b, String c) => 'Welcome to $a$b$c!';
         ^^^^
main.dart:8:31:
Error: Can't use 'call' because it is declared more than once.
  var geek_output = geek_input('Geeks', 'For', 'Geeks');
                              ^^^^
main.dart:8:31:
Error: The method 'call' isn't defined for the class 'GeeksForGeeks'.
 - 'GeeksForGeeks' is from 'main.dart'.
  var geek_output = geek_input('Geeks', 'For', 'Geeks');
                              ^
Error: Compilation failed.

A callable class in Dart is a class that can be invoked like a function. To create a callable class, you must define a call method inside the class. The call method can take any number of arguments and return any type of value.

Dart

class Adder {
  int add(int a, int b) {
    return a + b;
  }
}

void main() {
  var adder = Adder();
  var sum = adder(1, 2);
  print(sum); // prints 3
}

You can also define a call method inside an anonymous function, which allows you to create a callable function on the fly. Here is an example:

Dart

void main() {
  var adder = (int a, int b) {
    return a + b;
  };
  var sum = adder(1, 2);
  print(sum); // prints 3
}

Example: 

Dart

void main() {
  var adder = Adder();
  var sum = adder(1, 2);
  print(sum); // prints 3
}

class Adder {
  int add(int a, int b) {
    return a + b;
  }

  // Define the call method
  int call(int a, int b) {
    return add(a, b);
  }
}

Output: 3