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.

Implementing a callable class in Dart

Example :

Output:

Welcome to GeeksForGeeks!

Note: 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.

Implementing Multiple callable functions in a class of Dart

Example :

Output:

compileDDC
main.dart:7:10: Error: 'call' is already declared in this scope.
 String call(String a) => 'Welcome to $a!';
 ^^^^
main.dart:4:10: Context: Previous declaration of 'call'.
 String call(String a, String b, String c) => 'Welcome to $a$b$c!';
 ^^^^

- 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.

- 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:

Example: 

Output:

3

Conclusion

• Dart allows the creation of callable classes through the use of the call() method.
• A callable class allows an instance to be used and invoked like a function.
• Each class can have only one call() method (method overloading is not permitted).
• Callable functions can also be created using anonymous functions, although these do not qualify as callable classes.