In which case do we need to use typedef in dart

when I used the typedef in the code it didn't do anything extra except increase my line of code. I need to know when to use typedef and when to avoid it?

Code snippet using typedef

typedef SaySomething = void Function(String name);
    
    void sayHello(String name){
      print('Hello to $name');
    }
    
    void sayGoodbye(String name){
      print('Goodbye to $name');
    }
    
    void main() {
      // Using alias
      SaySomething myFunction;
      
      myFunction = sayHello;
      myFunction('[email protected]');
      
      myFunction = sayGoodbye;
      myFunction('[email protected]');
    }

Code snippet without using typedef

void sayHello(String name){print('Hello to $name');}
        
        void sayGoodbye(String name){ print('Goodbye to $name');}
        
        void main() {        
          sayHello('[email protected]');            
          sayGoodbye('[email protected]');
        }

CodePudding user response：

So, it is my belief that there are very few situations in which you need typedef, but it can help to avoid long code repetition, take a look at this example class:

class MyClass {
  MyClass(this.value, {
    this.onChange,
    this.onSubmit,
    this.onCancel,
    this.onDelete,
    this.onReset,
  });

  Map<String, Map<int, List<double>>> value;
  
  void Function(Map<String, Map<int, List<double>>>)? onChange;
  void Function(Map<String, Map<int, List<double>>>)? onSubmit;
  void Function(Map<String, Map<int, List<double>>>)? onCancel;
  void Function(Map<String, Map<int, List<double>>>)? onDelete;
  void Function(Map<String, Map<int, List<double>>>)? onReset;
}

The above code is ugly to say the least, I think it is obvious how to use typedef here, but I will show it nonetheless.

typedef MyCallback = void Function(Map<String, Map<int, List<double>>>);

class MyClass {
  MyClass(this.value, {
    this.onChange,
    this.onSubmit,
    this.onCancel,
    this.onDelete,
    this.onReset,
  });

  Map<String, Map<int, List<double>>> value;

  MyCallback? onChange;
  MyCallback? onSubmit;
  MyCallback? onCancel;
  MyCallback? onDelete;
  MyCallback? onReset;
}

or even

typedef MyClassValue = Map<String, Map<int, List<double>>>;

class MyClass {
  MyClass(this.value, {
    this.onChange,
    this.onSubmit,
    this.onCancel,
    this.onDelete,
    this.onReset,
  });

  MyClassValue value;

  void Function(MyClassValue)? onChange;
  void Function(MyClassValue)? onSubmit;
  void Function(MyClassValue)? onCancel;
  void Function(MyClassValue)? onDelete;
  void Function(MyClassValue)? onReset;
}

Yes, it is more code, but the code is better looking.

In general, typedef is suited for functional programming patterns where you find yourself passing functions as arguments and returning functions from functions.