Why do we pass functions to widgets?

• Functions are first class objects in Dart and can be passed as parameters to other functions.
• We pass a function to a widget essentially saying, “invoke this function when something happens”.
• Callbacks using interfaces like Android (<Java 8) have too much boilerplate code for a simple callback.

Java Functions are first class objects in Dart and can be passed as parameters to other functions.callback:

button.setOnClickListener(new View.OnClickListener() {  
    @override  
    public void onClick(View view) {  
      // Do something here  
    }  
  }  
);

(Notice that this is only the code for setting up a listener. Defining a button requires separate XML code.)

Dart equivalent:

FlatButton(  
  onPressed: () {  
    // Do something here  
  }  
)

(Dart does both declaration as well as setting up the callback.) This becomes much cleaner and organised and helps us avoid unnecessary complication.