Functions

Remarks#

Dart is a true object-oriented language, so even functions are objects and have a type, Function. This means that functions can be assigned to variables or passed as arguments to other functions. You can also call an instance of a Dart class as if it were a function.

Functions with named parameters

When defining a function, use {param1, param2, …} to specify named parameters:

void enableFlags({bool bold, bool hidden}) {
  // ...
}

When calling a function, you can specify named parameters using paramName: value

enableFlags(bold: true, hidden: false);

Function scoping

Dart functions may also be declared anonymously or nested. For example, to create a nested function, just open a new function block within an existing function block

void outerFunction() {
  
    bool innerFunction() {
        /// Does stuff
    }
}

The function innerFunction may now be used inside, and only inside, outerFunction. No other other functions has access to it.

Functions in Dart may also be declared anonymously, which is commonly used as function arguments. A common example is the sort method of List object. This method takes an optional argument with the following signature:

int compare(E a, E b)

The documentation states that the function must return 0 if the a and b are equal. It returns -1 if a < b and 1 if a > b.

Knowing this, we can sort a list of integers using an anonymous function.

List<int> numbers = [4,1,3,5,7];

numbers.sort((int a, int b) {
   if(a == b) {
      return 0;
   } else if (a < b) {
      return -1;
   } else {
      return 1;
   }
});

Anonymous function may also be bound to identifiers like so:

Function intSorter = (int a, int b) {
   if(a == b) {
      return 0;
   } else if (a < b) {
      return -1;
   } else {
      return 1;
   }
}

and used as an ordinary variable.

numbers.sort(intSorter);