Extension Methods

For Dart see here, for Kotlin see here

Kotlin

fun String.parseInt():Int {
    //kotlin already have a similar extension method: redirect to it
    return this.toInt()
}
fun String.parseDouble():Double {
    //kotlin already have a similar extension method: redirect to it
    return this.toDouble()
}

fun <T> MutableList<T>.swap(index1: Int, index2: Int) {
    val tmp = this[index1] // 'this' corresponds to the list
    this[index1] = this[index2]
    this[index2] = tmp
}

println("123".parseInt()+1); //124
println("123.0".parseDouble()+0.5); //123.5

var list= mutableListOf(0,1,2,3)
list.swap(0,1);
println(list); //1,0,2,3

Dart

// extension method are defined as part of a named extension block
// it is used in Dart to help resolving conflicting extension method definitions
extension NumberParsing on String {
  int parseInt() {
    return int.parse(this);
  }
  double parseDouble() {
    return double.parse(this);
  }
  // ···
}

// it is possible to define extension methods on a generic type. 
// This is the syntax
extension ListSwap<T> on List<T> {
  void swap(int index1, int index2) {
    final tmp= this[index1];
    this[index1]=this[index2];
    this[index2]=tmp;
  }
}

// To create a local extension that’s visible only in the library 
// where it’s declared, either omit the extension name
// or give it a name that starts with an underscore (_)
extension on String {
  String wrappedWithA() {
    return "A${this}A";
  }
}

print('123'.parseInt()+1); //124
print('123.0'.parseDouble()+0.5); //123.5

var list= [0,1,2,3];
list.swap(0,1);
print(list); //1,0,2,3
print ('BB'.wrappedWithA()); //ABBA

extensions are resolved statically

Both in Dart and in Kotlin extensions are resolved statically. In other words the dynamic type at runtime does not count. What counts is the resolved static type at a compile time.

Extension Methods Visibility

There are several important details about extension methods visibility and disambiguation. For Dart see here for Kotlin see here