Dart's noSuchMethod Feature

In Dart, a call to a non-existent method on an object will be routed to a special method named noSuchMethod(..) which all objects inherit from Object. For example, if you run this code:

p1.jump();

and p1 doesn't have a method named jump then Dart will instead call:

p1.noSuchMethod(..)

The default implementation of noSuchMethod (the one from Object) just throws a NoSuchMethodException, which is typically what you want. But you can override noSuchMethod to some useful things. First, I'll show you a trivial example, to illustrate the basic mechanics then I'll show a slightly more practical use-case.

Here is the simplest possible example:

class Box{

  noSuchMethod(InvocationMirror invocation) {

    print(invocation.memberName);

  }

}

main(){

  var b = new Box();

  b.foo(); //prints foo, the method name

}

Since b does not have a foo method, it calls our noSuchMethod method. This is obviously not very useful. Below is a more practical example.

A more realistic use-case
Many developers like to access and update database records as objects. This is what Java's Hibernate and Rail's Active Record and Django's Object-relational mapper do. One way to do this is to wrap a generic map or database record with a class as in the example below:

class Person{

  

  DbRecord record;

  

  String get firstName() => record.getFieldValue("firstName"); 

  String get lastName() => record.getFieldValue("lastName"); 

  String get age() => record.getFieldValue("age"); 

  String get fullName() => firstName + " " + lastName; 

  

}

The thing to notice here is that 3 of the 4 getter methods are completely generic. Here is how this could be made easier using noSuchMethod:

class Person{

  DbRecord record;

  String get fullName() => firstName + " " + lastName; 

  noSuchMethod(InvocationMirror invocation) => record.getFieldValue(invocation.memberName);

}

By extracting this into a generic base class it become a bit more useful.

Like many of Dart's features, this is not something ground breaking. Dart's nosuchMethod capability is similar to Ruby's method_missing and Python's __setattr__.