How to avoid escaping references in Dart? - dart

Question

Dart is pass-by and return-by reference, meaning that if I have a class which contains a member instance of another class, and I want to return this using a get function, the caller of this function has direct access to mutate the data inside that member instance.
For instance, take the following example:
class Mother {
String _name;
Mother() {
_name = "Default";
}
String get name {
return _name;
}
set name (String name) {
_name = name;
}
}
class Child {
Mother _mother = Mother();
Mother get mother {
return _mother;
}
}
void main() {
Child child = Child();
Mother localMother = child.mother;
localMother.name = "Samantha";
print(localMother.name); // Prints Samantha - expected
print(child.mother.name); // Prints Samantha - bad. The original instance inside child was mutated, without the ability for me to react inside my setter
}
How can I prevent the caller of child.mother from modifying the data inside the mother class?

Answer 1

Something like this will make sure the user gets an error on compile time:
class Mother {
String _name;
Mother() : _name = "Default";
String get name {
return _name;
}
}
class _Mother extends Mother {
set name(String name) {
_name = name;
}
}
class Child {
_Mother _mother = _Mother();
Mother get mother {
return _mother;
}
}
void main() {
Child child = Child();
Mother localMother = child.mother;
localMother.name = "Samantha"; // There isn’t a setter named 'name' in class 'Mother'.
print(localMother.name); // Prints Samantha - expected
print(child.mother
.name); // Prints Samantha - bad. The original instance inside child was mutated, without the ability for me to react inside my setter
}
An alternative design (which are used a lot in the Dart SDK) is to make something like this. A problem here is we cannot give a compile error about the use of set since the exception are thrown at runtime. The same behavior can be observed when using UnmodifiableListView from dart:collection:
abstract class Mother {
String get name;
set name(String name);
factory Mother() => _Mother();
}
class _Mother implements Mother {
String _name;
_Mother() {
_name = "Default";
}
String get name {
return _name;
}
set name(String name) {
_name = name;
}
}
class MotherView implements Mother {
final Mother _src;
MotherView(this._src);
#override
String get name => _src.name;
#override
set name(String name) {
throw UnsupportedError('Setting name');
}
}
class Child {
Mother _mother = Mother();
Mother get mother => MotherView(_mother);
}
void main() {
Child child = Child();
Mother localMother = child.mother;
localMother.name = "Samantha"; // UnsupportedError: Setting name
print(localMother.name); // Prints Samantha - expected
print(child.mother
.name); // Prints Samantha - bad. The original instance inside child was mutated, without the ability for me to react inside my setter
}