What is 'Private properties' in Dart? - flutter

Question

While studying Dart, I noticed that private properties start with an underscore. So what do private properties mean?
class OfficialName extends Name {
// Private properties begin with an underscore
final String _title;
OfficialName(this._title, String first, String last)
: super (first, last);
#override
String toString(){
return 'S_title. ${super.toString()}';
}
}

Answer 1

Private properties are those properties that are only accessible in the file in which they were declared. In other words, only dart code in the dart file, where these private properties are found, "know" the private properties.
Consider the following example
In lib/foo.dart
class Foo {
var _foo = 'foo';
var bar = 'bar';
}
main() {
print(Foo()._foo); // foo
print(Foo().bar); // bar
}
Then in lib/bar.dart
import './foo.dart';
main() => {
// print(Foo()._foo); // this won't work
print(Foo().bar); // bar
}
Running both files will show the commented results. But if the print(Foo()._foo); line is uncommented in bar.dart, the compiler will throw an Error, that the getter '_foo' isn't defined for the class 'Foo'. This is because the _foo property on the Foo class is private to the foo.dart file.
In Dart, every file is a library. So it makes sense that privacy or private properties are scoped to each library.
To create a private property, be it a field or a method, simple prefix its name with an underscore _.
So when coding, if there are properties you feel that other dart files (or libraries) shouldn't access, (maybe because of isolating logic), then you can make them private (by prefixing with underscore _).

Answer 2

Dart privacy is indeed only on a per library basis.

Answer 3

I understand this way, let's talk about your example.
final String _title; means our title here is private.
If we declare a variable with underscore, we can't change its property.
Let's create an object of OfficialName and print it.
OfficialName ofName = OfficialName("Dr", "John", "kim");
print("From constructor=> title: ${ofName._title} first: ${ofName.first} last: ${ofName.last}");
Now if we like to change the attribute value of this object, we can do
ofName.first= "new value" however, we can't change _title here. I think private property as an immutable object.
Example code
class Name {
String first;
String last;
Name(this.first, this.last);
}
class OfficialName extends Name {
// Private properties begin with an underscore
final String _title;
OfficialName(this._title, String first, String last) : super(first, last);
#override
String toString() {
return 'S_title. ${super.toString()}';
}
}
// Main function
void main() {
OfficialName ofName = OfficialName("Dr", "John", "kim");
print(
"From constructor=> title: ${ofName._title} first: ${ofName.first} last: ${ofName.last}");
///let's change some property
/// this will show errors
ofName._title = "changed title";
ofName.first = "firstName";
ofName.last = "lastName";
print(
"After changed=> title: ${ofName._title} first: ${ofName.first} last: ${ofName.last}");
}
For clear information visit language-tour

Answer 4

The underscore determines the visibility of that identifier to the outside:
The import and library directives can help you create a modular and shareable code base. Libraries not only provide APIs, but are a unit of privacy: identifiers that start with an underscore (_) are visible only inside the library. Every Dart app is a library, even if it doesn’t use a library directive.
From the dart documentation