flutterdartrequireddart-null-safety

Flutter required keyword


I don't really understand how required works. For example I've seen this code:

class Test{
  final String x;
  Test({
    required this.x
  });

  factory Test.initial(){
    return Test(x: "");
  }
}

But what should required do here? Seems like it makes an optional parameter a non optional parameter.


Solution

  • Dart 2.12 (null safety):

    Beginning with Dart 2.12, the @required annotation is now replaced by the required keyword. You should mark your field required if it is mandatory for others to pass some value to it.

    For example:

    class Foo {
      final int a; // Mandatory? Use 'required'
      final int b; // Not mandatory? Don't use 'required'
    
      Foo({
        required this.a, // Marked 'required'
        this.b = 1, 
      });
    }
    

    Usage:

    Foo(); // Error: 'a' is required
    Foo(a: 0); // Good
    Foo(a: 0, b: 1); // Good