Initializing members in a Constructor

user_5761604 0
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Let's assume a simple class like this

class MyClass {
  String aString;
  DateTime aDate;

  MyClass({this.aString = '', this.aDate = DateTime.now()});
}

This code does not work because optional parameters must be constant. I do not want aDate be nullable.

I can do this:

class MyClass {
  String aString;
  DateTime aDate;

  MyClass({this.aString = ''}) : this.aDate = DateTime.now();
}

This is accepted by the compiler but I cannot set aDate when constructing an instance of MyClass like so: var c = MyClass(aString: 'bla bla', aDate: DateTime(2021, 9, 20));

Instead it seams I have to write:

var c = MyClass(aString: 'bla bla');
c.aDate = DateTime(2021, 9, 20);

Is there really no better way to initialize complex objects with optional named parameters in constructors with null safety turned on? This is just a simplified sample. In reality, I have classes with lots of complex objects that I get from a remote server. The method shown above forces me to write hundreds of lines of assignment statements.

dart-null-safety dart
1 Answers

You can use the following:

class MyClass {
  String aString;
  DateTime aDate;

  MyClass({this.aString = '', DateTime? aDate}) : aDate = aDate ?? DateTime.now();
}

You were on the right track, you just needed to combine the two methods you provided above. This uses an optional named parameter, which must be nullable, and only assigns it to this.aDate when it's not null using the ?? operator. If it is null, it uses your desired default value of DateTime.now().

user_132501420
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