Building object trees #

Constructing object trees is very important in Flutter (framework).

Dart (language) has a couple of tricks you can use to write constructors for classes, so that they are both clear and concise.

Constructors #

Constructor in C#

This is how you are used to writing constructors in C#.

class Task
{
    string name;
    bool done;
    Task(string name, bool done)
    {
        this.name = name;
        this.done = done;
    }
}

Constructor in Dart

In Dart we need to assign fields/instance-variables a bit different.

class Task {
  String name;
  bool done;
  Task(String name, bool done):
    this.name = name,
    this.done = done;
}

Notice that colon is used followed by a list of assignments.

Shorthand parameters

Because assigning parameters to fields/instance-variables is so common, there is a shorthand for it.

class Task {
  String name;
  bool done;
  Task(this.name, this.done);
}

Task("Learn Dart", false);

Optional parameters

We can make a parameter optional wrapping it in [] and making the field nullable.

class Task {
  String name;
  bool? done;
  Task(this.name, [this.done]);
}

Task("Learn Dart"); // `done` is null
Task("Learn Dart", false);

Optional with default value

Instead of having the field nullable, you could specify a default value.

class Task {
  String name;
  bool done;
  Task(this.name, [this.done = false]);
}

Task("Learn Dart"); // `done` is true
Task("Learn Dart", true);

Named parameter

It can be difficult to remember what the meaning is of a positional parameter. If you just saw Task("Learn Dart", true) without seeing the class definition, would you be able to tell what the 2nd parameter means? Maybe true means that the task is done, or it could indicate that the task is important.

Named parameters can be used to make the purpose of a parameter more explicit.

class Task {
  String name;
  bool? done;
  Task(this.name, {this.done});
}

Task("Learn Dart", done: true);

Now there is no doubt what true means.

Named with default value

class Task {
  String name;
  bool? done;
  Task(this.name, {this.done = false});
}

Task("Learn Dart"); // `done` is false

Required named parameter

Named parameters are optional by default. It can be useful to make a named parameter required.

class Task {
  String name;
  bool done;
  Task(this.name, {required this.done});
}

Task("Learn Dart"); // Compile error, since `done` is required