'MyhomePage({Key key, this.title}) : super(key: key);' in Flutter - what would be a clear explanation with an example?

Question

In Flutter, what would be a clear explanation with an example?

My confusion is about key, as in the code below.

MyHomepage({Key key, this.title}) : super(key: key);

Answer 1

The code is the constructor of the MyHomepage widget.

{Key key, this.title}

It declares two optional named parameters (optional named because of {}) where

• the first is of name key with type Key

• the second is of name title with the type of the field this.title and automatically initializes this.title with the passed value. This is nice syntactic sugar that saves some writing.

: starts the initializer list. The initializer list allows some to execute some expressions before the call is forwarded to the constructor of the super class.

When a class is initialized, read access to this is forbidden until the call to the super constructor is completed (until the body of the constructor is executed - in your example the constructor has no body).

The initializer list is often use to validate passed parameter values with assert(key != null) or to initialize final fields with calculated values (final fields can't be initialized or updated later).

super(key: key) forwards to the constructor of the super class and passes the parameter key passed to MyHomepage to the super constructors key parameter (same as for MyHomepage({Key key})).

Answer 2

Thanks for Günter's detailed explanation which helped me at the very beginning. Here I would like to explain the background for this question, and especially the punctuation as syntax a little bit.

The mentioned line of code,

MyHomepage({Key key, this.title}) : super(key: key);

should come from the auto-generated Flutter application boilerplate.

The complete context is:

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

For me, the weird punctuation (curly brackets{ } and two colons :) is what prevented me from comprehending its syntax.

Curly brackets of {Key key, this.title}: is the syntax for declaring optional parameters while defining function in Dart.

The first colon of MyHomepage(...) : super(key: key) is a separator that specifies the initializer list (super(key: key)) of constructor function MyHomepage(...)

The second colon within super(key: key) is the way how you pass a parameter to a named function (super() in this case).

• For example, a function enableFlags is defined as follows

void enableFlags({bool bold, bool hidden}) {...}

• To call the function, the way Dart passes parameter to the function, is by declaring parameterName before value, separated with colon :, which is safer for developer than the Pythonic way. The counterpart syntax in Swift should be external parameter.

enableFlags(bold: true, hidden: false);

I wish this could help.

All the definitions and examples can be found in Dart's official document.

Answer 3

Flutter uses Keys for a lot of things inside the framework. Since your widget extends StatelessWidget or StatefulWidget, this is the way of your widget having a key so that the framework knows what to do with it (the super function is passing the key to the superclass widget - which can be understood by the framework).

Except for that is only Dart syntax (which is awesome by the way).

You can learn more about Keys here: https://www.youtube.com/watch?v=kn0EOS-ZiIc

Another example is the testing framework. Flutter driver needs a key to know which widget to look for and interact with it. With this syntax your widget will have a key and it will work!