Constructors and named parameters in Flutter - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

Identify the correct way to define a named parameter in a Flutter constructor

Which constructor correctly uses a named parameter for the title property in a Flutter widget class?

Flutter

class MyWidget extends StatelessWidget {
  final String title;

  MyWidget({this.title});

  @override
  Widget build(BuildContext context) {
    return Text(title);
  }
}

A MyWidget({required this.title});

B MyWidget({title});

C MyWidget(title);

D MyWidget(this.title);

💡 Hint

Remember that named parameters use curly braces and can be marked as required.

Option A correctly defines a named parameter title with the required keyword, ensuring it must be provided. Option A uses a positional parameter. Option A is invalid syntax. Option A defines a named parameter but without this. and without required, which is not recommended.

❓ ui_behavior

intermediate

What will be displayed when using a constructor with a default named parameter?

Given this widget code, what text will appear on the screen if MyWidget() is used without arguments?

Flutter

class MyWidget extends StatelessWidget {
  final String title;

  MyWidget({this.title = 'Hello'});

  @override
  Widget build(BuildContext context) {
    return Text(title);
  }
}

A null

B An empty string

C Compilation error

D Hello

💡 Hint

Check the default value assigned to the named parameter.

The constructor sets a default value 'Hello' for the named parameter title. If no argument is passed, the default is used, so the text 'Hello' appears.

❓ lifecycle

advanced

How does using named parameters with required affect widget rebuilds?

Consider a Flutter widget with a constructor using named parameters marked as required. What is the impact on widget rebuilds when the parameter values change?

A The widget rebuilds regardless of parameter changes because named parameters do not affect rebuilds.

B The widget never rebuilds if parameters are named and required.

C The widget rebuilds only if the required named parameters change values.

D The widget rebuilds only if positional parameters change, not named ones.

💡 Hint

Think about how Flutter compares widget properties to decide rebuilds.

Flutter rebuilds widgets when their properties change. Using named parameters marked as required ensures those properties are set and tracked. If their values change, Flutter triggers a rebuild.

❓ navigation

advanced

How to pass named parameters when navigating to a new screen in Flutter?

You want to navigate to DetailScreen and pass a named parameter id. Which code correctly passes the parameter?

Flutter

class DetailScreen extends StatelessWidget {
  final int id;
  DetailScreen({required this.id});

  @override
  Widget build(BuildContext context) {
    return Text('ID: $id');
  }
}

A Navigator.push(context, MaterialPageRoute(builder: (_) => DetailScreen(42)));

B Navigator.push(context, MaterialPageRoute(builder: (_) => DetailScreen(id: 42)));

C Navigator.pushNamed(context, '/detail', arguments: 42);

D Navigator.pushNamed(context, '/detail', id: 42);

💡 Hint

Check how named parameters are passed in constructors inside the builder function.

Option B correctly uses the named parameter id when creating DetailScreen. Option B passes a positional parameter which is invalid. Option B passes arguments but not as named parameters in constructor. Option B uses an invalid parameter id in pushNamed.

🔧 Debug

expert

Why does this Flutter constructor cause a runtime error?

Examine this Flutter widget constructor. Why does it cause a runtime error when instantiated without arguments?

Flutter

class MyWidget extends StatelessWidget {
  final String title;

  MyWidget({required this.title});

  @override
  Widget build(BuildContext context) {
    return Text(title);
  }
}

// Usage:
// MyWidget();

A Because the required named parameter <code>title</code> was not provided when creating <code>MyWidget</code>.

B Because the <code>title</code> field is not initialized with a default value.

C Because StatelessWidget requires a positional parameter in the constructor.

D Because the <code>build</code> method is missing a return statement.

💡 Hint

Check the meaning of required in named parameters.

The constructor marks title as required, so it must be provided when creating an instance. Calling MyWidget() without title causes a runtime error.