Скачать или смотреть How to Properly Initialize Instance Variables in Dart Child Classes from Parent Classes

Learn how to effectively `initialize instance variables` in Dart child classes using parent class variables. This guide provides practical examples and solutions.
---
This video is based on the question https://stackoverflow.com/q/74325887/ asked by the user 'Shahood ul Hassan' ( https://stackoverflow.com/u/7983864/ ) and on the answer https://stackoverflow.com/a/74326348/ provided by the user 'Hossein Yousefi' ( https://stackoverflow.com/u/3785400/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: How to initialize an instance variable in constructor of child class, from an instance variable of parent class in Dart

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
How to Properly Initialize Instance Variables in Dart Child Classes from Parent Classes

In object-oriented programming, it is common to create child classes that extend the functionality of their parent classes. In Dart, as in other programming languages like Java, we often need to initialize instance variables of a child class based on the instance variables of its parent class. However, this can present challenges, as seen in a typical error you might encounter. In this guide, we'll dive into how to properly handle this situation in Dart.

The Problem

Consider the following scenario: You need to initialize an instance variable in a child class's constructor from an instance variable of the parent class. In Java, this is achieved smoothly, but when attempting to replicate the same structure in Dart, you might run into errors. For instance:

[[See Video to Reveal this Text or Code Snippet]]

In your child class, you might want to access db like this:

[[See Video to Reveal this Text or Code Snippet]]

However, if you run this code, you'll encounter an error stating:

"The instance member 'db' can't be accessed in an initializer."

This can be frustrating, especially after spending a lot of time searching for a solution. But don’t worry; there’s a clear path forward!

The Solution

To solve the issue of initializing instance variables in the constructor of a child class from a parent class variable in Dart, we can use the late keyword, which allows you to defer the initialization of a variable until later in the code.

Step-by-Step Breakdown

Declare with late: The first step is to declare your instance variable with the late modifier. This tells Dart that the variable will be initialized later, but you assure the compiler that it will be set before it’s accessed.

[[See Video to Reveal this Text or Code Snippet]]

Initialize in Constructor: Next, in the constructor of your child class, you can initialize this variable after calling the parent class's constructor. This avoids the initializer error because now the variable _calculationDao is assigned after db is already initialized through the parent constructor.

Final Implementation: Your child class should now look like this:

[[See Video to Reveal this Text or Code Snippet]]

Why Use late?

The late keyword is useful because it allows for enhanced flexibility in your code. It ensures that you can access member variables that depend on other initialized values without running into initialization errors. However, use late cautiously, as accessing an uninitialized variable will lead to runtime exceptions.

Conclusion

Navigating the nuances of initializing instance variables from parent classes in Dart can be tricky, but understanding how to use late effectively can save you time and headaches. By following this approach, you can ensure your child classes are set up properly without encountering errors, allowing them to leverage the properties of their parent classes seamlessly.

If you have any more questions on Dart programming or object-oriented design, feel free to reach out. Happy coding!