Understanding Kotlin and JavaFX: Why Do Bindings Behave Strangely?

Описание: Discover the peculiar behavior of bindings in Kotlin and JavaFX, and learn how to prevent premature garbage collection of your objects with effective listener management.
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This video is based on the question https://stackoverflow.com/q/78107979/ asked by the user 'MakerC' ( https://stackoverflow.com/u/23536512/ ) and on the answer https://stackoverflow.com/a/78108267/ provided by the user 'James_D' ( https://stackoverflow.com/u/2189127/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Kotlin and JavaFX: Why Do Bindings Behave Strangely?

When working with JavaFX in Kotlin, you may encounter unexpected behavior with bindings, particularly when interacting with UI components like buttons and combo boxes. A common issue arises when changing a combo box selection seems to trigger a listener only when a button is present in the layout. What causes this strange behavior? In this guide, we'll unravel the mechanics behind this and provide you with clear solutions to ensure your bindings work as intended.

The Problem

Consider the following scenario: You create a simple JavaFX application with a combo box and a button. You bind the combo box's value property to a property of an object (let's call it Holder) that has a listener attached to it. You notice that when you remove the button from the layout, the listener fails to trigger when the combo box selection changes.

Example Code Snippet

Here's a simplified version of the application structure:

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

If the button is included, the listener prints the updated value whenever the combo box selection changes. However, if you remove the button, the listener seems to be unresponsive.

The Explanation

Understanding Bindings and Garbage Collection

The key to the problem lies in how JavaFX manages bindings and listener references:

Weak Listeners: JavaFX uses weak listeners internally. This means that if there are no strong references to the object holding the bound property, it can be garbage collected.

In our example, when the start() method of the Application class exits, there may be no strong reference to the Holder instance anymore, apart from its bidirectional binding to the combo box's property. This makes it eligible for garbage collection, thus disabling the listener.

Why Does the Button Matter?

The button creates a strong reference to the Holder object through its action listener. Here's how the hierarchy works:

The button is added to the VBox, which is part of the Scene.

The Scene is managed by the Stage, which remains in scope even after start() completes.

Because the button holds a direct reference to the Holder instance via its listener, the Holder object is not garbage collected, and the listener stays active.

Solution

Maintain Strong Reference Outside start()

To ensure that the Holder instance remains in memory and its listeners remain active, you can keep a strong reference to it at the class level, rather than within the method scope.

Updated Code Example

Here’s how you could modify your application to avoid the premature garbage collection issue:

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

Conclusion

The strange behavior you observed with bindings in Kotlin and JavaFX stems from how object references and garbage collection work within the framework. By ensuring that your listener's backing object maintains a strong reference, you can prevent it from being prematurely garbage collected. Understanding these nuances can greatly improve how you design and implement your JavaFX applications, leading to more reliable and predictable behavior.

If you have any further questions or need clarification on this topic, feel free to drop a comment!