# Class WeakChangeListener<T>

java.lang.Object
javafx.beans.value.WeakChangeListener<T>

Type Parameters:
  `T` - The type of the observed value

All Implemented Interfaces:
  `ChangeListener<T>, WeakListener`

public final class WeakChangeListener<T> extends Object implements ChangeListener<T>, WeakListener
A `WeakChangeListener` can be used if an `ObservableValue` should only maintain a weak reference to the listener. This helps to avoid memory leaks which can occur if observers are not unregistered from observed objects after use.

`WeakChangeListener` instances are created by passing in the original `ChangeListener`. The `WeakChangeListener` should then be registered to listen for changes of the observed object.

Note: You have to keep a reference to the `ChangeListener` that was passed in for as long as it is in use, otherwise it will be garbage collected too soon.

Since:
  JavaFX 2.0
See Also:
  - `ChangeListener`
  - `ObservableValue`

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `WeakChangeListener(ChangeListener<T> listener)`
  The constructor of `WeakChangeListener`.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `void`
  `changed(ObservableValue<? extends T> observable, T oldValue, T newValue)`
  Called when the value of an `ObservableValue` changes.
  `boolean`
  `wasGarbageCollected()`
  Returns `true` if the linked listener was garbage-collected.

  ### Methods declared in class Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- ## Constructor Details

  - ### WeakChangeListener

    public WeakChangeListener(ChangeListener<T> listener)
    The constructor of `WeakChangeListener`.
    Parameters:
      `listener` - The original listener that should be notified

- ## Method Details

  - ### wasGarbageCollected

    public boolean wasGarbageCollected()
    Returns `true` if the linked listener was garbage-collected. In this case, the listener can be removed from the observable.
    Specified by:
      `wasGarbageCollected` in interface `WeakListener`
    Returns:
      `true` if the linked listener was garbage-collected.

  - ### changed

    public void changed(ObservableValue<? extends T> observable, T oldValue, T newValue)
    Called when the value of an `ObservableValue` changes.

    In general, it is considered bad practice to modify the observed value in this method.

    Specified by:
      `changed` in interface `ChangeListener<T>`
    Parameters:
      `observable` - The `ObservableValue` which value changed
      `oldValue` - The old value
      `newValue` - The new value