Package com.luciad.model

Class ALcdWeakModelListener<T>

java.lang.Object
com.luciad.model.ALcdWeakModelListener<T>
Type Parameters:
T - The type of the short-lived object
All Implemented Interfaces:
ILcdModelListener, EventListener
public abstract class ALcdWeakModelListener<T> extends Object implements ILcdModelListener

An ILcdModelListener implementation which only keeps a WeakReference to the object it wants to update when model changes are received. This avoids memory leaks, even without removing the listener.

As an example, suppose we have a table that shows the contents of some model. Assume the table is created in a dialog that a user can show and dismiss at will. Here's the code: 


   ILcdModel longLiving = ...;
   JTable shortLiving = ...;

   longLiving.addModelListener(new ModelContentsListener(shortLiving));

   private static final class ModelContentsListener extends ALcdWeakModelListener<JTable> {
     public ModelContentsListener(JTable aToModify) { super(aToModify); }

     @Override
     protected void modelChangedImpl(JTable aToModify, TLcdModelChangedEvent aModelChangedEvent){
       ILcdModel model = aModelChangedEvent.getModel();
       aToModify.setModel( createTableModel(model) );
     }
   }

The table can be garbage collected as soon as it is removed from the UI (so the dialog is closed), while the model still exists. There is no need to explicitly remove the listener, which makes it convenient to use.

Rules on using weak listeners:

  • Use a weak listener when listening to an object that can exist longer than the object that needs to be updated.
  • The listener must not keep a strong reference to the object to update (directly or indirectly).
  • It is OK for the listener to keep a strong reference to the long living object (was not needed in example).
  • Use a static inner class for the listener (or a regular class). The compiler can add extra fields to non-static or anonymous inner classes, so it is safer and simpler to use static inner classes.
  • There is no need to remove the listener. When the object to modify gets garbage collected, the listener will remove itself (the moment the next event is received by the listener).

Since:
2015.1

  • Constructor Summary

    Constructors
    Modifier
    Constructor
    Description
    protected
    ALcdWeakModelListener(T aObjectToModify)
    Creates a new ALcdWeakModelListener instance which can update aObjectToModify when it receives events.

  • Method Summary

    Modifier and Type
    Method
    Description
    final void
    modelChanged(TLcdModelChangedEvent aEvent)
    Notifies this model listener that a model has changed.
    protected abstract void
    modelChangedImpl(T aToModify, TLcdModelChangedEvent aModelChangedEvent)
    This method is called each time a TLcdModelChangedEvent is received by this listener, and the short-living object to modify passed in the constructor of this listener is not yet GC-ed.

    Methods inherited from class java.lang.Object

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

  • Constructor Details

    • ALcdWeakModelListener

      protected ALcdWeakModelListener(T aObjectToModify)

      Creates a new ALcdWeakModelListener instance which can update aObjectToModify when it receives events.

      Parameters:
      aObjectToModify - The object you want to modify when a TLcdModelChangedEvent is received. This is a short-lived object, while the listener will be attached to a long-living ILcdModel object.

  • Method Details

    • modelChanged

      public final void modelChanged(TLcdModelChangedEvent aEvent)
      Notifies this model listener that a model has changed.

      See the template code example in TLcdModelChangedEvent on how to handle model change events.

      This method will check whether the short-living object is still available. If it is, this method will pass the short-living object and the event to the modelChangedImpl(Object, TLcdModelChangedEvent) method. If the short-living object is already garbage collected, this method will de-register the listener from the source model.

      Specified by:
      modelChanged in interface ILcdModelListener
      Parameters:
      aEvent - the TLcdModelChangedEvent that contains relevant information on the change(s).
      See Also:

    • modelChangedImpl

      protected abstract void modelChangedImpl(T aToModify, TLcdModelChangedEvent aModelChangedEvent)

      This method is called each time a TLcdModelChangedEvent is received by this listener, and the short-living object to modify passed in the constructor of this listener is not yet GC-ed.

      Parameters:
      aToModify - The object to modify, as passed in the constructor of this class.
      aModelChangedEvent - The received event