Reflecting updates to a ListCell that contains a mutable object

Question

Hi,
I've seen many variants of this question, but unfortunately have not found the desired answer, so I thought to ask here. Apologies if missing something obvious!
Objective:
I start individual Tasks in batches. A ListCell reflects each Task, from initial request to eventual result. More batches can be submitted while one set is processing. When all processes of any batch is finished, they will eventually disappear from the ListView.
As a result, I want a ListCell to reflect a Task, and reflect the transition from initial conception to eventual outcome.
Predicament:
I currently try this with an ObservableList of my own POJOs, each reflected using a custom ListCell extension.
I have achieved this result, but it does not feel correct. First of all, I read that it is best practice not to change an ObservableList's objects under its feet. However, I have multiple threads working against the same list. With objects being added, removed and updated, it seemed safer to update the referenced object rather than try to manage synchronization to prevent concurrent modification issues. After all, I'm not really adding a new item, I'm wanting to update the representation of an item that is now in a finished state.
Attempt details:
I have achieved this by constructing an 'observableArrayList' with a Callback extractor. This Callback call method provides an Observable array containing an ObjectProperty, the property being a member variable of my POJO used in each ListCell. Each Task updates this object property with some result information at the end of its processing. This I believe ensure that change listeners are notified of updates to this POJO, via its ObjectProperty. (https://javafx-jira.kenai.com/browse/RT-15029)
The ListCell constructed with this POJO is listening for updates, but I believe this is really to reflect additions and removals on the ObservableList that the ListView represents. However, in the case of updates, the private implementation of the ListCell updateItem method (javafx.scene.control.ListCell#updateItem) does not call the overridable updateItem method. This is because the object that caused the update is seen as equal to the object the ListCell currently holds (they're the same instance, so this is true). However, if the overridable updateItem method is never invoked, I can't get the custom ListCell to update its representation of the ListCell object that has changed since its last representation was rendered.
If I make my custom POJO always return false in its overriden equals method, the overridable updateItem method is invoked and I get the opportunity to detect the change in the object and render a new representation. This works, but this feels wrong and like a hack.
Can anyone help point me at the correct way of doing this please? Happy to provide more information if needed.
Thanks,
Louis

Answer 1

If the changes in the ObservableList are to be reflected in the UI, you need to make these changes on the JavaFX Application Thread. Ensuring this happens (using Platform.runLater(...) if necessary) will also prevent any concurrent modification issues.
I would approach this by binding the text property or graphic property (or both) of the custom ListCell to the appropriate properties of your POJO. Here's a simple example where I have a list view displaying a bunch of "counters" which count in a background thread. The cell display is updated when the state of the task changes. I introduced a counter property (which is a bit redundant; I could have used the progress property or message property) to demonstrate updating a property on the FX Application thread using Platform.runLater(...).
Because ListCells can be reused, you need to account for the fact that the item (task) displayed may change during the lifecycle of the cell. You can do this using a listener to the itemProperty (which is what I did), or by overriding the updateItem(...) method.