TableView API, no lazy retrieval of visible cell content possible?

[Tomas Mikula]

If the cells don't ever get removed from the scene, then I guess your
best bet is to start and cancel loading when the item changes, i.e.
listening to itemProperty(). This is similar to your original
updateItem() approach, but also don't forget to cancel loading of the
old item: cancel loading of the old item, if not null, and start
loading the new item, if not null. I don't think you can yourself do
much about the inefficiency that more cells than needed are created
and hold non-null items.

Let's see if others have something to suggest.

Tomas

On Tue, Jan 27, 2015 at 3:49 PM, Robert Krüger <krueger at lesspain.de> wrote:
> Hi Tomas,
>
> When I do that, the property is never set to null. If I use it for
> triggering the loading (when it is set to a non-null value) it is set for a
> lot more rows than those actually visible. In my test with a model
> containing 200 items of which the first 4 were visible, it was set to
> non-null for the first 23 items.
>
> Am Dienstag, 27. Januar 2015 schrieb Tomas Mikula :
>
>> Hi Robert,
>>
>> instead of listening to visibleProperty(), listen to sceneProperty()
>> and cancel loading when scene becomes null.
>>
>> Tomas
>>
>> On Tue, Jan 27, 2015 at 1:16 PM, Robert Krüger <krueger at lesspain.de>
>> wrote:
>> > Hi,
>> >
>> > either I don't see the forest for the trees or something is missing in
>> > the
>> > TableView API as I cannot seem to implement something that seems a
>> > common
>> > requirement and was rather easy (not pretty though) in Swing.
>> >
>> > Imagine an application like OSX finder in its list view, i.e. something
>> > that displays a possibly very long list of files and displays thumbnails
>> > for them generated on the fly. For many files (e.g. video files)
>> > extracting
>> > these thumbnails is an expensive operation and has to be performed in
>> > the
>> > background. My application does a very similar thing and uses a
>> > TableView.
>> > What I want is to begin extracting video thumbnails as soon as their
>> > corresponding table row has become visible. I have done that in Swing in
>> > the past without any problem (for more info, you can read
>> > https://community.oracle.com/message/12810930).
>> >
>> > With JavaFX I have tried the following:
>> >
>> > 1) Trigger loading the thumbnail in the table cell when it is updated
>> > and
>> > the corresponding thumbnail isn't already there
>> >
>> > Result: Triggering works more or less as desired but how do I stop the
>> > loading process if the cell becomes invisible? If the user quickly
>> > scrolls
>> > through a large number of rows and puts tons of thumbnail loading jobs
>> > on
>> > the queue I have not found a way to dequeue them, so this is unusable. I
>> > added output to the calls to the update method of the TableCell to see
>> > which instances are used and how their data is reset but the pattern I
>> > see
>> > is not suitable for deciding which cell is currently visible.
>> >
>> > 2) Register a change listener to the TableCell’s visible property to
>> > make
>> > that control the image loading.
>> >
>> > Result: I don’t seem to get any change events when I do that, so it does
>> > not work at all for my purpose.
>> >
>> > I see no public API in table view to explicitly compute the items
>> > visible
>> > in the current viewport either. What am I missing. It can't be an
>> > oversight
>> > in the API design, as the thing, I am trying to achieve appears rather
>> > basic.
>> >
>> > Thanks in advance for any hints,
>> >
>> > Robert
>
>
>
> --
> Robert Krüger
> Managing Partner
> Lesspain GmbH & Co. KG
>
> www.lesspain-software.com
>