Use ScenePulseListener to avoid expensive recalculations?

[Jonathan Giles]

You're right in that controls don't tend to use ScenePulseListener. This
may be due to an oversight on our part, or just that our requirements
differ (I don't honestly know).

You're also right that it is important to batch up property changes and
do them once per pulse, rather than as they come in. If this is not done
you do indeed get a severe performance hit (I remember back in the day
before I optimised many of the controls in this way, the performance
impact of not doing this was staggering).

The way I do it is simple: in places where I receive events from
properties / bindings / listeners, where I know that there is
potentially a lot of changes coming through, I create a boolean (e.g.
'refreshView' or somesuch). I then set this boolean to true, and in the
appropriate place in the code (most commonly layoutChildren(), but
sometimes in the computePref*() methods), I start the method with code
like this:

if (refreshView) {
    doExpensiveOperationThatShouldHappenOnlyOncePerPulse();
    refreshView = false;
}

The reason why I sometimes use layoutChildren() and sometimes the
computePref*() methods comes down to the (new) rule in JavaFX 8.0 that
you can not change the children list of a node within the
layoutChildren() method. Because of this, if I need to modify the
children list, I put the above code in the computePref*() method.

I hope that helps.

-- Jonathan

On 6/11/2013 3:58 a.m., John Hendrikx wrote:
> Hi List,
>
> I'm considering using a ScenePulseListener to avoid expensive
> recalculations when multiple bindings trigger an action.
>
> My problem is this:
>
> I like to build Views (Controls) that are dumb and expose properties
> that can be manipulated by whatever is using the View.  The View
> listens to its own Properties and adjusts its state accordingly.  Some
> of these adjustments are related (like width + height) and can be
> expensive when calculated immediately.  So I would like to mark the
> View as "invalid" and recalculate its state (if invalid) on the next
> Pulse.
>
> My current use case I'm looking at is a View that wraps (amongst
> others) a TreeView.  The View exposes an ObservableList and a
> BooleanProperty that decides whether the first level of the Tree
> should be displayed as Tabs or as Nodes (which has an impact on what
> Nodes actually are added to the TreeView, and which are added as
> Tabs).  User code will thus often set a new list of nodes + change the
> boolean to show tabs or nodes.  The View currently naively has
> InvalidationListeners on both of these properties which cause
> TreeNodes to be created after the first change... then discarded and
> recreated after the second change by the user code, ie:
>
>   view.nodesProperty().setAll(nodes);   // Recreates all
> Tabs/TreeNodes with the current value of expand top level, as we donot
> know another change might be incoming...
>   view.expandTopLevelProperty().set(false);   // Recreates all
> Tabs/TreeNodes again if expand top level was changed...
>
> This specific problem might be done in a better way, but the point
> remains, how do I avoid expensive calculations when multiple
> properties get changed one after the other by the user code?  I'm
> assuming that JavaFX controls already avoid these kinds of things, and
> I'd like to know whether using a ScenePulseListener is the way to go,
> or that it can/should be done in a different way -- examining the code
> for TreeView (and its superclasses), I couldn't find uses of
> ScenePulseListener...
>
> --John
>
>
>
>