Thanks Jonathan, it's good to get your insight.
You did finish by muddying the waters again though - to do something
complex with zooming and scrolling you'd "be tempted" to fall back
into Java2D paint-style programming, and use Canvas for this, not the
Scene graph? It's more a couldbe/maybe comment though and is in
contrast to your earlier suggestion that there is very little that a
scenegraph-based approach can't do. What's the trigger to switch from
one approach to the other?
Previously there have been comments about the Canvas not really being
intended for highly dynamic stuff (that was my interpretation of
comments on here when Canvas was first released), and Nodes should be
used for most real things. Richard wanted to use Nodes in the TD game
for sprites. To add to the confusion, Canvas currently has some
drastic z-order bugs, and some clipping issues, so using it combined
with Nodes is currently a no-go.
I'm not expecting Jonathan to have an answer here really, just
highlighting the fact that there is no clear answer on this. I'm still
confused and I imagine many others are too. I think we'll see this
question again.
On Tue, Aug 6, 2013 at 11:00 AM, Jonathan Giles
<jonathan.gi...@oracle.com <mailto:jonathan.gi...@oracle.com>> wrote:
I don't think there is any particular secret sauce going on in
what I do compared with the general guidelines that have been
spelled out numerous times. It's the same old, same old: don't
create more nodes than you need, don't modify the scenegraph
needlessly, don't update the scenegraph multiple times in a single
pulse, change state as little as possible, use as few listeners as
possible, etc. I wish I had something more groundbreaking for you,
but that is about it :-)
With respect to TableView (and ListView, TreeView, and
TreeTableView), they are all based on the same virtualisation code
(VirtualFlow for those of you playing at home). We don't rubber
stamp, we create separate cell instances for the visible area of
the control, and reuse these exact same cells as the user scrolls.
Therefore, if the visible area requires 20 cells, we may create
~22 cells, and as the user scrolls downwards we take the cells
that disappear out the top of the view and place them at the
bottom of the view, with their new content in place before it is
shown on screen.
Because all cells come from a single cell factory, and all cells
can be used in any location, it is up to the cell to respond to
the item placed into it and draw itself appropriately. Therefore,
we don't have 1000's of types of cells in a single control, we
only have one type of cell that needs to handle all the visual
approaches required in the app. Realistically, there aren't 1000's
of styles in a single control, normally there are only one, or two
at most. All this takes place in the Cell.updateItem(T, boolean)
method, and so people overriding this method need to be smart and
not do heavy lifting in there. The biggest mistake I see people
doing in updateItem(...) is throw away their entire cell
scenegraph and recreate the nodes and update the scenegraph. This
is unwise.
If you have a ListView with 100 nodes, and they are all equally
sized except for one (say the 50th), which is _significantly_
bigger, you will see the scrollbar jump in size and other
weirdness happen when it is scrolled into view, precisely for the
reason you state - we can't go off and measure every row as we'd
be doing a lot of busy work. We only measure what is in the visual
area, and we don't know where we are in the scroll range in terms
of pixels but rather in terms of a 0.0 - 1.0 range (which is
translated back to pixels when needed). Up to this point I've
known about this issue but I've not spent the cycles to resolve it
- it is a relatively rare use case (although it still happens).
Priority #1 for these virtualised controls is always speed.
If zooming were required on TableView, the implication (I presume)
is that there would be that less cells that were visible at any
one time, and so we would end up having less cells in the
scenegraph. Other than that, things would work as above.
In a past life I did a lot of work in Java 2D. This worked really
well for use cases like you suggest at the end of your email,
namely zooming and direct mouse manipulation of nodes on screen.
If I were to write something like you show in the screenshot, I
would be tempted to take a Canvas-based route nowadays, but of
course that decision would also be driven by the requirements and
use cases, and it is possible a scenegraph-based approach with
absolute node positioning would work just as well.
Hope that helps.
-- Jonathan
On 6/08/2013 12:38 p.m., Daniel Zwolenski wrote:
Sneaking in here, as you've given an opening with "if implemented
wisely, there is very little that a scenegraph-based approach
can't do". The question I've been asking for a while is what does
"implemented wisely" look like in JFX.
This has come up in the performance conversations, the game
conversations, the CAD conversations, and many other places. No
one seems to have an answer, but you're building extremely
complex stuff on a regular basis - what's your tips?
When you say you only have "20 visible nodes" out of 1000's in
general are the other nodes:
a) in the scenegraph and set to not visible
b) in memory but not in the scenegraph - added/removed when
scrolled into view and out of view
c) not in memory, created, added and then removed, destroyed when
scrolled into view and out of view
d) something else?
I know Table uses a rubber stamp approach, where it re-uses cell
views where possible, but let's say every row in my 100,000 row
Table was uniquely rendered using a different cell. What would
happen under the covers?
How do you work out the scroll range as well? Each cell can be a
unique height right? How do you know the extents of the vertical
scrolling without instantiating and rendering everything? Is this
what you do? What if a cell is changing size (has a collapsable
pane in it, etc) - what happens to the vertical scroll range?
Do any of the controls have zooming on them? Have you had to deal
with this and have you got a strategy for handling this with
respect to scroll bounds, working out which nodes are in view,
scaling fonts, etc? Could you hazard a guess at what you would do
if you had to implement zooming on a Table for example?
Maybe the Table is lucky with its restrictive grid like layout
but imagine you had to build a visualisation of the same data but
in a diagram, maybe something like
http://www.novell.com/communities/files/img/groupwise_8_protocol_flow_diagram_v1.3.jpg
but with x100 nodes, with zooming and panning - could you outline
a general strategy?
On Tue, Aug 6, 2013 at 10:10 AM, Jonathan Giles
<jonathan.gi...@oracle.com <mailto:jonathan.gi...@oracle.com>> wrote:
I think it would pay to take a step back and understand why
you think a 'traditional' scenegraph-based (or retained mode)
control is not sufficient for your needs?
Unfortunately you've not detailed your use case, so it is
hard to give any specific advice. Are you able to give any
details about what it is you're trying to build and why you
think the normal approach to building controls is not sufficient?
We've built some fairly complex controls using this approach,
and if implemented wisely, there is very little that a
scenegraph-based approach can't do. Specifically, do you
think your control will render all of the 'thousands of
nodes' at once, or will many of these nodes be off screen or
otherwise not visible at any one time? For things like the
TableView we only render the nodes that are visible. This
means that regardless of whether there are 100 or 1,000,000
rows of data, we only have visual nodes for the 20 visible
rows, for example. Keeping your scenegraph as minimal as
possible is always a very wise idea, if performance is a concern.
As you note, the other problem is that you will run into
issues if you want to mix canvas rendering with the
scenegraph-based controls like Button. The best you're likely
to achieve (having not tried it personally) is to position
the control on top of the canvas, rather than attempting to
render the control inside the canvas (and having to then deal
with event handling, etc). This will likely prove to be
finicky, and more cumbersome than simply using an entirely
canvas-based or entirely scenegraph-based approach.
-- Jonathan
On 5/08/2013 10:11 p.m., Felix Bembrick wrote:
I am investigating the feasibility of developing a JavaFX
8 control based
on Canvas. I have chosen Canvas as the base class as
this control is of a
very dynamic nature and would not be easy to implement
with a retained mode
style ancestor (at least as far as I can tell).
So is this feasible? While I can readily see how to
render the visual
aspects of the control, I am not sure how to best "embed"
other controls
within it should that become necessary (and almost
certainly will).
For example, how would I go about embedding a Button
within my control? It
looks to me like I would need to create an actual Button
node somewhere
else in the scenegraph and then perhaps render it within
my control using
gc.drawImage() passing in a snapshot of the Button node.
That's OK but
then I have to somehow handle events and I am not sure
how best to do that.
Another issue I see is that there seems to be no way to
apply effects to
individual graphic elements within the Canvas as the
applyEffect() method
applies to the entire Canvas.
Finally, a significant obstacle is this issue:
https://javafx-jira.kenai.com/browse/RT-23822
This issue relates to the lack of support for LCD font
smoothing within
Canvas. This may not sound that serious but the
difference between LCD
font-smoothed text in other controls and the grey-scale
text in Canvas is
so distinct on my current machine that a control based on
Canvas would
really stick out like a sore thumb and appear
significantly less appealing
than a "standard" control.
So, am I wasting my time?
Are there any other issues I am likely to face?
Are there other ways to develop dynamic controls which
may involve
thousands of nodes (such as lines, curves etc.)?
Thanks,
Felix
