Developing controls based on Canvas?

[Jonathan Giles]

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