API Review RT:17407 Canvas Node

Jim Graham james.graham at oracle.com
Tue Apr 24 17:08:38 PDT 2012

This issue is that if it can be resized fairly commonly by standard 
facilities then it really needs to be designed around a repaint 
mechanism.  This is why awt.Canvas and swing.Component have 
"paint[Content](Graphics)" style methods that are the intended way to 
render your content.

Canvas is acting like a retained "painting" in that you render once and 
then leave it alone for its lifetime.  While it is true that you can 
directly call the setWidth/Height() methods and it will gain or lose 
pixels, the scene graph is not in the habit of doing that for you.  If 
this were designed as a managed node then any time you put it in a 
layout container it would suddenly get resized by 3rd parties.  The most 
common example of:

	Canvas cv = new Canvas(w,h);
	GraphicsContext gc = cv.getGraphicsContext2D();

would fail because your canvas would not yet have gone through layout.

So, yes, it is possible for them to detect resizing with the current 
scheme, but it is not enough for them to "be able to" deal with resizing 
and damage - the "manageable" model would invite a significant amount of 
resizing, scrolling, and other types of damage that the above rendering 
practice just wouldn't be able to deal with.

Also, scrolling would tend to require you to re-render.  You can't 
listen to size properties to discover when to repaint for scrolling.  I 
suppose we could modify the position of the canvas somehow to indicate 
scrolling, but that isn't as direct as "this part is damaged - repair it 
now" and the location would only be loosely tied to potential damage and 
any loose association there that we promote would not cover all possible 
cases where the Canvas could be damaged.

So, if we want to invite a layout-able "pane" like awt.Canvas or 
swing.Panel, then we have to create a fundamentally different top-level 
design.  I'm not saying we can't do it - but it isn't something to 
lightly graft onto the current Canvas API...


On 4/23/12 3:27 PM, Daniel Zwolenski wrote:
> Sorry, I'm not following the problem.
> Wouldn't the height and width properties provide the callback to say 'I have been resized'? This would include the amount resized-by so the developer could just render the new area if they want (or redraw the whole scene, etc). If the developer chooses not to patch/repaint then that's their choice.
> HTML has never had good layout manager concepts so for me it is not the best benchmark in this topic.
> What's the requirement to repaint on every frame? I don't follow that.
> On 24/04/2012, at 6:11 AM, Kevin Rushforth<kevin.rushforth at oracle.com>  wrote:
>> I wrote:
>>> I can't think of any reason it should not be resizable.
>> Good think I'm not the only one thinking about it, then. :)
>> Yes, this would be an issue. So unless/until we are ready to deal with this, my earlier suggestion of binding to the width/height property, and adding your own listeners is probably the way to go.
>> -- Kevin
>> Jim Graham wrote:
>>> We would have to deal with damage repair.  Right now it is a persistent rendering, not an area to be "repainted" on every frame.
>>> If we wanted it to be dynamically and asynchronously resizable we would have to switch to (or provide?) a call-back model of rendering and you would have to keep enough state to be able to render it again on every frame.  HTML5 Canvas demos are not rendered that way, though they can be at their own will - they simply clear the entire canvas and re-render on every pulse.  Or, they just render the new stuff.  Their call.  If they change size, like you can with Canvas, then they are responsible for changing the size and rendering the new stuff - just like if you manually modify the size of our Canvas.
>>>             ...jim
>>> On 4/23/12 5:36 AM, Kevin Rushforth wrote:
>>>> If Canvas were a "Resizable" node then yes, it could do that. Off-hand,
>>>> I can't think of any reason it should not be resizable.
>>>> -- Kevin
>>>> Daniel Zwolenski wrote:
>>>>>> A Canvas node is a fixed size, so it only resizes when you tell it
>>>>>> to. If you want it to track the size of the Scene (or similar), you
>>>>>> could bind its width and height properties and react to change
>>>>>> notifications.
>>>>> Any reason why Canvas wouldn't just adhere to the Layout algorithm of
>>>>> the Container it is in? Eg if in a BorderPane center then fill, if in
>>>>> a GridPane take up that cell, etc. If we want to fix size then we
>>>>> would use min/max constraints.
>>>>> That would seem the most sensible and useful thing to me.

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