Skin layoutChildren: when to get bounds of child nodes?

Martin Sladecek martin.sladecek at
Mon Jul 28 05:38:41 UTC 2014

The super.layoutChildren should size every child of the control (which 
is VBox), but not child's children. The control must finish the layout 
before children can do theirs. If you need to do layout on some child 
before that, you can call .layout() on it. It will do it's layout using 
it's current size. You should have all the bounds correct after that call.

But that would not work in your case anyway. You have both childs in a 
HBox, which takes care of resizing the children. This means you need to 
layout the HBox to get children size and in order to do that, you need 
HBox to be at it's final size, which will happen during the VBox layout. 
So your steps would be:
1) super.layoutChildren() - VBox is resized to Controls content size
2) now the VBox is resized, you can call vbox.layout()
3) now HBox is resized, so call hbox.layout()
4) children are resized. They have correct layout bounds now. But in 
order to get correct boundsInParent (but maybe you really need layout 
bounds?), you need to call .layout() on children too.

Even if you do all these steps, calling setPrefWidth() on child2 marks 
the whole tree dirty again. Because HBox needs to resize child2 using 
it's new PrefWidth. This also means, HBox prefwidth will be different, 
so it's parent (VBox) must do the same. Ditto with the control. Also, 
the HBox (VBox, control) may not have enough size to resize child2 to 
it's pref width, so child1 might be shrinked as a result, which breaks 
your invariant. You are basically changing the input for HBox's layout 
(child2.pref size) based on it's output (child1 size), which makes this 
a loop.

So in order to really make this work, you need to manage the child nodes 
directly and compute your layout by yourself. This can be done either by 
using your own subclass of Pane and overriding it's layoutChildren. Or 
if you want to do everything in Skin's layoutChildren, you can make the 
children unmanaged, but then it doesn't really matter where they are in 
the scenegraph, HBox won't be managing them.

Hope this helps!


On 25.7.2014 18:56, Richard Bair wrote:
> Hmmm. The first question I have is whether boundsInParent is really what you want to use, vs. layout bounds. But assuming it is what you want, why are the bounds zero? This is during the layout pass, which is the right place to be doing what you’re doing. The super layoutChildren call will size everything based on the contentX, contentY, contentWidth, contentHeight ( Is it possible that the parent itself is size 0? boundsInParent should always be invalidated automatically whenever the width/height/transforms/etc changes. If not that is definitely a bug (that you can write a simple test case for to prove).
> But my first guess is maybe the parent is size 0 as well, due to something else (maybe the pref size of the control is 0 to start with or something…)
> Richard
> On Jul 24, 2014, at 3:34 AM, Werner Lehmann <lehmann at> wrote:
>> Hi,
>> inside a control skin I have the following code pattern:
>>   protected void layoutChildren(...)
>>   {
>>     super.layoutChildren(...);
>>     Node child1 = ...
>>     Bounds bip = child1.getBoundsInParent();
>>     if (!animating) {
>>       Node child2 = ...
>>       child2.setTranslateX(bip.getMinX();
>>       child2.setPrefWidth(bip.getWidth());
>>     }
>>   }
>> The skin scene graph looks roughly like this:
>> VBox
>>   HBox { ..., child1, ...}
>>   child2
>>   ...
>> Everything is layouted just fine but I want to adjust child2.translateX and prefWidth based child1 bounds. This does not work initially because boundsInParent returns zero components leading to incorrect initial display of the control.
>> Seems as if boundsInParent is not yet updated. I guess I could use a binding for that but it would conflict with an animation I also have on translateX and prefWidth.
>> Maybe there is a better time to make those adjustments on child2?
>> Rgds
>> Werner

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