Layout issue

John Hendrikx hjohn at
Wed Feb 12 02:34:00 PST 2014

On 11/02/2014 21:09, Martin Sladecek wrote:
> The rule of thumb in case you modify content during the layout is that 
> content should depend on layout pane size, not the other way around. 
> It means that changing the content won't modify the min/pref/max size 
> of the pane as that would trigger another layout pass (possibly 
> falling into a loop).

   +- (TOP) Filter (subclass of FlowPane)
   +- (CENTER) TreeView

I modify the content during the compute* method of the BorderPane.  For 
TreeView I change the root; for the Filter (FlowPane) I add/remove children.

For some reason that seems to be the "wrong" time to do it, despite the 
fact that this happens almost as the first thing during the layout 
process.  LayoutChildren will get called later on BorderPane, and no 
more changes were made, and the layout is incorrect.  Then when the 
focus changes, the compute* methods get called again (but I change 
nothing since everything is valid still) and layoutChildren gets called 
again on BorderPane... and then everything looks correct.

> This is why it's not recommended to change children during the layout, 
> as default computation of min/pref/max size depends on the children. 
> But if you modify compute* methods and return something that is not 
> dependent on the children, you can safely modify them while doing the 
> layout.
> So, e.g. your filter control might return pref size that is approx. 
> for 2-3 "tabs" (or whatnot). Since BorderPane resizes top to the full 
> width, you would always get the maximum width you can get. Based on 
> the assigned width, you can then compute the layout and add as much 
> tabs as you can. Since that would not change the pref width, the 
> layout is done.
It is actually a FlowPane -- I'd like it to accomodate the tabs as well 
as possible, and even add an extra row if they don't all fit.  Since I 
donot myself override its compute* methods, I didn't think the issue 
might be in that area (see the Filter class included in the JIRA I 
created -- it's really simple).
> Anyway, as Jonathan already noted, a JIRA issue with some sample would 
> be great.
Yes, I just did, RT-35830 has a nice sample.

Thanks for giving some more insight in the layout process.

> Thanks,
> -Martin
> On 02/11/2014 05:50 PM, John Hendrikx wrote:
>> From an earlier posting on this list, I came to understand that in 
>> JavaFX 8 it is no longer allowed to modify the children list in 
>> layoutChildren, and that such modifications may need to be moved to 
>> the computerPref* methods.
>> However, I get a different odd issue, and I'm wondering exactly what 
>> is allowed and what isn't when it comes to layout (any documentation 
>> on this?)
>> What's happening is the following:
>> I got a (subclass of) BorderPane, at the top I have a tab-like 
>> control (let's call it a FilterControl).  At the center I got a 
>> TreeView.
>> When the BorderPane gets laid out, I set up the content for both the 
>> TreeView and the FilterControl.  I do this in layoutChildren or in 
>> computerPref* of the BorderPane -- it makes no difference. Setting up 
>> the content for the FilterControl involves changing its children 
>> list.  The TreeView probably will do the same (adding/removing Cells 
>> as needed).
>> Now, I'm seeing layout issues.  The BorderPane for example is often 
>> putting the TreeView right on top of the FilterControl (stuff is 
>> transparent, so I can see it).  With other attempts, the 
>> FilterControl is not visible at all (zero height).  Forcing an update 
>> (by changing focus) usually clears up the issue and the FilterControl 
>> will take its rightful spot and force the TreeView to be a little 
>> less high.
>> I'm thinking this is somekind of issue with layout and that I'm 
>> approaching this wrong causing the layout problems.  However, I don't 
>> quite understand what I'm possibly doing wrong -- modifying the 
>> children of the Top node of the BorderPane during BorderPane's own 
>> computePref/layoutChildren call should be perfectly fine right?
>> Any help appreciated!
>> --John

More information about the openjfx-dev mailing list