java - javafx custom layout pane: label always returns -1 for its preferred width and height -

i try implement custom layout pane in javafx position nodes in circle around center. problem nodes return -1.0 preferredwidth , minwidth , height properties. here class:

public class circlepane extends region {  @override protected void layoutchildren() {     double middlex = getwidth() / 2;     double middley = getheight() / 2;     double factor = math.min(getwidth(), getheight()) * 0.45;// how far middle nodes arranged     double degreesperchild = 2 * math.pi / getchildren().size();     (int = 0; < getchildren().size(); i++) {         if(getchildren().get(i) instanceof region){             region r = (region) getchildren().get(i); // believe need region sizes?             double prefw = math.max(r.getminwidth(), r.getprefwidth()); //my attempt reliable preferredwidth , height             double prefh = math.max(r.getminheight(), r.getprefheight());              double degrees = * degreesperchild;             double xpos = math.sin(degrees) * factor + middlex - prefw / 2;             double ypos = math.cos(degrees) * factor + middley - prefh / 2;             layoutinarea(r, xpos , ypos, prefw, prefh, 0.0d, hpos.left, vpos.center);         }     } }  @override public observablelist<node> getchildren() {     return super.getchildren(); } 

it works, nodes moved right (a few pixels, approximately width / 2). also, want make sure other components fit correctly.

i found solution myself. right width , height, have call region.prefwidth(-1) , region.prefheight(-1)respectively.

the purpose of these 2 functions be:

called during layout determine preferred width/height node.