sorry, a new problem.
when i create lots of box ,one is rootnode,the other is subnode,like this:
--------------------- 
                  o   |
o o o o o o o o o o o o o o o o o o o o o 
                      |
                       |
---------------------
this line "------------|"is screen, view's left and right sides in document.
when i run program,initial view is this picture,but i think,the program running,
the viewer is on the root box,also.the rootbox is in the middle of viewer like this:

          ---------------------- 
         |                           |
         |           o              |
         |                           |
oo o o o o o o o o o o o o o o o o o o 
         |                           |                              
      -------------------------- ----->this is our viewer, on the initial ,we can see the rootbox.

i don't know , i say it very clear?

yes,in the default,the view is left, we can't see the middle node,i think
when the programe is run, the view is in the middle of tree,also is root node ,like box. how i can do it ?

It's OK,thank you .