谁能帮我分析一段代码？

用vb语言

public declare function sendmessage lib "user32" alias "sendmessageA (byval hwnd as long ,byval wmsg as long ,byval wparam as long, byval lparam as any) as long.

public sub trichear( tr treeview)

dim x as integer

with tr

sendmessage .hwnd,&HB,0,0

for x=.nodes.count to 1 step -1

   .nodes.removed x

next x

sendmessage .hwnd,&HB,1,0

end with

end sub

上面的sendmessage函数是什么意思啊，它的各个参数是什么意思？还有那个hwnd属性，我是初手，还望大家帮忙.