一次展開Tree元件的所有節點(Expand All Children of Tree)
展開一個Tree元件的所有節點的方法，有兩個階段：
1. 走訪所有節點
2. 如果是枝幹(branch)結點，就打開它( 有預設的方法可以使用, expandChildrenOf( node, open) )
要提醒的地方是，所有ListBase類型的元件，裡頭都有一個collectionChangeHandler的涵式，我們可以複寫他，讓每次資料重讀之後，就打開全部節點。
請見下方原始碼 
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All ListBase-components handle collection with a collectionChangeHandler(event). All we have to do is override it by adding a expandAll() function. How will we do with this expandAll()? See following sample:
override protected function collectionChangeHandler(event:Event):void
{
super.collectionChangeHandler( event );
if( expandChildrenAfterReset )
{
if( CollectionEvent( event ).kind == CollectionEventKind.RESET )
{
expandAll();
}
}
}
public function expandAll():void
{
if( collection )
{
var c:IViewCursor = collection.createCursor();
var o:Object;
while( !c.afterLast )
{
o = c.current;
if( dataDescriptor.isBranch( o ) )
expandChildrenOf( o ,true );
c.moveNext();
}
}
}
{
super.collectionChangeHandler( event );
if( expandChildrenAfterReset )
{
if( CollectionEvent( event ).kind == CollectionEventKind.RESET )
{
expandAll();
}
}
}
public function expandAll():void
{
if( collection )
{
var c:IViewCursor = collection.createCursor();
var o:Object;
while( !c.afterLast )
{
o = c.current;
if( dataDescriptor.isBranch( o ) )
expandChildrenOf( o ,true );
c.moveNext();
}
}
}
As you see, we just iterate all tree node, determining if it is a branch and then call the built-in expandChildreOf(node,open) method.