Ext tree的异步加载 求解!

<script>
Ext.onReady(function(){
var root=new Ext.tree.TreeNode({
id:"root",
text:"树的根"
}); var c1=new Ext.tree.TreeNode({
text:"子节点1"
}); var c2=new Ext.tree.AsyncTreeNode({
text:"子节点2"
}); root.appendChild(c1);
root.appendChild(c2); var tree=new Ext.tree.TreePanel({
renderTo:"hello",
root:root,
width:300,
loader:new Ext.tree.TreeLoader({
applyLoader:false,
url:"tree.txt"
})
});
});
</script>tree.txt的内容为:[
{
id:1,
text:'leaf',
leaf:true
}
]
异步加载显示不出来,到底错哪里了呢?有好的例子给我看看也行...
小弟初学ext,恳请各位高手指点!谢过大家了...