为什么必须在listener属性中的方法名称之前加上“listener”前缀(看起来多余，是不是？)
Why do you have to preface the method name in the listener attribute with "listener" (it seems redundant, doesn't it)?
如果将命名空间置于关系名称之前，将发生错误。
An error occurs when you prepend the namespace to the relationship name.
注意第3行中，如果元素存在某些属性，就应该恰好在元素名称之后，元素内容(第4行)之前告知。
Notice in line 3 that, if there are attributes to an element, they should be informed right after the element's name, followed by the element content (line 4).
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