null set

基本解释

When a change is made to a relationship, the related foreign-key property is set to a null value.
對關系進行更新時，相關的外鍵的屬性設置爲空值。
If the pszText member is non-NULL, return TRUE to set the item's label to the edited text. Return FALSE to reject the edited text and revert to the original label.
処理這個通知，查找是否有同名項，如果是則返廻FALSE，否則將編輯後的文字複制到節點。
However in this case, the link's ref attribute will be set to the value "null," and the link's text content will be hidden unless a flag to show null-link content is set to true.
但是，在這種情況下，這個鏈接的 ref 屬性會設置爲 “null” 值，而且鏈接的文本內容會隱藏起來，除非將顯示空鏈接內容的標志設置爲 true。