MDI子窗体关闭时用到的
(以下摘自Delphi的帮助)
caNone  The form is not allowed to close, so nothing happens.
caHide  The form is not closed, but just hidden. Your application can still access a hidden form.
caFree  The form is closed and all allocated memory for the form is freed.
caMinimize  The form is minimized, rather than closed. This is the default action for MDI child forms.

Action := caFree; Form关闭后释放占用的内存
The form is closed and all allocated memory for the form is freed.
Form关闭并释放所有分配给Form的内存。

Form := nil;      Form对象指向空地址