tailieunhanh - programming XML by Example phần 3

Ngay cả nếu địa chỉ IP của bạn không trực tiếp nhận dạng bạn, nó chắc chắn Đã WebTV đã đạt được một số thành công với một trình duyệt trong một máy truyền hình. Điện thoại di động và PDA, chẳng hạn như PalmPilot phổ biến, sẽ ngày càng được sử dụng cho các trình duyệt Web. | Designing DTDs 91 Conditional Sections o 1 z VỆ EXAMPLE As your DTDs mature you might have to change them in ways that are partly incompatible with previous usage. During the migration period when you have new and old documents it is difficult to maintain the DTD. To help you manage migrations and other special cases XML provides conditional sections. Conditional sections are included or excluded from the DTD depending on the value of a keyword. Therefore you can include or exclude a large part of a DTD by simply changing one keyword. Listing shows how to use conditional sections. The strict parameter entity resolves to INCLUDE. The lenient parameter entity resolves to IGNORE. The application will use the definition of name in the strict section fname lname and ignores the definition in the lenient section PCDATA fname lname . Listing Using Conditional Sections ENTITY strict INCLUDED ENTITY lenient IGNORE strict -- a name is a first name and a last name -- ELEMENT name fname lname lenient -- name is made of string first name and last name. This is a very flexible model to accommodate exotic name -- ELEMENT name PCDATA fname lname However to revert to the lenient definition of name it suffices to invert the parameter entity declaration ENTITY strict IGNORE ENTITY lenient INCLUDE Designing DTDs Now that you understand what DTDs are for and that you understand how to use them it is time to look at how to create DTDs. DTD design is a creative and rewarding activity. 92 Chapter 3 XML Schemas It is not possible in this section to cover every aspect of DTD design. Books have been devoted to that topic. Use this section as guidance and remember that practice makes proficient. Yet I would like to open this section with a plea to use existing DTDs when possible. Next I will move into two examples of the practical design of practically designing DTDs. Main Advantages of Using Existing DTDs There are many XML DTDs available already and it seems more are being made .