Entry units support form-based data entry. They are used for gathering input, which is typically employed to do the following:
Perform searches over the objects of an entity, for example to locate the instances of an entity whose attributes contain a given keyword.
Supply parameters to operations like content updates, login, and external services.
Entry units are characterized by the following properties: Name:the user-defined name for the entry unit. Fields:the set of fields for inputting values.
Figure 3.10 shows the graphic notation used for an entry unit (ArtistInput).
3.2 Units 89 ArtistInput Artist
ARTIST INPUT
FirstName:
LastName:
BirthDate:
DeathDate:
Louis Armstrong 07-06-1971 08-04-1901As shown in the rendition example, the entry unit in Figure 3.10 has four fields, for inputting the first name, last name, birth date, and death date of an artist. Fields are not shown in the graphical notation, but appear in the textual one:
EntryUnit ArtistInput (fields FirstNameField String; LastNameField String; BirthDateField Date; DeathDateField Date)
Entry units fields correspond to the input fields normally found in the form constructs of mark-up languages. Input fields have a number of properties:
Name:the name of the field.
Type:the data type of the value input in the field (for example string, text, integer, date and so on).
Initial value (optional):a default value to be initially proposed to the user. Modifiability:a flag which specifies if the user can modify the initial field value or not; by default all fields are modifiable.
Validity predicate:a Boolean condition applicable to the value input by the user, to check its validity. The validity predicate can be any logical expres- sion constructed using the field name, an operator applicable to the data type of the field, and a constant or variable term. The variable term can be the name of another field, which permits the comparison of values input by the user in different fields, for example, to ensure that the death date of an artist is greater than the birth date. The special keyword notnull can be used to require that the user specify some value for a field. The complete syntax for defining validity predicates is reported in Appendix B. For example, the specification of the fields of the ArtistInput entry unit of Figure 3.10 can be enriched with all the relevant field properties, including validity pred- icates, as follows:
EntryUnit ArtistInput (fields
FirstNameField string, notnull; LastNameField string, notnull; BirthDateField date, notnull;
DeathDateField date, DeathDateField > BirthDateField)
3.3 Pages
Pagesare the actual interface elements delivered to the user, who browses the hy- pertext by accessing its pages in the desired sequence. A page typically consists of several units, grouped together to accomplish a well-defined communication pur- pose. In the rest of this chapter, we will show several examples of pages and will present frequently adopted page design patterns.
Figure 3.11 shows the graphic notation for pages, which is simply a la- beled box surrounding the units that belong to the page. In the example, the page called AlbumPage contains two index units, one for displaying the list of all artists, and one for displaying the list of all albums. The figure also shows a possible rendition of the AlbumPage in HTML, which is simply the aggrega- tion of the renditions of the two index units. Note that, although in Figure 3.11 the position of the index units in the HTML rendition is the same as that of the index unit icons in the WebML specification, this fact is purely coinciden- tal. A WebML page specification is abstract, and has nothing to do with pre- sentation aspects, like the relative position of content elements in the HTML rendition.
Like units, pages also admit a textual specification. The textual syntax for specifying page Album includes the page name and the list of its enclosed units:
Page AlbumPage
(units AlbumIndex, ArtistIndex)
3.3 Pages 91 Album AlbumIndex Album Album ALBUMS
• All the way • Let's talk about... •Sing-a-long • These are ... ARTISTS • Celine Dion • Frank Sinatra • Nirvana • Lenny Kravitz ArtistIndex Artist
3.4 Links
Neither pages nor units exist in isolation, because real-world hypertexts are made of connected pages, which contain several interrelated pieces of content and com- mands permitting the user to interact with the application. To express these fea- tures, pages and units can be linked, to specify the allowed navigation paths between pages, the selections offered to the user, and the effect of the user’s in- teraction on the content of the units displayed in the page.
Navigation modeling is the part of hypertext modeling that deals with the specification of the links between units and pages, and of the properties of such links. The central notions of navigation modeling are the concepts of link, link pa- rameters,andparametric selectors:
A link is an oriented connection between two units or pages.
A link parameter is the specification of a piece of information, which is transported from the source to the destination of the link.
A parametric selector is a unit selector whose predicates contain a refer- ence to a link parameter.
These concepts and their applications are treated in detail in the rest of this section.