Design Patterns

CHAPTER 9 ■ THE COMPOSITE PATTERN 127
Example: Form Validation
In this example, let’s say you get a new project at work. Initially it seems simple: create a form
whose values can be saved, restored, and validated. Any half-rate web developer could pull this
off, right? The catch is that the contents and number of elements in the form are completely
unknown and will change from one user to the next. Figure 9-2 shows a typical example.
A validate function that is tightly coupled to specific form fields, such as Name and Address,
won’t work because you won’t know at development time what fields to look for. This is a perfect
job for the composite.
Figure 9-2. Each user can potentially see a different form.
First, let’s identify the elements of a form and label them as either a composite or a leaf
(see Figure 9-3 for the identification). The most basic building blocks of a form are the fields
where the user enters data: input, select, and textarea tags. Fieldset tags, which group related
fields together, are one level up. The top level is the form itself.
Figure 9-3. Identifying the basic form elements as composite or leaf