HomeApache Page 3 - Introducing Simple Components in Apache Tapestry
Implicit vs. declared components - Apache
In the previous article, you witnessed the interplay between a Tapestry page and its components (granted, we had only one simple component there, but you get my point). Page class, when rendering its page, finds any components mentioned in the template and asks those components to display themselves, as they know better how to do that. Components, in their turn, might need some information to display themselves, and they ask the page class to provide the necessary information by calling some of its methods.
You have seen just one Tapestry component so far, so this discussion will be unavoidably abstract, but I still think it will be useful to prepare you to understand what will come later.
The obvious advantage of the implicit components (fully described in the HTML template) is that you can see everything related to this component in one place: how and where it is used on the page, what its type is and what it is bound to in the page class. You don’t have to look into another file or page specification to find out any additional information.
The obvious disadvantage of this approach is that if a component has several bindings (in some cases there can be three or even more bindings), the page's HTML code gets cluttered with weird nonstandard attributes and maybe also complex OGNL expressions.
The advantage of the declared components is that you have them listed all neat and orderly in the page specification. Having multiple bindings is not a problem and complex OGNL expressions are welcome here.
The disadvantage of the declared components is obvious when you have a component without any bindings (you will see one such component soon), or with only one simple binding. The proper specification for such a component will be more verbose than its implicit declaration.
In practice, I find myself combining the two approaches. When a component has no bindings, I define it implicitly. When it has more than one binding, I declare it in page specification. In the case of simple components, like Insert, I might define them either way depending on the complexity of the page design. If the page is large and its design is complex, I will prefer declared components over implicit ones.
It is the time now to have more practice and to meet more components. Let’s create another project, this time having two Tapestry pages.
Apache 
