表示层模式:Dispatcher View—分发者视图模式

Context
System controls flow of execution and access to presentation processing, which is
responsible for generating dynamic content.
Note
The Dispatcher View pattern, like the Service to Worker pattern, describes a common
combination of other patterns from the catalog. Both of these macro patterns describe the
combination of a controller and dispatcher with views and helpers. While describing this
common structure, they emphasize related but different usage patterns.
Problem
The problem is a combination of the problems solved by the Front Controller and
View Helper patterns in the presentation tier. There is no centralized component for
managing access control, content retrieval or view management, and there is duplicate
control code scattered throughout various views. Additionally, business logic and
presentation formatting logic are intermingled within these views, making the system less
flexible, less reusable, and generally less resilient to change.
Intermingling business logic with view processing also reduces modularity and
provides a poor separation of roles among Web production and software development
teams.
Forces
Authentication and authorization checks are completed per request.
Scriptlet code within views should be minimized.
Business logic should be encapsulated in components other than the view.
Control flow is relatively simple and is typically based on values encapsulated with
the request.
View management logic is limited in complexity.
Solution
Combine a controller and dispatcher with views and helpers (see "Front Controller"
on page 172 and "View Helper" on page 186) to handle client requests and prepare a
dynamic presentation as the response. Controllers do not delegate content retrieval to
helpers, because these activities are deferred to the time of view processing. A dispatcher is
responsible for view management and navigation and can be encapsulated either within a
controller, a view, or a separate component.
Dispatcher View describes the combination of the Front Controller and View Helper
patterns with a dispatcher component. While this pattern and the Service to Worker pattern
describe a similar structure, the two patterns suggest a different division of labor among the
components. The controller and the dispatcher typically have limited responsibilities, as
compared to the Service to Worker pattern, since the upfront processing and view
management logic are basic. Furthermore, if centralized control of the underlying resources
is considered unnecessary, then the controller is removed and the dispatcher may be moved
into a view.
Since the Service to Worker and Dispatcher View patterns represent a common
combination of other patterns from the catalog, each warrants its own name to promote
efficient communication among developers. Unlike the Service to Worker pattern, the
Dispatcher View pattern suggests deferring content retrieval to the time of view processing.
In the Dispatcher View pattern, the dispatcher typically plays a limited to moderate
role in view management. In the Service to Worker pattern, the dispatcher typically plays a
moderate to large role in view management.
A limited role for the dispatcher occurs when no outside resources are utilized in
order to choose the view. The information encapsulated in the request is sufficient to
determine the view to dispatch the request. For example:
http://some.server.com/servlet/Controller?next=login.jsp
The sole responsibility of the dispatcher component in this case is to dispatch to the
view login.jsp.
An example of the dispatcher playing a moderate role is the case where the client
submits a request directly to a controller with a query parameter that describes an action to
be completed:
http://some.server.com/servlet/Controller?action=login
The responsibility of the dispatcher component here is to translate the logical name
login into the resource name of an appropriate view, such as login.jsp, and dispatch to that
view. To accomplish this translation, the dispatcher may access resources such as an XML
configuration file that specifies the appropriate view to display.
On the other hand, in the Service to Worker pattern, the dispatcher might be more
sophisticated. The dispatcher may invoke a business service to determine the appropriate
view to display.
The shared structure of these two patterns, as mentioned above, consists of a
controller working with a dispatcher, views, and helpers.