表示层模式:Service to Worker—工作者服务模式

Context
The system controls flow of execution and access to business data, from which it
creates presentation content.
Note
The Service to Worker pattern, like the Dispatcher View 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 complex and based on values from dynamic content.
View management logic is relatively sophisticated, with multiple views potentially
mapping to the same request.
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 delegate content retrieval to helpers,
which manage the population of the intermediate model for the view. A dispatcher is
responsible for view management and navigation and can be encapsulated either within a
controller or a separate component.
Service to Worker describes the combination of the Front Controller and View Helper
patterns with a dispatcher component.
While this pattern and the Dispatcher View pattern describe a similar structure, the
two patterns suggest a different division of labor among the components. In Service to
Worker, the controller and the dispatcher have more responsibilities.
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 Service to Worker and Dispatcher View consists of a
controller working with a dispatcher, views, and helpers.