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Customizing View Selection with View States and End States
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Recall from 11 that view states instruct Spring Web Flow to pause the execution of a flow and render a view allowing the user to participate in the flow. Spring Web Flow creates logical view selections (consisting of a view name and a set of model data), but the resolution of those view selections to a renderable View is the responsibility of the calling web framework. That said, Spring Web Flow allows you full control over view selection logic via the org.springframework.webflow.ViewSelector interface. Listing 12-25. org.springframework.webflow.ViewSelector public interface ViewSelector { ViewSelection makeSelection(RequestContext context); } This interface allows you to customize how the view name is calculated and/or what model data is available to the view. The ViewSelection returned by the ViewSelector is then converted into a renderable artifact by the calling web framework (e.g., a Struts ActionForward or a Spring MVC ModelAndView). This conversion is performed by the appropriate Front Controller (FlowAction for Struts or FlowController for Spring MVC). Figure 12-3 illustrates this conversion.
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CHAPTER 12 ADVANCED SPRING WEB FLOW
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FlowExecutionManager
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ViewSelection
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Web Framework (Servlet, JSF, Portlet)
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Figure 12-3. Conversion of a ViewSelection to a View There are two implementations of ViewSelector provided out of the box, shown in Table 12-10. Table 12-10. ViewSelector Implementations
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org.springframework.webflow. support.RedirectViewSelector
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Behavior
Represents a redirect view.
Model Exposed
Only exposes parameters explicitly specified in the redirect Expression (e.g., redirect:/url.htm param0=$ {flowScope.foo}¶m1=value1). Exposes all data in both flow and request scope data in the model.
org.springframework. webflow.support. SimpleViewSelector
Refers to a static view name.
The FlowBuilder decides which ViewSelector to use based upon the value of the view property. Table 12-11 describes the criteria Spring Web Flow uses to choose a ViewSelector. Table 12-11. ViewSelector Selection
ViewSelector
RedirectViewSelector YourViewSelector SimpleViewSelector Indicating Transitions
Criteria
If the viewName contains a redirect: prefix. If the viewName is bean:YourViewSelector. If none of the other two conditions are met.
When Spring Web Flow encounters a view state it will 1. render the view; 2. pause the flowExecution; 3. wait for a user-supplied eventId to resume.
CHAPTER 12 ADVANCED SPRING WEB FLOW
The view can submit the eventId in one of two ways. The first way is to submit a request parameter whose name is FlowExecutionManagerParameterExtractor.getEventIdParameterName() (the default is _eventId) and whose value will be the actual user eventId (like submit).
Note For the sake of brevity and readability, FlowExecutionManagerParameterExtractor may be
referred to as FEMPE.
Alternatively, the second way is to have the view submit a parameter whose name has the format FEMPE.getEventIdParameterName()FEMPE.getParameterDelimiter()value (the default value for FEMPE.getParameterDelimiter() is _ ). This form is primarily used with the name of an HTML input button to support multiple buttons per form without JavaScript. In this case, the eventId is derived fully from the parameter name, and the value of this parameter is ignored. To illustrate these two approaches, to signal the submit event you may provide either of the following request parameters: _eventId=submit or _eventId_submit=ignored.
Note
FlowExecutionManagerParameterExtractor will also support image buttons that submit
parameters of type eventId.x or eventId.y.
Decision States
Although the example decision state in 11 defined a simple, single if/else expression, decision states can do more. As well as supporting multiple if conditions, the decision state can also delegate the criteria for the decision to Java application code. If multiple if conditions are supplied, they are evaluated one by one. If none of the conditions evaluates to true, then a NoMatchingTransitionException is thrown. You can implement a chain of if conditions, but realize that any if condition that defines an else clause will by definition evaluate to true, and none of the remaining if conditions will be evaluated. For example, the fragment in Listing 12-26 will never evaluate the second condition. Listing 12-26. Badly Defined Decision State <decision-state> <if test="${flowScope.object.booleanProperty}" then="stateA" else="stateB"/> <if test="${this.will.never.be.called}" then="neverGetCalled"/> </decision-state> Listing 12-27 shows a chain of conditions that behaves as you would expect.
CHAPTER 12 ADVANCED SPRING WEB FLOW
Listing 12-27. Correctly Defined Decision State <decision-state> <if test="${flowScope.object.booleanProperty}" then="stateA" /> <if test="${this is called if the above test evaluates to false}" then="stateB"/> <if test="${this is called if the above test evaluates to false}" then="stateC" else ="stateD"/> </decision-state>
Caution It is generally wise to put an else branch on the last if; otherwise, you run the risk of Spring
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