SPRING IN ACTION 第4版笔记-第八章Advanced Spring MVC-002-SpringFlow的组件(state\<transition>\<var>\<set>\<evaluate>)

一、

In Spring Web Flow, a flow is defined by three primary elements: states, transitions,and flow data.如果把webflow当做是一次旅行，则state是旅途中每个景点，而transitions是连接景点的路径，data是在每个景点中购买的纪念品。

1.state

(1)支持的状态

(2)view-state

View states are used to display information to the user and to offer the user an opportunity to play an active role in the flow. The actual view implementation could be any of the views supported by Spring MVC but is often implemented in JSP .

<view-state id="welcome" />

因为没有指定view，所以对应的view以id为准，即welcome.jsp。也可明确指定view

<view-state id="welcome" view="greeting" />

If a flow presents a form to the user, you may want to specify the object to which the form will be bound. To do that, set the model attribute:

<view-state id="takePayment" model="flowScope.paymentDetails"/>

Here you specify that the form in the takePayment view will be bound to the flowscoped paymentDetails object.

(3)action-state

Whereas view states involve the users of the application in the flow, action states are where the application itself goes to work. Action states typically invoke some method on a Spring-managed bean and then transition to another state depending on the outcome of the method call.

 <action-state id="saveOrder">

     <evaluate expression="pizzaFlowActions.saveOrder(order)" />

     <transition to="thankYou" />

 </action-state>

The <evaluate> element gives an action state something to do. The expression attribute is given an expression that’s evaluated when the state is entered. In this case, expression is given a SpEL expression indicating that the
saveOrder() method should be called on a bean whose ID is pizzaFlowActions

(4)DECISION STATES

Decision states enable a binary branch in a flow execution. A decision state evaluates a Boolean expression and takes one of two transitions, depending on whether the expression evaluates to true or false .

 <decision-state id="checkDeliveryArea">

     <if test="pizzaFlowActions.checkDeliveryArea(customer.zipCode)" then="addCustomer"

     else="deliveryWarning" />

 </decision-state>

(5)SUBFLOW STATES

在一个应用中，你不会把所有逻辑都写到一个方法里，你会拆分，SUBFLOW STATES就像是在一个方法里调用另一个方法来完成任务。

The<subflow-state> element lets you call another flow from within an executing flow.

 <!-- Order -->

     <subflow-state id="order" subflow="pizza/order">

       <input name="order" value="order"/>

       <transition on="orderCreated" to="payment" />

     </subflow-state>

Here, the <input> element is used to pass the order object as input to the subflow.And if the subflow ends with an <end-state> whose ID is orderCreated , then the flow will transition to the state whose ID is payment

(6)END STATES

<end-state id="customerReady" />

When the flow reaches an <end-state> , the flow ends. What happens next depends on a few factors:

 If the flow that’s ending is a subflow, the calling flow will proceed from the <subflow-state> . The <end-state> ’s ID will be used as an event to trigger the transition away from the <subflow-state> .
 If the <end-state> has its view attribute set, the specified view will be rendered.The view may be a flow-relative path to a view template, prefixed withexternalRedirect: to redirect to some page external to the flow, or prefixed

with flowRedirect: to redirect to another flow.
 If the ending flow isn’t a subflow and no view is specified, the flow ends. The browser lands on the flow’s base URL , and, with no current flow active, a new instance of the flow begins.

2.transitions

transitions connect the states within a flow. Every state in a flow, with the exception of end states, should have at least one transition so that the flow will know where to go once that state has completed

<transition to="customerReady" />

The to attribute is used to specify the next state in the flow. When <transition> is declared with only a to attribute, the transition is the default transition for that state and will be taken if no other transitions are applicable.

More commonly, transitions are defined to take place on some event being fired.In a view state, the event is usually an action taken by the user. In an action state, theevent is the result of evaluating an expression. In the case of a subflow state, the eventis determined by the ID of the subflow’s end state. In any event (no pun intended),you can specify the event to trigger the transition in the on attribute:

<transition on="phoneEntered" to="lookupCustomer"/>

In this example, the flow will transition to the state whose ID is lookupCustomer if a phoneEntered event is fired.

The flow can also transition to another state in response to some exception being thrown.

<transition

on-exception="com.springinaction.pizza.service.CustomerNotFoundException" to="registrationForm" />

The on-exception attribute is much like the on attribute, except that it specifies an exception to transition on instead of an event.

(2)GLOBAL TRANSITIONS

<global-transitions>

    <transition on="cancel" to="endState" />

</global-transitions>

With this global transition in place, all states in the flow will have an implicit cancel transition.

3.flow data

(1)<var>

<var name="customer" class="com.springinaction.pizza.domain.Customer"/>

a new instance of a Customer object is created and placed into the variable whose name is customer . This variable is available to all states in a flow.

(2)<evaluate>

As part of an action state or on entry to a view state, you may also create variables using the <evaluate> element. For example,

<evaluate result="viewScope.toppingsList" expression="T(com.springinaction.pizza.domain.Topping).asList()" />

In this case, the <evaluate> element evaluates an expression (a SpEL expression) and places the result in a variable named toppingsList that’s view-scoped.

(3)<set>

<set name="flowScope.pizza" value="new com.springinaction.pizza.domain.Pizza()" />

The <set> element works much the same as the <evaluate> element, setting a vari able to the resulting value from an evaluated expression. Here, you’re setting a flow- scoped pizza variable to a new instance of a Pizza object.

4.scope

When you declare a variable using the <var> element, the variable is always flow-scoped in the flow defining the variable. When you use <set> or <evaluate> , the scope is specified as a prefix for the name or result attribute. For example, here’s how you would assign a value to a flow-scoped variable named theAnswer :

<set name="flowScope.theAnswer" value="42"/>

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