5 Flow Scopes

Scope Basics

You'll notice from previous examples that we used a special object called flow to store objects within "flow scope". Grails flows have five different scopes you can utilize:

• request - Stores an object for the scope of the current request
• flash - Stores the object for the current and next request only
• flow - Stores objects for the scope of the flow, removing them when the flow reaches an end state
• conversation - Stores objects for the scope of the conversation including the root flow and nested subflows
• session - Stores objects in the user's session

Grails service classes can be automatically scoped to a web flow scope. See the documentation on guide:services for more information.

Returning a model Map from an action will automatically result in the model being placed in flow scope. For example, using a transition action, you can place objects within flow scope as follows:

enterPersonalDetails {
    on("submit") {
        [person: new Person(params)]
    }.to "enterShipping"
   on("return").to "showCart"
}

Be aware that a new request is always created for each state, so an object placed in request scope in an action state (for example) will not be available in a subsequent view state. Use one of the other scopes to pass objects from one state to another. Also note that Web Flow:

1. Moves objects from flash scope to request scope upon transition between states;
2. Merges objects from the flow and conversation scopes into the view model before rendering (so you shouldn't include a scope prefix when referencing these objects within a view, e.g. GSP pages).

Flow Scopes and Serialization

When placing objects in flash, flow or conversation scope they must implement java.io.Serializable or an exception will be thrown. This has an impact on guide:GORM in that domain classes are typically placed within a scope so that they can be rendered in a view. For example consider the following domain class:

class Book {
    String title
}

To place an instance of the Book class in a flow scope you will need to modify it as follows:

class Book implements Serializable {
    String title
}

This also impacts associations and closures you declare within a domain class. For example consider this:

class Book implements Serializable {
    String title
    Author author
}

Here if the Author association is not Serializable you will also get an error. This also impacts closures used in guide:eventsAutoTimestamping such as onLoad, onSave and so on. The following domain class will cause an error if an instance is placed in a flow scope:

class Book implements Serializable {
    String title
    def onLoad = {
        println "I'm loading"
    }
}

The reason is that the assigned block on the onLoad event cannot be serialized. To get around this you should declare all events as transient:

class Book implements Serializable {
    String title
    transient onLoad = {
        println "I'm loading"
    }
}

or as methods:

class Book implements Serializable {
    String title
    def onLoad() {
        println "I'm loading"
    }
}

The flow scope contains a reference to the Hibernate session. As a result, any object loaded into the session through a GORM query will also be in the flow and will need to implement Serializable.

If you don't want your domain class to be Serializable or stored in the flow, then you will need to evict the entity manually before the end of the state:

flow.persistenceContext.evict(it)