The Direct to Web Context

The Direct to Web Context

As mentioned earlier, a Direct to Web template is rendered using runtime information about the entities it displays. To translate that information into something you can bind to the template's dynamic elements, Direct to Web uses an instance of the D2WContext class called the Direct to Web context. This object has two functions: it maintains a state dictionary that holds the state of a Direct to Web template as it renders, and it provides values that you can bind directly to attributes of dynamic elements. Each instance of a Direct to Web template has an associated Direct to Web context.

Maintaining State

As the Direct to Web template changes state as it is rendered, the Direct to Web context changes state with it. Specifically, the Direct to Web context uses an NSDictionary containing

the current task

the current entity

the current property (attribute or relationship)

The task and the entity remain constant while the template renders (with the exception of the query all template, for which only the task remains constant). The property does not stay constant, however. Consider an edit page. It displays the entity name and the entity's visible properties. To display the properties, the Direct to Web template iterates through them using a WORepetition. As it iterates, the Direct to Web context updates the information about the current property in its dictionary.

Providing Binding Values

Each Direct to Web template has a Direct to Web context called d2wContext, which implements the EOKeyValueCoding interface. Thus you can bind directly to keys that the context responds to. For example, Listing 3-1 shows the bindings file for a Direct to Web template that displays the name of the entity.

Listing 3-1 Bindings file for a Direct to Web template

String1 : WOString {
    value = d2wContext.entity.name;
};

The Direct to Web context determines the values for the keys (d2wContext.entity.name for example) in one of three ways:

it looks it up in its state dictionary

it accesses application configuration information. (The Web Assistant is the primary way to modify the application configuration.)

it derives values from the state and configuration information

Resolving Keys With the State Dictionary

The state dictionary contains the following entries:

Key	Description of Value
`task`	A string representing the current task.
`entity`	An EOEntity representing the current entity.
`propertyKey`	A string representing the key of the current property.
`attribute`	An EOAttribute representing the current attribute (`null` if the current property is a relationship.)
`relationship`	An EORelationship representing the current relationship (`null` if the current property is an attribute.)

If the dictionary contains the key the template needs, the Direct to Web context resolves the key by returning the value in the dictionary. Otherwise the Direct to Web context resolves the key using one of the other ways.

Resolving Keys With the Application Configuration

Some keys can only be resolved using the application configuration information, which is stored as a database of rules. For example a rule to determine the property-level component for the dateReleased attribute might be "If the task is 'edit', the entity name is 'Movie', and the property key is 'dateReleased' then the value for the componentName key is 'D2WQueryDateOperator'."

The Direct to Web context uses the rule engine to resolve keys that aren't in its dictionary. Figure 3-3 shows how the rule engine relates to the Direct to Web context. "The Rule System" contains detailed information on how the rule engine works.

Figure 3-3 Direct to Web Architecture

Resolving Derived Values

The rule engine also provides objects that have methods that derive values from the Direct to Web context's state dictionary. An example of a derived value is the name displayed for a property: a method converts a property name like dateReleased to a display string "Date Released". Using derived values is discussed in more detail in "The Rule System".