PATH Documentation > WebObjects 4.5 > EOControl Reference

EOClassDescription

Inherits from:
NSObject

Declared in:
EOControl/EOClassDescription.h

Class Description

The EOClassDescription class provides a mechanism for extending classes by giving them access to metadata not available in the run-time system. This is achieved as follows:

EOClassDescription provides a bridge between enterprise objects and the metadata contained in an external source of information, such as an EOModel (EOAccess). It defines a standard API for accessing the information in an external source. It also manages the registration of EOClassDescription objects in your application.
The EOEnterpriseObject informal protocol declares several EOClassDescription-related methods that define basic enterprise objects behavior, such as undo and validation. The Enterprise Objects Framework extends NSObject by providing implementations of these methods. An enterprise object class can either accept the default implementations or it can provide its own implementation by overriding. This is discussed in more detail in the section "Using EOClassDescription" .

Enterprise Objects Framework implements a default subclass of EOClassDescription in EOAccess, EOEntityClassDescription. EOEntityClassDescription extends the behavior of enterprise objects by deriving information about them (such as NULL constraints and referential integrity rules) from an associated EOModel.

For more information on using EOClassDescription, see the sections

Constants

In EOClassDescription.h, EOControl defines the enumeration type EODeleteRule. It's constants are:

Constant	Description
`EODeleteRuleNullify`	When the source object is deleted, any references a destination object has to the source are removed or "nullified." For example, suppose a department has a to-many relationship to multiple employees. When the department is deleted, any back references an employee has to the department are set to nil.
`EODeleteRuleCascade`	When the source object (department) is deleted, any destination objects (employees) are also deleted.
`EODeleteRuleDeny`	If the source object (department) has any destination objects (employees), a delete operation is refused.
`EODeleteRuleNoAction`	When the source object is deleted, its relationship is ignored and no action is taken to propagate the deletion to destination objects.This rule is useful for tuning performance.To perform a deletion, Enterprise Objects Framework fires all the faults of the deleted object and then fires any to-many faults that point back to the deleted object. For example, suppose you have a simple application based on the sample Movies database. Deleting a Movie object has the effect of firing a to-one fault for the Movie's studio relationship, and then firing the to-many movies fault for that studio. In this scenario, it would make sense to set the delete rule `EODeleteRuleNoAction` for Movie's studio relationship. However, you should use this delete rule with great caution since it can result in dangling references in your object graph.

EOClassDescription.h also defines string constants for the names of the notifications it posts. For more information, see the section "Notifications" .

Method Types

Managing EOClassDescriptions

+ invalidateClassDescriptionCache

+ registerClassDescription:forClass:

Getting EOClassDescriptions

+ classDescriptionForClass:

+ classDescriptionForEntityName:

Creating new object instances

- createInstanceWithEditingContext:globalID:zone:

Propagating delete

- propagateDeleteForObject:editingContext:

Returning information from the EOClassDescription

- entityName

- attributeKeys

- classDescriptionForDestinationKey:

- toManyRelationshipKeys

- toOneRelationshipKeys

- inverseForRelationshipKey:

- ownsDestinationObjectsForRelationshipKey:

- deleteRuleForRelationshipKey:

Performing validation

- validateObjectForDelete:

- validateObjectForSave:

- validateValue:forKey:

Providing default characteristics for key display

- defaultFormatterForKey:

- defaultFormatterForKeyPath:

- displayNameForKey:

Handling newly inserted and newly fetched objects

- awakeObject:fromFetchInEditingContext:

- awakeObject:fromInsertionInEditingContext:

Setting the delegate

+ classDelegate

+ setClassDelegate:

Getting an object's description

- userPresentableDescriptionForObject:

Getting fetch specifications

- fetchSpecificationNamed:

Class Methods

classDelegate

+ (id)classDelegate

Returns the delegate for the EOClassDescription class (as opposed to EOClassDescription instances).

See Also: + setClassDelegate:

classDescriptionForClass:

+ (EOClassDescription *)classDescriptionForClass:(Class)aClass

Invoked by the default implementations of the EOEnterpriseObject informal protocol method classDescription to return the EOClassDescription for aClass. It's generally not safe to use this method directly-for example, individual EOGenericRecord instances can have different class descriptions. If a class description for aClass isn't found, this method posts an EOClassDescriptionNeededForClassNotification on behalf of the receiver's class, allowing an observer to register a an EOClassDescription.

classDescriptionForEntityName:

+ (EOClassDescription *)classDescriptionForEntityName:(NSString *)entityName

Returns the EOClassDescription registered under entityName.

invalidateClassDescriptionCache

+ (void)invalidateClassDescriptionCache

Flushes the EOClassDescription cache. Because the EOModel objects in an application supply and register EOClassDescriptions on demand, the cache continues to be repopulated as needed after you invalidate it. (The EOModel class is defined in EOAccess.)

You'd use this method when a provider of EOClassDescriptions (such as an EOModel) has newly become available, or is about to go away. However, you should rarely need to directly invoke this method unless you're using an external source of information other than an EOModel.

registerClassDescription:forClass:

+ (void)registerClassDescription:(EOClassDescription *)description forClass:(Class)class

Registers an EOClassDescription object for class in the EOClassDescription cache. You should rarely need to directly invoke this method unless you're using an external source of information other than an EOModel (EOAccess).

setClassDelegate:

+ (void)setClassDelegate:(id)delegate

Sets the delegate for the EOClassDescription class (as opposed to EOClassDescription instances) to delegate, without retaining it. For more information on the class delegate, see the EOClassDescription ClassDelegate informal protocol specification.

See Also: + classDelegate

Instance Methods

attributeKeys

- (NSArray *)attributeKeys

Overridden by subclasses to return an array of attribute keys (NSStrings) for objects described by the receiver. "Attributes" contain immutable data (such as NSNumbers and NSStrings), as opposed to "relationships" that are references to other enterprise objects. For example, a class description that describes Movie objects could return the attribute keys "title," "dateReleased," and "rating."

EOClassDescription's implementation of this method simply returns.

awakeObject:fromFetchInEditingContext:

- (void)awakeObject:(id)object fromFetchInEditingContext:(EOEditingContext *)anEditingContext

Overridden by subclasses to perform standard post-fetch initialization for object in anEditingContext. EOClassDescription's implementation of this method does nothing.

awakeObject:fromInsertionInEditingContext:

- (void)awakeObject:(id)object fromInsertionInEditingContext:(EOEditingContext *)anEditingContext

Assigns empty arrays to to-many relationship properties of newly inserted enterprise objects. Can be overridden by subclasses to propagate inserts for the newly inserted object in anEditingContext. More specifically, if object has a relationship (or relationships) that propagates the object's primary key and if no object yet exists at the destination of that relationship, subclasses should create the new object at the destination of the relationship. Use this method to put default values in your enterprise object.

classDescriptionForDestinationKey:

- (EOClassDescription *)classDescriptionForDestinationKey:(NSString *)detailKey

Overridden by subclasses to return the class description for objects at the destination of the to-one relationship identified by detailKey. For example, the statement:

[movie classDescriptionForDestinationKey:@"studio"];

might return the class description for the Studio class. EOClassDescription's implementation of this method returns nil.

createInstanceWithEditingContext:globalID:zone:

- (id)createInstanceWithEditingContext:(EOEditingContext *)anEditingContext globalID:(EOGlobalID *)globalID zone:(NSZone *)zone

Overridden by subclasses to create an object of the appropriate class in anEditingContext with globalID and in zone. In typical usage, all three of the method's arguments are nil. If the object responds to initWithEditingContext:classDescription:globalID subclasses should invoke that method, otherwise they should invoke init. Implementations of this method should return an autoreleased object. Enterprise Objects Framework uses this method to create new instances of objects when fetching existing enterprise objects or inserting new ones in an interface layer EODisplayGroup. EOClassDescription's implementation of this method returns nil.

defaultFormatterForKey:

- (NSFormatter *)defaultFormatterForKey:(NSString *)key

Returns the default NSFormatter to use when parsing values for assignment to key. EOClassDescription's implementation returns nil. The access layer's EOEntityClassDescription's implementation returns an NSFormatter based on the Objective-C value class specified for key in the associated model file. Code that creates a user interface, like a wizard, can use this method to assign formatters to user interface elements.

defaultFormatterForKeyPath:

- (NSFormatter *)defaultFormatterForKeyPath:(NSString *)keyPath

Similar to defaultFormatterForKey:, except this method traverses keyPath and returns the formatter for the key at the end of the path (using defaultFormatterForKey:).

deleteRuleForRelationshipKey:

- (EODeleteRule)deleteRuleForRelationshipKey:(NSString *)relationshipKey

Overridden by subclasses to return a delete rule indicating how to treat the destination of the given relationship when the receiving object is deleted. The delete rule is one of:

EODeleteRuleCascade
EODeleteRuleDeny
EODeleteRuleNullify
EODeleteRuleNoAction

EOClassDescription's implementation of this method returns the delete rule EODeleteRuleNullify. In the common case, the delete rule for an enterprise object is defined in its EOModel. (The EOModel class is defined in EOAccess.)

See Also: - propagateDeleteWithEditingContext: (EOEnterpriseObject)

displayNameForKey:

- (NSString *)displayNameForKey:(NSString *)key

Returns the default string to use in the user interface when displaying key. By convention, lowercase words are capitalized (for example, "revenue" becomes "Revenue"), and spaces are inserted into words with mixed case (for example, "firstName" becomes "First Name"). This method is useful if you're creating a user interface from only a class description, such as with a wizard or a Direct To Web application.

entityName

- (NSString *)entityName

Overridden by subclasses to return a unique type name for objects of this class. For example, the access layer's EOEntityClassDescription returns its EOEntity's name. EOClassDescription's implementation of this method returns nil.

fetchSpecificationNamed:

- (EOFetchSpecification *)fetchSpecificationNamed:(NSString *)name

Overridden by subclasses to return the fetch specification associated with name. For example, the access layer's EOEntityClassDescription returns the fetch specification in its EOEntity named name (if any). EOClassDescription's implementation returns nil.

inverseForRelationshipKey:

- (NSString *)inverseForRelationshipKey:(NSString *)relationshipKey

Overridden by subclasses to return the name of the relationship pointing back at the receiver from the destination of the relationship specified by relationshipKey. For example, suppose an Employee object has a relationship called department to a Department object, and Department has a relationship called employees back to Employee. The statement:

[employee inverseForRelationshipKey:@"department"];

returns the string "employees".

EOClassDescription's implementation of this method returns nil.

ownsDestinationObjectsForRelationshipKey:

- (BOOL)ownsDestinationObjectsForRelationshipKey:(NSString *)relationshipKey

Overridden by subclasses to return YES or NO to indicate whether the objects at the destination of the relationship specified by relationshipKey should be deleted if they are removed from the relationship (and not transferred to the corresponding relationship of another object). For example, an Invoice object owns its line items. If a LineItem object is removed from an Invoice it should be deleted since it can't exist outside of an Invoice. EOClassDescription's implementation of this method returns NO. In the common case, this behavior for an enterprise object is defined in its EOModel. (The EOModel class is defined in EOAccess.)

propagateDeleteForObject:editingContext:

- (void)propagateDeleteForObject:(id)object editingContext:(EOEditingContext *)anEditingContext

Propagates a delete operation for object in anEditingContext, according to the delete rules specified in the EOModel. This method is invoked whenever a delete operation needs to be propagated, as indicated by the delete rule specified for the corresponding EOEntity's relationship key. (The EOModel and EOEntity classes are defined in EOAccess.) For more discussion of delete rules, see the EOEnterpriseObject informal protocol specification.

See Also: - deleteRuleForRelationshipKey:

toManyRelationshipKeys

- (NSArray *)toManyRelationshipKeys

Overridden by subclasses to return the keys for the to-many relationship properties of the receiver. To-many relationship properties contain arrays of enterprise objects. EOClassDescription's implementation of this method returns nil.

toOneRelationshipKeys

- (NSArray *)toOneRelationshipKeys

Overridden by subclasses to return the keys for the to-one relationship properties of the receiver. To-one relationship properties are other enterprise objects. EOClassDescription's implementation of this method returns nil.

userPresentableDescriptionForObject:

- (NSString *)userPresentableDescriptionForObject:(id)anObject

Returns a short (no longer than 60 characters) description of anObject based on its data. This method enumerates anObject's attributeKeys and returns each attribute's value, separated by commas and with the default formatter applied for numbers and dates.

validateObjectForDelete:

- (NSException *)validateObjectForDelete:(id)object

Overridden by subclasses to determine whether it's permissible to delete object. Subclasses should return nil if the delete operation should proceed, or return an exception containing a user-presentable (localized) error message if not. EOClassDescription's implementation of this method returns nil.

validateObjectForSave:

- (NSException *)validateObjectForSave:(id)object

Overridden by subclasses to determine whether the values being saved for object are acceptable. Subclasses should return nil if the values are acceptable and the save operation should proceed, or return an exception containing a user-presentable (localized) error message if not. EOClassDescription's implementation of this method returns nil.

validateValue:forKey:

- (NSException *)validateValue:(id *)valueP forKey:(NSString *)key

Overridden by subclasses to validate the value pointed to by valueP. Subclasses should return nil if the value is acceptable, or return an exception containing a user-presentable (localized) error message if not. Implementations can replace valueP with a converted value (for example, an EOAttribute might convert an NSString to an NSNumber). EOClassDescription's implementation of this method returns nil.

An enterprise object performs custom attribute specific validation with a method of the form validateKey. See the EOValidation protocol specification for more information.

Notifications

The following notifications are declared by EOClassDescription and posted by enterprise objects in your application.

EOClassDescriptionNeededForClassNotification

EOCONTROL_EXTERN NSString *EOClassDescriptionNeededForClassNotification

One of the EOClassDescription-related methods that Enterprise Objects Framework adds to NSObject to extend the behavior of enterprise objects is classDescription. The first time an enterprise object receives a classDescription message (for example, when changes to the object are being saved to the database), it posts EOClassDescriptionNeededForClassNotification to notify observers that a class description is needed. The observer then locates the appropriate class description and registers it in the application. By default, EOModel objects are registered as observers for this notification and register EOClassDescriptions on demand.

Notification Object	Enterprise object class
userInfo Dictionary	None

EOClassDescriptionNeededForEntityNameNotification

EOCONTROL_EXTERN NSString *EOClassDescriptionNeededForEntityNameNotification

When classDescriptionForEntityName: is invoked for a previously unregistered entity name, this notification is broadcast with the requested entity name as the object of the notification. By default, EOModel objects are registered as observers for this notification and register EOClassDescriptions on demand.

Notification Object	Entity name (NSString)
userInfo Dictionary	None