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WOAssociation

Inherits from: NSObject

Package: com.apple.yellow.webobjects

Class Description

The WOAssociation class declares the programmatic interface to objects that represent the values of WebObject attributes, as specified in a declarations file. You rarely need to create subclasses of WOAssociation, except in situations where you need to subclass WODynamicElement.

The purpose of a WOAssociation object is to provide a unified interface to values of different types. For example, consider these declarations:

TREENAME1:WOString {value = "Ash"};
TREENAME2:WOString {value = treeName};
TREENAME3:WOString {value = selectedTree.name};

At runtime, the WebObjects parser scans an HTML template and these declarations and creates three WOString dynamic element objects. In the first case, the WOString's value attribute is assigned a constant string. In the second, it's associated with the treeName variable of the component in which the dynamic element is declared. In the third, value is associated with the name attribute of the component's selectedTree variable. The search path for the value can be arbitrarily deep, depending on the needs of your application:

MAYOR:WOString {value = country.state.city.mayor.name};

To resolve a path such as this, WebObjects accesses each part in turn. First, it looks for the component's country variable. If the component responds to a country message, it sends one to determine the value; otherwise, it directly accesses the component's country instance variable to determine the value. Next, it checks the country object for a state attribute, using the same strategy of looking for an accessor method named state and then, if necessary, accessing the state variable's value directly. It continues in this way until the ultimate value is determined.

WOAssociation objects present the WebObjects framework with a unified interface to attribute values, whether their values are static or dynamic. The value attribute for TREENAME1 in the example above will never change during the course of program execution, but the other WOStrings have values that are potentially dynamic, and so will have to be determined at runtime. Since the value of any WOAssociation can be determined by sending it a valueInComponent message, objects that use WOAssociation objects don't have to be concerned with how values are resolved. The WODynamicElement class makes extensive use of this feature. See the WODynamicElement class specification for more information.

Method Types

Creation

associationWithKeyPath

associationWithValue

Obtaining association attributes

isValueConstant

isValueConstantInComponent

isValueSettable

isValueSettableInComponent

Setting and retrieving value

setValue

valueInComponent

Static Methods

associationWithKeyPath

public static WOAssociation associationWithKeyPath(String aKeyPath)

Creates and returns a WOAssociation object whose value is determined by evaluating aKeyPath. This method is used when a dynamic element's attribute is set to a variable from the component's script. For example, when the WebObjects parser sees a declaration of this sort,

TREENAME3:WOString {value = selectedTree.name};

it invokes associationWithKeyPath to create a WOAssociation whose key is "selectedTree.name". When the resulting WOAssociation is asked for its value, it searches for the value of the name attribute of in the current component's selectedTree attribute.

If aKeyPath is null, the value of the WOAssociation is also null.

See Also: associationWithValue

associationWithValue

public static WOAssociation associationWithValue(Object aValue)

Creates and returns a WOAssociation object whose value is aValue, a constant value. This method is used when a dynamic element's attribute is set to a constant. For example, when the WebObjects parser sees a declaration of this sort,

TREENAME3:WOString {value = "Time Flies!"};

it invokes this method to create a WOAssociation whose value is "Time Flies!".

See Also: associationWithKeyPath

Instance Methods

isValueConstant

public boolean isValueConstant()

Returns true if the WOAssociation's value is a constant, false otherwise.

See Also: associationWithValue, isValueSettable

isValueConstantInComponent

public boolean isValueSettableInComponent(WOComponent aComponent)

Returns false when the association is "constant." Use this for checking bindings at runtime.

isValueSettable

public boolean isValueSettable()

Returns false if the receiver's value is constant, true otherwise.

See Also: associationWithKeyPath, isValueConstant

isValueSettableInComponent

public boolean isValueSettableInComponent(WOComponent aComponent)

Returns true when the association is "constant." Use this for checking bindings at runtime.

See Also: associationWithKeyPath, isValueConstant

setValue

public void setValue( Object aValue, WOComponent aComponent)

Finds the attribute of aComponent pointed to by the left-hand-side of the receiver and sets its value to aValue. This method throws an exception if the receiver's value is not settable. For example, sending a setValue message to a WOAssociation created from this declaration,

USER:WOTextField {value = userName};

sets the current component's userName variable to the value typed into the WOTextField.

One way in which the WebObjects framework uses this method is to synchronize the values of nested components. When attributes in child and parent components are associated with one another and changes occur in one component, this method is invoked to migrate those changes to the other component. See the reusable components chapter in the WebObjects Developer's Guide for more information.

See Also: valueInComponent

valueInComponent

public Object valueInComponent(WOComponent aComponent)

Returns a value based on the receiver's association and the current component. For example, sending a value message to a WOAssociation created from this declaration,

DOWNPAYMENT:WOString {value = downpayment};

returns the value of the current component's downpayment variable.

Sending a value message to a WOAssociation created from this declaration,

DOWNPAYMENT:WOString {value = "$5000.00"};

returns the value "$5000.00" (independent of the current component).

This method raises an exception if it cannot resolve the WOAssociation's value with the current component.

See Also: setValue