Implementing Properties

- An alternative properties implementation - - The following discussion focusses on the relationship between - Flow Objects in the Flow Object tree, and properties. There - is no (or only passing) discussion of the relationship between - properties and traits, and by extension, between properties - and the Area tree. - -

- Property handling is complex and expensive. Varying numbers of - properties apply to individual Flow Objects - (FOs) in the FO tree but - any property may effectively be assigned a value on any - element of the tree. If that property is inheritable, its - defined value will then be available to any children of the - defining FO. -

- - (XSL 1.0 Rec) 5.1.4 Inheritance - ...The inheritable properties can be placed on any formatting - object. - -

- Even if the value is not inheritable, it may be accessed by - its children through the inherit keyword or the - from-parent() core function, and potentially by - any of its descendents through the - from-nearest-specified-value() core function. -

- In addition to the assigned values of properties, almost every - property has an initial value which is used - when no value has been assigned. -

- The history problem -

- The difficulty and expense of handling properties comes from - this univeral inheritance possibility. The list of properties - which are assigned values on any particular FO - element will not generally be large, but a current value is - required for each property which applies to the FO - being processed. -

- The environment from which these values may be selected - includes, for each FO, for each applicable - property, the value assigned on this FO, - the value which applied to the parent of this FO, - the nearest value specified on an ancestor of this element, - and the initial value of the property. -

- The construction hierarchy -

- Properties are resoved in the FO tree in a - strictly hierarchical manner. Nodes are detected in the - input in a pre-order traversal, and are - built in the same order. This imples that there are two - phases, or states, of property resolution and construction. - Any particular FO node is either in a state of constructing - its own subtree, or in a stable state where the subtree - construction is complete. These states have differenct data - requirements. -

Subtree building: - In this state, all properties defined on this node, or any - of its ancestors must be available to the subtree. In - effect, any property defined on this node must be - available to its descendants, as all properties defined on - any ancestor are available to this node. -
Stable: subtree building complete: - In this state, only the properties applicable to - this node need be available. -

- Representing properties: <property> classes -

- Class vs instance -

- What information is required of property objects? - More particularly, what information is particular to the - property classes, and what to the instantiated - objects? The answer to this question depend largely on - how the property objects are used in the context - of layout and Area tree construction. The approach taken - in this implementation is that properties are simply flags - on certain data values associated with FOs. The semantics - of these flags are determined within the layout engine. -

- Certain constant information attaches to individual - property classes. This information is detailed in - the descriptions of individual properties in Section - 7 of the specification. Such information is - represented in class fields and data - structures within the classes. -

- The "instance" information content of a property - is: -

- explicitly, the PropertyValue datum of - the property, and -
- implicitly, the Flow Object to which - the property is attached. -

- Properties, then, serve essentially to link FO - instances with PropertyValue instances, - attaching certain invariant semantic markers to the - PropertyValues in the process. In this implementation, - these functions can be realised entirely within the - property classes themselves, - without the need to instantiate any objects. In practice, - property singletons are - instantiated to make access to some invariants simpler. -

- Next: property classes overview. -