1 Properties

1.8 Exporting Inheritable Properties

The property package provides most of the code required to make inheritance work, and so all that is required to implement an inheritable property is to provide some simple template code within its exporting class. We will illustrate this with an example.

Suppose we have a property called “width” that is to be made inheritable. Then addition to it’s value variable and set/get accessors, the host class should provide a PropertyMode variable along with set/get accessors:

⬇
   int myWidth;
   PropertyMode myWidthMode = PropertyMode.Inherited;
   public PropertyMode getWidthMode() {
      return myWidthMode;
   }
   public void setWidthMode (PropertyMode mode) {
      myWidthMode = PropertyUtils.setModeAndUpdate (
         this, "width", myWidthMode, mode);
   }

The call to PropertyUtils.setModeAndUpdate() inside the set method ensures that inherited values within the hierarchy are properly whenever the mode is changed. If the mode is set to PropertyMode.Explicit, then the property’s value needs to be propagated to any descendent nodes for which it is inherited. If the mode is set to PropertyMode.Inherited, then the property’s value needs to be obtained from the ancestor nodes, and then also propagated to any descendent nodes for which it is inherited.

As mentioned in the previous section, explicitly setting a property’s value using the set accessor should cause it’s property mode to be set to Explicit and the new value to be propagated to hierarchy descendents. This can be accomplished by using PropertyUtils.propagateValue within the set accessor:

⬇
   public void setWidth (int w) {
      myWidth = w;
      myWidthMode = PropertyUtils.propagateValue (
         this, "width", myWidth, myWidthMode);
   }

The actual creation of an inherited property can be done using the PropertyList methods

⬇
   void addInheritable (
      String nameAndMethods, String description,
      Object defaultValue)
   void addInheritable (
      String nameAndMethods, String description,
      Object defaultValue, String options)

instead of the add or addReadOnly methods. The nameAndMethods argument may now specify up to five method names, corresponding, in order, to the get/set accessors for the property value, the getRange accessor, and the get/set accessors for the property’s mode. If any of these are omitted or specified as ’*’, then the system searches for names of the form getXxx, setXxx, getXxxRange, getXxxNode, and setXxxMode, where xxx is the property name.

Finally, the host objects which actually correspond to hierarchy nodes must implement the HierarchyNode interface as described in the previous section, and any routine which adds a node to the hierarchy must also implement the following code fragment:

⬇
   public void addChild (HierarchyNode node) {
      ... add node to the hierarchy ...
      PropertyUtils.updateInheritedProperties (node);
   }

This ensures that when a node is added, all property values within and beneath it are made consistent with the inheritance hierarchy.