6.5 Properties

• Property types and subtypes
• Units
• Virtual properties

The kernel objects store various data of various types and various meaning:

• geometric properties, like eccentricity of an ellipse, control point weights of a Bézier curve
• graphic properties, like stroke color, fill color, line width, invisibility
• text or TeX text source code
• any additional user-defined data

All these data can be accessed using an uniform kernel interface. Some of the properties are just regular data structure members, while the others are virtual with read and write callbacks which can cause complex recomputations.

One property is stored in a struct prop structure. Its contents are:

• string key – a unique identifier of the property in the given object. It contains the displayed name of the property.
• prop_value value – a union for storing various property values.
• u8 type specifies the data type of the property value stored.
• u8 subtype provides a more subtle differentiation of property values within the data type.
• u8 unit – an index into unit array See Units. It defines the display multiplier of the property value (used only for real number values).
• u8 flags – a combination of the following:
  • PTF_VIRTUAL means that the property is virtual. See Virtual properties.
  • PTF_READ_ONLY means that changing value of the property always fails.
  • PTF_SAVE means that the property is saved when saving the object.
  • PTF_RECYCLABLE means that the property is recycled by the GUI recycler (see Property Recycler).