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Dokka Data Model

There a four data models that Dokka uses: Documentable Model, Documentation Model, Page Model and Content Model.

Documentable Model

Documentable model represents parsed data, returned by compiler analysis. It retains basic order structure of parsed Psi or Descriptor models.

After creation, it is a collection of trees, each with DModule as a root. After the Merge step, all trees are folded into one.

The main building block of this model is Documentable class, that is a base class for all more specific types that represents elements of parsed Kotlin and Java classes with pretty self-explanatory names: DPackage, DFunction and so on. DClasslike is a base for class-like elements, such as Classes, Enums, Interfaces and so on.

There are three non-documentable classes important for the model: DRI, SourceSetDependent and ExtraProperty.

  • DRI (Dokka Resource Identifier) is a unique value that identifies specific Documentable. All references to other documentables different than direct ownership are described using DRIs. For example, DFunction with parameter of type X has only X's DRI, not the actual reference to X's Documentable object.
  • SourceSetDependent is a map that handles multiplatform data, by connecting platform-specific data, declared with either expect or actual modifier, to a particular Source Set
  • ExtraProperty is used to store any additional information that falls outside of regular model. It is highly recommended to use extras to provide any additional information when creating custom Dokka plugins. This element is a bit more complex, so you can read more about how to use it below.

ExtraProperty class usage

ExtraProperty classes are used both by Documentable and Content models. To declare a new extra, you need to implement ExtraProperty interface.

interface ExtraProperty<in C : Any> {
    interface Key<in C : Any, T : Any> {
        fun mergeStrategyFor(left: T, right: T): MergeStrategy<C> = MergeStrategy.Fail {
            throw NotImplementedError("Property merging for $this is not implemented")

    val key: Key<C, *>

It is advised to use following pattern when declaring new extras:

data class CustomExtra( [any values relevant to your extra ] ): ExtraProperty<Documentable> {
    companion object : CustomExtra.Key<Documentable, CustomExtra>
    override val key: CustomExtra.Key<Documentable, *> = CustomExtra
Merge strategy for extras is invoked only if merged objects have different values for same Extra. If you don't expect it to happen, you can omit implementing mergeStrategyFor function.

All extras for ContentNode and Documentable classes are stored in PropertyContainer<C : Any> class instances. The C generic class parameter limits the type of properties, that can be stored in the container - it must match generic C class parameter from ExtraProperty interface. For example, if you would create DFunction-only ExtraProperty, it will be limited to be added only to PropertyContainer<DFunction>.

In following example we will create Documentable-only property, store it in the container and then retrieve its value:

data class CustomExtra(val customExtraValue: String) : ExtraProperty<Documentable> {

    companion object: ExtraProperty.Key<Documentable, CustomExtra>

    override val key: ExtraProperty.Key<Documentable, *> = CustomExtra

val extra : PropertyContainer<DFunction> = PropertyContainer.withAll(
    CustomExtra("our value")

val customExtraValue : String? = extra[CustomProperty]?.customExtraValue

You can also use extras as markers, without storing any data in them:

object MarkerExtra : ExtraProperty<Any>, ExtraProperty.Key<Any, MarkerExtra> {
    override val key: ExtraProperty.Key<Any, *> = this

val extra : PropertyContainer<Any> = PropertyContainer.withAll(MarkerExtra)

val isMarked : Boolean = extra[MarkerExtra] != null

Documentation Model

Documentation model is used along Documentable Model to store data obtained by parsing code commentaries.

There are three important classes here:

  • DocTag describes a specific documentation syntax element, for example: header, footer, list, link, raw text, paragraph, etc.
  • TagWrapper described a whole comment description or a specific comment tag, for example: @See, @Returns, @Author; and holds consisting DocTag elements
  • DocumentationNode acts as a container for TagWrappers for a specific Documentable

DocumentationNodes are references by a specific Documentable

Page Model

Page Model represents the structure of future generated documentation pages and is independent of the final output format, which each node corresponding to exactly one output file. Renderer is processing each page separately.Subclasses of PageNode represents different kinds of rendered pages for Modules, Packages, Classes etc.

The Page Model is a tree structure, with RootPageNode being the root.

Content Model

Content Model describes how the actual page content is presented. It organizes it's structure into groups, tables, links, etc. Each node is identified by unique DCI (Dokka Content Identifier) and all references to other nodes different than direct ownership are described using DCIs.

DCI aggregates DRIs of all Documentables that make up specific ContentNode.

Also, all ExtraProperty info from consisting Documentables is propagated into Content Model and available for Renderer.