# Architecture overview

Normally, you would think that a tool like `Dokka` simply parses some programming language sources and generates
`HTML` pages for whatever it sees along the way, with little to no abstractions. That would be the simplest and
shortest way to implement an API documentation engine.

However, it was clear that `Dokka` may need to generate documentation from various sources (not only `Kotlin`), that users
might request additional output formats (like `Markdown`), that users might need additional features like supporting
custom `KDoc` tags or rendering `mermaid.js` diagrams - all these things would require changing a lot of code inside
`Dokka` itself if all solutions were hardcoded.

For this reason, `Dokka` was built from the ground up to be easily extensible and customizable by adding several layers
of abstractions to the data model, and by providing pluggable extension points, giving you the ability to introduce
selective changes on a single level.

## Overview of data model

Generating API documentation begins with `Input` source files (`.kts`, `.java`, etc) and ends with some `Output` files
(`.html`/`.md` pages, etc). However, to allow for extensibility and customization, several input and output independent
abstractions have been added to the data model.

Below you can find the general pipeline of processing data gathered from sources and the explanation for each stage.

```mermaid
flowchart TD
    Input --> Documentables --> Pages --> Output
```

* `Input` - generalization of sources, by default `Kotlin`/`Java` sources, but could be virtually anything
* `Documentables` - unified data model that represents _any_ parsed sources as a tree, independent of the source
  language. Examples of a `Documentable`: class, function, package, property, etc
* `Pages` - universal model that represents output pages (e.g a function/property page) and the content it's composed of
  (lists, text, code blocks) that the users needs to see. Not to be confused with `.html` pages. Goes hand in hand
  with so-called `Content` model.
* `Output` - specific output format like `HTML`/`Markdown`/`Javadoc`/etc. This is a mapping of pages/content model to
  some human-readable and visual representation. For instance:
    * `PageNode` is mapped as 
        * `.html` file for `HTML` format
        * `.md` file for `Markdown` format
    * `ContentList` is mapped as
        * `<li>` / `<ul>` for `HTML` format
        * `1.` / `*` for `Markdown` format
    * `ContentCodeBlock` is mapped as
        * `<code>` or `<pre>` with some CSS styles in `HTML` format
        * Text wrapped in triple backticks for `Markdown` format

    
You, as a `Dokka` developer or a plugin writer, can use extension points to introduce selective changes to the
model on one particular level without touching everything else. 

For instance, if you wanted to make some annotation/function/class invisible in the final documentation, you would only
need to modify the `Documentables` model by filtering undesirable members out. If you wanted to display all overloaded
methods on the same page instead of on separate ones, you would only need to modify the `Page` model by merging multiple
pages into one, and so on.

For a deeper dive into Dokka's model with more examples and details,
see sections about [Documentables](data_model/documentables.md) and [Page/Content](data_model/page_content.md)

For an overview of existing extension points that let you transform Dokka's models, see 
[Core extension points](extension_points/core_extensions.md) and [Base extensions](extension_points/base_extensions.md).

## Overview of extension points

An extension point usually represents some pluggable interface that performs an action during one of the stages of
generating documentation. An extension is therefore an implementation of that interface which is extending the
extension point.

You can create extension points, provide your own implementations (extensions) and configure them. All of
this is possible with Dokka's plugin/extension point API.

Here's a sneak peek of the DSL:

```kotlin
class MyPlugin : DokkaPlugin() {
    // create an extension point for other developers
    val signatureProvider by extensionPoint<SignatureProvider>()

    // provide a default implementation
    val defaultSignatureProvider by extending {
        signatureProvider with KotlinSignatureProvider()
    }

    // register our own extension in base plugin and override its default
    val dokkaBasePlugin by lazy { plugin<DokkaBase>() }
    val multimoduleLocationProvider by extending {
        (dokkaBasePlugin.locationProviderFactory
                providing MultimoduleLocationProvider::Factory
                override dokkaBasePlugin.locationProvider)
    }
}

// use a registered extention, pretty much dependency injection
class MyExtension(val context: DokkaContext) {
    
    val signatureProvider: SignatureProvider = context.plugin<MyPlugin>().querySingle { signatureProvider }

    fun doSomething() {
        signatureProvider.signature(..)
    }
}

interface SignatureProvider {
    fun signature(documentable: Documentable): List<ContentNode>
}

class KotlinSignatureProvider : SignatureProvider {
    override fun signature(documentable: Documentable): List<ContentNode> = listOf()
}
```

For a deeper dive into extensions and extension points with more examples and details, see
[Introduction to Extensions](extension_points/introduction.md).

For an overview of existing extension points, see [Core extension points](extension_points/core_extensions.md) and
[Base extensions](extension_points/base_extensions.md).

## Historical context

This is a second iteration of Dokka that was built from scratch.

If you want to learn more about why Dokka has been designed this way, watch this great talk by Paweł Marks:
[New Dokka - Designed for Fearless Creativity](https://www.youtube.com/watch?v=OvFoTRhqaKg). The general principles 
and general architecture are the same, although it may be outdated in some areas, so please double-check.