path: root/docs/modules/extend/pages/use-cases.adoc

= Extended Converter Use Cases
:navtitle: Use Cases

In xref:create-converter.adoc[], we only touched on what you can do with an extended converter.
This page presents more realistic use cases that can be accomplished by extending and customizing the converter.
Each section introduces a different use case and presents the code for an extended converter you can use as a starting point.

TIP: An extended converter can access predefined or custom theme keys via the `theme` accessor.
The segments in a key are always separated by an underscore character (e.g., `theme.title_page_font_color`).
Consulting the value of theme keys allows the extra behavior provided by the extended converter to be styled using the theme.

The source code for each use case on this page can be found in the {url-project-repo}/tree/{page-origin-refname}/docs/modules/extend/examples[docs/modules/extend/examples] directory in the Asciidoctor PDF repository.
To use one of them, you can either download the script to your local disk or clone the Asciidoctor PDF repository.
Once you have done so, refer to xref:use-converter.adoc[] to learn how to register and use it with Asciidoctor PDF.
For example, to register and use the extended converter that allows you to theme admonitions by type, you'd use this command:

 $ asciidoctor-pdf -r pdf-converter-admonition-theme-per-type.rb doc.adoc

Refer to the sections below for details about how to use each extended converter.

== Custom thematic break

One of the simplest ways to extend the converter is to make a thematic break.
For this case, we'll override the convert handler method for a thematic break, which is `convert_thematic_break`.
The thematic break only consists of line graphics, no text.
That means we can make use of graphics fill and stroke methods provided by Asciidoctor PDF or Prawn.

.Extended converter with custom thematic break
[,ruby]
----
include::example$pdf-converter-custom-thematic-break.rb[tags=**]
----

The return value of the convert handler method for a block node is ignored, which is why there's no clear return value in this override.
If this were a convert handler method for an inline node, a return value would be required, which becomes the text to render.

== Custom title page

Every title page is as unique as the work itself.
That's why Asciidoctor PDF gives you the ability to customize the title page by overriding the `ink_title_page` method in an extended converter.
The `ink_title_page` method is called after the title page has been created and the background applied, so it can focus on writing content.
In this method, you can choose to honor the `title-page` settings from the theme, or go your own way.
The one rule is that the `ink_title_page` method *must not* start a new page.
If it tries to start a new page, that request will be ignored and a warning will be generated.

Let's create a custom title page that shows the document title and subtitle between two lines in the top half and a logo in the bottom half.

.Extended converter with custom title page
[,ruby]
----
include::example$pdf-converter-custom-title-page.rb[]
----

The methods `move_cursor_to` and `move_cursor` advance the cursor on the page where the next content will be written.
The method `theme_font` applies the font from the specified category in the theme (with hyphens in the category name replaced by underscores).
The method `stroke_horizontal_rule` draws a horizontal line using the specified color and line width.
The method `ink_prose` is provided by Asciidoctor PDF to make writing text to the page easier.
Finally, the method `convert` will convert and render the Asciidoctor node that is passed to it, in this case a block image.

== Custom part title

A common need is to add extra styling to the title page for a part in a multi-part book.
Since this is a specialized section element, there's a dedicated method named `ink_part_title` that you can override.
The converter already allocates a dedicated page for the part title (so there's no need to worry about doing that).
The extended converter can override the method that inks the part title to add extra decoration or content to that page.

Let's customize the part title page by making the background orange, making the font white, aligning the title to the right, adding a line below it, and switching off the running content.

.Extended converter with custom part title
[,ruby]
----
include::example$pdf-converter-custom-part-title.rb[]
----

The method `ink_prose` is provided by Asciidoctor PDF to make writing text to the page easier.
If you wanted, you could just use the low-level `text` method provided by Prawn.

TIP: It's also possible to override the `start_new_part` method if all you want to do is called `page.imported` to turn off the running content.

Now let's look at how to center the part title both vertically and horizontally.
For this, we first need to compute the height of the title using the `height_of_typeset_text` helper, taking into account the vertical padding as well.
Then, we use that height to position the cursor so that the title falls in the vertical center of the page.
Next, we set the text alignment to center (which alternately could be done using the theme).
Finally, we delegate to the super method to handle rendering the title in the new position.

.Extended converter with centered part title
[,ruby]
----
include::example$pdf-converter-centered-part-title.rb[]
----

To find all the methods available to an extended converter, consult the {url-api-docs}[API docs^].

== Custom chapter title

A similar need is to add extra styling to the title of a chapter, or to place it on a page by itself.
The extended converter can override the method that inks the chapter title to add extra decoration or content to that page, then insert a page break afterwards.

.Extended converter with custom chapter title
[,ruby]
----
include::example$pdf-converter-custom-chapter-title.rb[]
----

TIP: It's also possible to override the `start_new_chapter` method if all you want to do is called `page.imported` to turn off the running content.

== Chapter image

As another way to customize the chapter title, you may want to add an image above the chapter title if specified.
Once again, the extended converter can override the method that inks the chapter title and use it as an opportunity to insert an image.

.Extended converter with chapter image
[,ruby]
----
include::example$pdf-converter-chapter-image.rb[]
----

The path to the image is controlled using the `image` block attribute on the chapter.

[,asciidoc]
----
[image=gears.png]
== Chapter Title
----

== Per chapter TOC

In addition to (or instead of) a TOC for the whole book, you may want to insert a TOC per chapter immediately following the chapter title.
Inserting a TOC into the PDF is a two-step process.
First, you need to allocate the space for the chapter TOC using the `allocate_toc` method.
Then, you need to come back and ink the TOC after the chapter has been rendered using the `ink_toc` method.

.Extended converter with TOC per chapter
[,ruby]
----
include::example$pdf-converter-chapter-toc.rb[]
----

The chapter TOC can is activated by setting the `chapter-toc` attribute and the depth of the TOC is controlled using the `chapter-toclevels` attribute.
For example:

[,asciidoc]
----
= Book Title
:chapter-toc:
:chapter-toclevels: 2
----

== License page

Let's say you want to insert a license page into your documents, but you don't want to have to put a block macro for it in the document source.
You can use an extended converter to add new pages to the body of the document.

Let's consider the case of reading the license text from a file and inserting it into the first page of the body.

.Extended converter with license page
[,ruby]
----
include::example$pdf-converter-license-page.rb[]
----

The method `start_new_page` will create a new page in the document.
The `ink_prose` method provides a `normalize` option.
When this option is false, it will preserve the newlines in the content, which is what we want in the case of license text.
You may want to take this a bit further and allow the location of the license file to be configurable.

== Paragraph numbering

To help with content auditing or correlation, you may want to add a number in front of each paragraph.
You can do this first by assigning a number to each paragraph in the document in the `init_pdf` method.
Then, you can add this number in the left margin at the start of each paragraph by overriding the `convert_paragraph` method.

.Extended converter with paragraph numbering
[,ruby]
----
include::example$pdf-converter-numbered-paragraphs.rb[]
----

== Change bars

If you have a preprocessor that adds change metadata to the content, you can use an extended converter to draw change bars to add a visual indicator in the rendered output.

.Extended converter with change bars
[,ruby]
----
include::example$pdf-converter-change-bars.rb[]
----

This converter will look for paragraphs like this one:

[,asciidoc]
----
[.changed]
This line has been changed.
----

== Avoid break after heading

This functionality is already provided by the converter if you set the `breakable` option on section title or discrete heading or you set the `heading-min-height-after` theme key to `auto`.
The code is presented here both to explain how it works and show how to do it programmatically (perhaps to tune it for specific headings).

If an in-flow heading is followed by content that doesn't fit on the current page, and the `breakable` option is not set on the heading, the converter will orphan the heading on the current page.
You can fix this behavior by overriding the `arrange_heading` method in an extended converter.

This extended converter takes this opportunity to use `dry_run` to make an attempt to write content in the remaining space on the page after the heading.
If no content is written, it advances to the next page before inking the heading (and its corresponding anchor).

.Extended converter that avoids a page break after a heading
[,ruby]
----
include::example$pdf-converter-avoid-break-after-heading.rb[]
----
<.> An optional optimization to skip this logic if the cursor is above the bottom third of the page.
<.> Initiate a dry run up to the end of the current page.
<.> Render the heading as normal.
<.> Proceed with converting content until the end of the page is reached. Returns true if content is written, false otherwise.
<.> Start new page before rendering heading if orphaned.

== Additional TOC entries

By default, the table of contents (TOC) only includes section references.
If you want to include additional entries in the TOC, or to filter the sections that are included, you can extend the converter and override the `get_entries_for_toc` method.
This method is invoked for each parent entry in the TOC, starting from the document.

.Extended converter that adds additional entries to the TOC
[,ruby]
----
include::example$pdf-converter-additional-toc-entries.rb[]
----

The depth of the TOC is automatically controlled by the `toclevels` attributes.
Once this limit is reached, the converter will not call `get_entries_for_toc` for that parent (as none of its children will be included in the TOC).

[#breakable-tables]
== Breakable tables

As explained on xref:ROOT:breakable-and-unbreakable.adoc[], tables are not configured with orphan prevention of the anchor and title by default.
In order to activate this behavior, the `breakable` option must be specified on the table.

To avoid having to add this option on every table, you can use an Asciidoctor extension to add it at runtime.
This use case employs a tree processor rather than an extended PDF converter, though its behavior does impact conversion.

.Extension that adds the breakable option to all tables
[,ruby]
----
include::example$breakable-tables-tree-processor.rb[]
----

This same technique can be used to add the `breakable` or `unbreakable` option at runtime to any blocks of your choosing.

== Narrow TOC

Let's say you want to make the content on the TOC page(s) really narrow.
You can do so by overriding the `ink_toc` method and squeezing the margins by applying extra indentation.

.Extended converter with narrow TOC
[,ruby]
----
include::example$pdf-converter-narrow-toc.rb[]
----

== Indent block image

If you want all (or some) block images to be indented by an amount specified in the theme, you can override the convert handler method for block images, `convert_image`, and call super within an indented context.

.Extended converter that indents block images
[,ruby]
----
include::example$pdf-converter-image-indent.rb[]
----

The `indent` DSL method adds padding to either side of the content area, delegates to the specified code block, then shaves it back off.

This converter works when a custom theme defines the `image-indent` key, as follows:

[,yaml]
----
extends: default
image:
  indent: [0.5in, 0]
----

== Look for images in multiple dirs

By default, an AsciiDoc converter only supports resolving images from a single location, the value of the `imagesdir` attribute.
You can use an extended converter to have Asciidoctor PDF look in multiple locations until it finds the image.

.Extended converter that resolve images from multiple locations
[,ruby]
----
include::example$pdf-converter-multiple-imagesdirs.rb[]
----

If you need the converter to support more than two locations, update the list of attribute names in the extended converter.

== Language label on code block

The built-in HTML converter inserts a source language label in the upper right corner of the code block, which appears on hover.
You can use an extended converter to imprint a fixed label in the PDF output.

To add this label, you'll need to override the `arrange_block` method of the converter.
This method arranges content blocks that have a border and/or background or support unbreakable, such as code blocks.
The override needs to filter the arguments for a node that has the `source` style and `language` attribute.
If the method detects that combination, it must decorate the callback passed via the `&block` argument to inject the extra logic.
Otherwise, the method should delegate directly to `super`.

When a code block is detected, the decorator should first call the block argument using `instance_exec`.
Then, it should look to see if the extent is set and that this is not a dry run.
The extent provides information about where the background and border of the code block started.
The extended converter should move to that page and cursor, reapply the code block padding, and ink the label using the code font settings.

.Extended converter that imprints a source langauge label on code blocks
[,ruby]
----
include::example$pdf-converter-source-language-label.rb[]
----

The way this extended converter is written, the label is inked on top of the inked code block.
You're free to customize where the label is placed.
The float method allows you to move the cursor around in absolute space without impacting the flow of the content.
The extent gives you the information about the location of the code block.

== Code block with wrap indicator

Since PDF is a fixed layout format, code blocks can not scroll left to right.
As a result, lines may wrap.
If you want to indicate to a reader when a line has wrapped, you can use an extended converter to add a wrap indicator at the end of these lines.

WARNING: This is a very low-level converter that relies on internal APIs in both Asciidoctor PDF and Prawn.
It's possible that it can break when either library is upgraded.

To add the wrap indicator, you need to intercept the `add_fragment_to_line` method on the line wrapper and register a callback on the last consumed fragment when it doesn't fit on the line.
That callback will then add a wrap indicator to the end of the line after the line is rendered.

.Extended converter that adds a wrap indicator to wrapped lines in a code block
[,ruby]
----
include::example$pdf-converter-code-with-wrap-indicator.rb[]
----

This converter uses the return key character as the line wrap indicator.
When using the built-in themes, the glyph for this character is only available in the fallback font.
Therefore, as written, this converter requires you to use the fallback font (i.e., `--theme default-with-font-fallbacks`) unless your theme uses a custom font that supports this character.

This extended converter does not attempt to change how lines are wrapped, and thus cannot make room for itself.
Rather, it adds the wrap indicator after the wrapped line has been rendered.
That means it has to use the remaining space available.
To do so, it overlays a wrap indicator character so it sits halfway into the gutter to the right of the printable area of the code block.

[#wrap-code-blocks-around-image]
== Wrap code blocks around an image float

Asciidoctor PDF provides basic support for image floats.
It will wrap paragraph text on the opposing side of the float.
However, if it encounters a non-paragraph, the converter will clear the float and continue positioning content below the image.

As a companion to this basics support, the converter provides a framework for broadening support for float wrapping.
We can take advantage of this framework in an extended converter.
By extending the converter and overriding the `supports_float_wrapping?` as well as the convert handler for the block you want to enlist (e.g., `convert_code`), you can arrange additional content into the empty space adjacent to the floated image.
In the following example, code (listing and literal) blocks are included in the float wrapping.

.Extended converter that additionally wraps code blocks around an image float
[,ruby]
----
include::example$pdf-converter-code-float-wrapping.rb[]
----

You can configure the gap next to and below the image using the `image-float-gap` key in the theme.

[,yaml]
----
extends: default
image:
  float-gap: [12, 6]
----

== Theme table using roles

The converter only supports custom roles on paragraphs and phrases.
You can use an extended converter to add this capability to tables.

.Extended converter that supports a custom role on a table
[,ruby]
----
include::example$pdf-converter-table-role.rb[]
----

This extended converter allows you to specify any theme key on the custom role that's supported for tables.
The role must be defined under a special role name `<table>` (to avoid clashing with other role names).
Here's an example of a custom table role named `heavy` that increases the width of the table border and grid lines and increases the font size.

[,yaml]
----
extends: default
role:
  <table>:
    heavy:
      border-width: 1.5
      grid-width: 1.5
      font-size: 12.5
----

You apply this role to a table by prepending `.heavy` to the first positional attribute in the block attribute line above the table.

[,asciidoc]
----
[.heavy,cols=2*]
|===
|big
|data
|===
----

As written, the extended converter only supports the first role on the table.
It could be enhanced to support an arbitrary number of roles, with each successive role cascading (like CSS).

TIP: You can use the technique shown in this extended converter to add role-based theming to any other block type recognized by the theme (e.g., code, sidebar, etc).

== Theme admonition per type

Similarly to the custom table role, we can use an extended converter to add support for theme keys per admonition type.

.Extended converter that supports theme keys per admonition type
[,ruby]
----
include::example$pdf-converter-admonition-theme-per-type.rb[]
----

This converter temporarily promotes keys under the `admonition-<type>` theme category to the `admonition` theme category, overriding any existing keys.
The placeholder `<type>` represents the admonition type; caution, important, note, tip, or warning.

Here's an example that shows how you'd use the theme to apply a border to the important admonition type when using this extended converter:

[,yml]
----
admonition:
  important:
    border-color: #BF0000
    border-width: 1
    column-rule-width: 0
    padding: 12
----

== Multiple columns

Starting with Asciidoctor PDF 2.1, this converter provides built-in support for multiple columns.
This feature is available when the doctype is article or manpage, but not book.
The columns get applied to the body of the document, which excludes the document title and TOC, if present.

The Asciidoctor PDF converter also provides the framework for making multi-column layouts in an extended converter.
This framework is accessible via the helper method `column_box`.
To make a multi-column layout, you put statements that ink content inside a code block and pass it to the `column_box` method as follows:

[,ruby]
----
column_box [bounds.left, cursor], columns: 2, width: bounds.width, reflow_margins: true do
  ink_prose 'left column'
  bounds.move_past_bottom
  ink_prose 'right column'
end
----

If you want a multi-column layout for a specific chapter or section, you can override the `traverse` method, look for the section you want to arrange, and wrap the call to `super` in a `column_box` enclosure.

== Access page number from inline macro

Although not an extended converter, this use case uses information from the converter in much the same way.
In this case, we're interested in retrieving the page number and inserting it into the content.

Let's create an inline macro named `pagenum` that inserts the current page number into the document when the macro is converted.

.inline-pagenum-macro.rb
[,ruby]
----
include::example$inline-pagenum-macro.rb[]
----

Here's how this macro would be used.

[,asciidoc]
----
= Document Title
:doctype: book

You're looking at page number pagenum:[].
----

We can build on this extension to show the start page of the current section by adding support for a scope parameter.
We can also have it show the page number label instead of the physical page number by subtracting the start page number (which is stored on the index catalog).

.advanced-inline-pagenum-macro.rb
[,ruby]
----
include::example$advanced-inline-pagenum-macro.rb[]
----

The macro can now be used to show the page number label for the current section:

[,asciidoc]
----
= Document Title
:doctype: book

== Chapter A

You're reading a section that begins on page pagenum:[section].
----

Taking inspiration from this extension, we develop another inline macro named `pageref` that resolves the page number of the closest parent section of a reference.

.inline-pageref-macro.rb
[,rb]
----
include::example$inline-pageref-macro.rb[]
----

The only caveat of this extension is that it has to use a two-phase conversion.
In other words, it has to convert the document a second time to resolve any forward references.
That's because the page number of a section is not known until it is rendered.
And not all sections are rendered until the first conversion is complete.

Here's how the `pageref` macro would be used:

[,asciidoc]
----
= Document Title
:doctype: book

== Chapter A

Content.

== Chapter B

Refer to <<_chapter_a>> on page pageref:_chapter_a[].
----

== Resources

To find even more examples of how to override the behavior of the converter, refer to the extended converter in the {url-infoq-template}[InfoQ Mini-Book template^].