# `Expresso.Media`
[🔗](https://github.com/rellen/expresso/blob/main/lib/expresso/media.ex#L1)

The files of the video element and of the audio element

The document holds a copy of each slide for the present view and one for
each page of the handout view. A file in each copy would make the document
many times larger. Therefore an element holds only its number in the deck,
and the renderer writes each file one time, as a data URI, into a JSON
element, such as `expresso-videos`. The presenter gives an element its
source when it plays. `assets/src/media.ts` gives the rules of the script.

An element module with the field `src` and the field `index` uses these
functions with its own struct and its own media types, such as
`%{".webm" => "video/webm"}`.

# `extension`

```elixir
@spec extension(String.t()) :: String.t()
```

Return the extension of a path in lower case, such as `".webm"`

    iex> Expresso.Media.extension("clips/Demo.WEBM")
    ".webm"

# `json`

```elixir
@spec json([String.t()]) :: String.t() | nil
```

Write a list of data URIs as JSON, or return `nil` for an empty list

The renderer writes the value into a `script` element with
`Expresso.Json.for_script/1`. A deck with no file of a kind gets `nil`, and
the document then holds no element.

# `number`

```elixir
@spec number(Expresso.Deck.t(), module(), (struct() -> term())) :: Expresso.Deck.t()
```

Give each element of a module in a deck its number

Each key gets a number in document order, from 0, and two elements with the
same key get the same number. The default key is the file, so `sources/4`
writes each file one time. `Expresso.Element.Embed` gives each embed a key
of its own with `make_ref/0`, so each embed gets its own number.

# `numbered`

```elixir
@spec numbered(Expresso.Deck.t(), module()) :: [struct()]
```

Return one element of a module for each number in a deck, in the order of
the numbers

`number/3` gives the numbers first.

# `source`

```elixir
@spec source(term(), %{required(String.t()) => String.t()}, String.t()) ::
  {:ok, String.t()} | {:error, String.t()}
```

Make sure that a value is the path of a local file with an extension of the
types

This function is the base of the custom type of the first argument of a
media element. It does not read the file, so a missing file gives an error
at render time, as for an image. A value with `:` is an address, and the
element does not take it.

# `sources`

```elixir
@spec sources(
  Expresso.Deck.t(),
  module(),
  %{required(String.t()) => String.t()},
  String.t()
) :: [
  String.t()
]
```

Return the data URI of each file of the elements of a module in a deck, in
the order of their numbers

The function reads each file through `Expresso.DeckFile`, so the watch mode
renders the deck again after a change to the file. It raises for a file
that it cannot read, with the `noun` of the element in the message, such as
`"video"`.

---

*Consult [api-reference.md](api-reference.md) for complete listing*