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

Gives each element that the auto morph uses a signature of its kind and its
content

The auto morph pairs an element of one slide with an element of the next
slide when the two have the same kind and the same content.
`Expresso.Renderer` calls `sign/1` before the render, and
`Expresso.Overlay.Render.attributes/1` writes the signature of an element
as `data-morph-signature`. The script in `assets/src/morph.ts` compares the
signatures, so it does not read the content from the page.

`Expresso.Element.kinds/0` gives the content of each kind:

  * `:text` - the text of the rendered element, with one space for each
    run of white space. The text of a `<style>` in an inline icon is not
    part of it, because the render gives each copy its own style rules.
  * `:source` - the `src` option of an image or of an embed.
  * `:file` - the file of a video or of a sound.
  * `:drawing` - the SVG of the element before the render gives it the
    ids of its copy, or the SVG of the rendered element.
  * `:svg` - the SVG of the file of a diagram, before the render gives it
    the ids of its copy. Two files with the same SVG have the same content.
  * `:shape` - the kind of a shape and its text.
  * `:language` - the language of a code element.
  * `:label` - the label of a stat.

A kind without a content gets no signature. An element with an empty
`:text`, `:drawing` or `:svg` content, such as a text area without a text
or a diagram with an empty `svg` element, gets no signature, because two
such elements are not the same element. A code without a language and a
stat without a label still get a signature. An element with the `morph` option gets no signature either,
because its key gives its partner.

# `sign`

```elixir
@spec sign(Expresso.Deck.t()) :: Expresso.Deck.t()
```

Write the signature of each element of a deck that the auto morph uses

# `signature`

```elixir
@spec signature(String.t(), String.t()) :: String.t()
```

Return the signature of a kind of element and its content

The slide template uses this function for the heading of a slide, with the
kind `"heading"` and the text of the heading.

    iex> Expresso.Morph.signature("heading", "Coffee") ==
    ...>   Expresso.Morph.signature("heading", "Coffee")
    true
    iex> Expresso.Morph.signature("heading", "Coffee") ==
    ...>   Expresso.Morph.signature("stat", "Coffee")
    false

---

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