# `Expresso.Overlay.Properties`
[🔗](https://github.com/rellen/expresso/blob/main/lib/expresso/overlay/properties.ex#L1)

The check of the custom properties of one slide

`slide/1` does the work of `Expresso.Overlay.PropertyVerifier`, and it is a
pure function on an `Expresso.Slide` struct. It reads the `state` option and
the `set` option of each `on` entity of the slide, and it gives one warning
for each property that `Expresso.Theme` reports as a defect.

`registered_state/2` does a part of the work of `Expresso.Overlay.Verifier`.
It refuses a state that takes the name of a property that the theme
registers.

`docs/overlays.md` gives the rules.

# `warning`

```elixir
@type warning() :: {String.t(), :erl_anno.anno()} | String.t()
```

A warning of the verifier, with the location of the `on` entity

# `registered_state`

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

Give an error for a state that takes the name of a property that the theme registers

The compiler registers each state as a number with the initial value 0, and
that registration replaces the registration of the theme. Each element of
the deck that reads the property then gets 0 in place of the initial value
of the theme, on each slide. The state `scale` is an example: each element
gets the size 0 × 0. The default registrations are those of the theme.

# `slide`

```elixir
@spec slide(Expresso.Slide.t(), Expresso.Css.names()) :: [warning()]
```

Give a warning for each custom property of a slide that is a defect

The names come from the theme and from the CSS of the deck, which
`Expresso.Css.merge/2` joins. The default is the theme alone. The function
reports these conditions:

- The `state` option, or a key of the `set` option, names a property that
  the theme does not use. The renderer writes the property, and no rule of
  the theme reads it. Therefore the value has no effect.
- The `state` option names a property that the theme declares, or that the
  theme registers with a different syntax than a number. The compiler
  registers each state as a number with the initial value 0. That
  registration makes the value of the theme invalid, and the theme then
  loses the property.
- A key of the `set` option names a registered property, and the value does
  not agree with the syntax of that registration. The browser then ignores
  the value, and the property keeps its initial value. A `<length>` needs a
  unit of length, an `<angle>` needs a unit of angle, and a `<number>` needs
  a plain number. A value with a CSS function, such as `var()` or `calc()`,
  and a CSS-wide keyword, such as `inherit`, get no warning, because the
  browser calculates them.

Each message holds the path of the slide. The function adds the location of
the `on` entity when Spark has it. A module of a test has no debug
information, and Spark then gives `nil`.

---

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