# `Expresso.Element.Diagram.Cache`
[🔗](https://github.com/rellen/expresso/blob/main/lib/expresso/element/diagram/cache.ex#L1)

Keeps the SVG of each graph and the MathML of each formula from one render
of the watch mode to the next

D2 with ELK takes some seconds for a graph, and the watch mode renders the
whole deck after each change. Without a cache, a change to the text of one
slide would run each tool of the deck again.

`Expresso.Watch` calls `run/2` around each render, with the cache of the
render before. `fetch/2` returns the value of a key from that cache, or it
calls its function. `run/2` returns the entries that the render used, so the
cache holds only the graphs of the deck as it is now. A render outside
`run/2`, such as a render from the command line, runs each tool one time
and keeps nothing.

The cache is in the process dictionary of the render, as the record of
`Expresso.DeckFile.track/1` is. A render runs in one process.

# `t`

```elixir
@type t() :: %{required(binary()) => term()}
```

The entries of a cache, by key

# `fetch`

```elixir
@spec fetch(term(), (-> value)) :: value when value: var
```

Return the value of a key, or call the function and keep its value

Outside `run/2`, the function calls `function` each time. The key is a term,
and the function keeps a hash of it.

# `fetch_many`

```elixir
@spec fetch_many([term()], ([term()] -> [value])) :: [value] when value: var
```

Return the value of each key, and call the function one time for the keys
that the cache does not hold

`function` takes the list of the missing keys, in the order of `keys`, and
returns one value for each. A tool that makes many values in one run, such
as Temml for the formulas of a deck, then runs one time. Outside `run/2`,
the function calls `function` with each key.

# `run`

```elixir
@spec run(t(), (-> result)) :: {result, t()} when result: var
```

Run a render with the entries of the render before

The function returns the result of the render and the entries that the
render used.

---

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