# `Delta.EmbedHandler`
[🔗](https://github.com/slab/delta-elixir/blob/main/lib/delta/embed_handler.ex#L1)

A module implementing `EmbedHandler` behaviour is required to make `compose`, `transform` and `invert` operations possible for custom embeds.

Suppose we want to have a custom `image` embed handler, that will always prefer image urls that start with `https://`

    defmodule ImageEmbed do
      @behaviour Delta.EmbedHandler

      @impl Delta.EmbedHandler
      def name, do: "image"

      @impl Delta.EmbedHandler
      def compose(url1, url2, _keep_nil?) do
        choose_https(url1, url2)
      end

      @impl Delta.EmbedHandler
      def transform(url1, url2, _priority? \ false) do
        choose_https(url1, url2)
      end

      @impl Delta.EmbedHandler
      def invert(change, base) do
        choose_https(change, base)
      end

      defp choose_https(url1, url2) do
        case {url1, url2} do
          {"https" <> _ , _} -> url1

          {_, "https" <> _} -> url2

          {_, _} -> url1
        end
      end
    end

Now we just need to add our module to config:

    config :delta, custom_embeds: [ImageEmbed]

## Examples
    iex> base = Op.insert(%{"image" => "update me"})
    iex> a = Op.retain(%{"image" => "http://quilljs.com/assets/images/icon.png"})
    iex> b = Op.retain(%{"image" => "https://quilljs.com/assets/images/icon.png"})
    iex> Delta.transform([base, a], [base, b])
    [
      %{"insert" => %{"image" => "update me"}},
      %{"retain" => 1},
      %{"retain" => %{"image" => "https://quilljs.com/assets/images/icon.png"}}
    ]

# `embed`

```elixir
@type embed() :: map()
```

# `t`

```elixir
@type t() :: module()
```

# `compose`

```elixir
@callback compose(any(), any(), keep_nil? :: boolean()) :: embed()
```

# `diff`
*optional* 

```elixir
@callback diff(Delta.Op.insert_op(), Delta.Op.insert_op()) :: Delta.t()
```

# `invert`

```elixir
@callback invert(any(), any()) :: embed()
```

# `name`

```elixir
@callback name() :: binary()
```

# `transform`

```elixir
@callback transform(any(), any(), priority? :: boolean()) :: embed()
```

---

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