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"}}
]