require "active_storage/blob"

# Image blobs can have variants that are the result of a set of transformations applied to the original.
# These variants are used to create thumbnails, fixed-size avatars, or any other derivative image from the
# original.
#
# Variants rely on `MiniMagick` for the actual transformations of the file, so you must add `gem "mini_magick"`
# to your Gemfile if you wish to use variants.
#
# Note that to create a variant it's necessary to download the entire blob file from the service and load it
# into memory. The larger the image, the more memory is used. Because of this process, you also want to be
# considerate about when the variant is actually processed. You shouldn't be processing variants inline in a
# template, for example. Delay the processing to an on-demand controller, like the one provided in
# `ActiveStorage::VariantsController`.
#
# To refer to such a delayed on-demand variant, simply link to the variant through the resolved route provided
# by Active Storage like so:
#
#   <%= image_tag url_for(Current.user.avatar.variant(resize: "100x100")) %>
#
# This will create a URL for that specific blob with that specific variant, which the `ActiveStorage::VariantsController`
# can then produce on-demand.
#
# When you do want to actually produce the variant needed, call `#processed`. This will check that the variant
# has already been processed and uploaded to the service, and, if so, just return that. Otherwise it will perform
# the transformations, upload the variant to the service, and return itself again. Example:
#
#   avatar.variant(resize: "100x100").processed.service_url
#
# This will create and process a variant of the avatar blob that's constrained to a height and width of 100.
# Then it'll upload said variant to the service according to a derivative key of the blob and the transformations.
#
# A list of all possible transformations is available at https://www.imagemagick.org/script/mogrify.php. You can
# combine as many as you like freely:
#
#   avatar.variant(resize: "100x100", monochrome: true, flip: "-90")
class ActiveStorage::Variant
  attr_reader :blob, :variation
  delegate :service, to: :blob

  def initialize(blob, variation)
    @blob, @variation = blob, variation
  end

  # Returns the variant instance itself after it's been processed or an existing processing has been found on the service.
  def processed
    process unless processed?
    self
  end

  # Returns a combination key of the blob and the variation that together identifies a specific variant.
  def key
    "variants/#{blob.key}/#{variation.key}"
  end

  # Returns the URL of the variant on the service. This URL is intended to be short-lived for security and not used directly
  # with users. Instead, the `service_url` should only be exposed as a redirect from a stable, possibly authenticated URL.
  # Hiding the `service_url` behind a redirect also gives you the power to change services without updating all URLs. And
  # it allows permanent URLs that redirec to the `service_url` to be cached in the view.
  #
  # Use `url_for(variant)` (or the implied form, like `link_to variant` or `redirect_to variant`) to get the stable URL
  # for a variant that points to the `ActiveStorage::VariantsController`, which in turn will use this `#service_call` method
  # for its redirection.
  def service_url(expires_in: 5.minutes, disposition: :inline)
    service.url key, expires_in: expires_in, disposition: disposition, filename: blob.filename, content_type: blob.content_type
  end


  private
    def processed?
      service.exist?(key)
    end

    def process
      service.upload key, transform(service.download(blob.key))
    end

    def transform(io)
      require "mini_magick"
      File.open MiniMagick::Image.read(io).tap { |image| variation.transform(image) }.path
    end
end