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