class ActiveStorage::Preview

install and use third-party software, make sure you understand the licensing implications of doing so.
These libraries are not provided by Rails. You must install them yourself to use the built-in previewers. Before you
and the other requires muPDF (version 1.8 or newer). To preview PDFs, install either Poppler or muPDF.
. Two PDF previewers are provided: one requires Poppler,
The built-in previewers rely on third-party system libraries. Specifically, the built-in video previewer requires
Outside of a Rails application, modify ActiveStorage.previewers instead.
# => [ ActiveStorage::Previewer::PopplerPDFPreviewer, ActiveStorage::Previewer::MuPDFPreviewer, ActiveStorage::Previewer::VideoPreviewer, DOCXPreviewer ]
Rails.application.config.active_storage.previewers << DOCXPreviewer
# Add a custom previewer for Microsoft Office documents:
# => [ ActiveStorage::Previewer::PopplerPDFPreviewer, ActiveStorage::Previewer::MuPDFPreviewer, ActiveStorage::Previewer::VideoPreviewer ]
Rails.application.config.active_storage.previewers
by manipulating Rails.application.config.active_storage.previewers in an initializer:
first previewer for which accept? returns true when given the blob. In a Rails application, add or remove previewers
To choose the previewer for a blob, Active Storage calls accept? on each registered previewer in order. It uses the
documentation for more details on what’s required of previewers.
subclassing ActiveStorage::Previewer and implementing the requisite methods. Consult the ActiveStorage::Previewer
and ActiveStorage::Previewer::MuPDFPreviewer are used for PDFs. Build custom previewers by
ActiveStorage::Previewer::VideoPreviewer is used for videos whereas ActiveStorage::Previewer::PopplerPDFPreviewer
A previewer extracts a preview image from a blob. Active Storage provides previewers for videos and PDFs.
extracting its first frame, and a PDF blob can be previewed by extracting its first page.
Some non-image blobs can be previewed: that is, they can be presented as images. A video blob can be previewed by

def download(&block)

if the preview has not been processed yet.
streamed and yielded in chunks. Raises ActiveStorage::Preview::UnprocessedError
of RAM for very large files. If a block is given, then the download is
given, the entire file is read into memory and returned. That'll use a lot
Downloads the file associated with this preview's variant. If no block is 
def download(&block)
  if processed?
    variant.download(&block)
  else
    raise UnprocessedError
  end
end

def image

Returns the blob's attached preview image. 
def image
  blob.preview_image
end

def initialize(blob, variation_or_variation_key) 

def initialize(blob, variation_or_variation_key)
  @blob, @variation = blob, ActiveStorage::Variation.wrap(variation_or_variation_key)
end

def key

Returns a combination key of the blob and the variation that together identifies a specific variant. 
def key
  if processed?
    variant.key
  else
    raise UnprocessedError
  end
end

def previewer 

def previewer
  previewer_class.new(blob)
end

def previewer_class 

def previewer_class
  ActiveStorage.previewers.detect { |klass| klass.accept?(blob) }
end

def process 

def process
  previewer.preview(service_name: blob.service_name) do |attachable|
    ActiveRecord::Base.connected_to(role: ActiveRecord.writing_role) do
      image.attach(attachable)
    end
  end
end

def processed

image is stored with the blob, it is only generated once.
Processing a preview generates an image from its blob and attaches the preview image to the blob. Because the preview

blob.preview(resize_to_limit: [100, 100]).processed.url

Processes the preview if it has not been processed yet. Returns the receiving Preview instance for convenience: 
def processed
  process unless processed?
  self
end

def processed? 

def processed?
  image.attached?
end

def url(**options)

a stable URL that redirects to the URL returned by this method.
This method synchronously processes a variant of the preview image, so do not call it in views. Instead, generate

preview has not been processed yet.
Returns the URL of the preview's variant on the service. Raises ActiveStorage::Preview::UnprocessedError if the 
def url(**options)
  if processed?
    variant.url(**options)
  else
    raise UnprocessedError
  end
end

def variant 

def variant
  image.variant(variation).processed
end