class ActiveStorage::Previewer

examples of concrete subclasses.
ActiveStorage::Previewer::MuPDFPreviewer and ActiveStorage::Previewer::VideoPreviewer for
This is an abstract base class for previewers, which generate images from blobs. See

def self.accept?(blob)

the previewer can generate an image.
Implement this method in a concrete subclass. Have it return true when given a blob from which 
def self.accept?(blob)
  false
end

def capture(*argv, to:) 

def capture(*argv, to:)
  to.binmode
  open_tempfile do |err|
    IO.popen(argv, err: err) { |out| IO.copy_stream(out, to) }
    err.rewind
    unless $?.success?
      raise PreviewError, "#{argv.first} failed (status #{$?.exitstatus}): #{err.read.to_s.chomp}"
    end
  end
  to.rewind
end

def download_blob_to_tempfile(&block) # :doc:

:doc:
Downloads the blob to a tempfile on disk. Yields the tempfile. 
def download_blob_to_tempfile(&block) # :doc:
  blob.open tmpdir: tmpdir, &block
end

def draw(*argv) # :doc:

:doc:
The output tempfile is opened in the directory returned by #tmpdir.

end
end
end
yield io: output, filename: "#{blob.filename.base}.png", content_type: "image/png"
draw "my-drawing-command", input.path, "--format", "png", "-" do |output|
download_blob_to_tempfile do |input|
def preview

generation. The resulting tempfile can be used as the +:io+ value in an attachable Hash:
Use this method to shell out to a system library (e.g. muPDF or FFmpeg) for preview image

Executes a system command, capturing its binary output in a tempfile. Yields the tempfile. 
def draw(*argv) # :doc:
  open_tempfile do |file|
    instrument :preview, key: blob.key do
      capture(*argv, to: file)
    end
    yield file
  end
end

def initialize(blob) 

def initialize(blob)
  @blob = blob
end

def instrument(operation, payload = {}, &block) 

def instrument(operation, payload = {}, &block)
  ActiveSupport::Notifications.instrument "#{operation}.active_storage", payload, &block
end

def logger # :doc:

:doc: 
def logger # :doc:
  ActiveStorage.logger
end

def open_tempfile 

def open_tempfile
  tempfile = Tempfile.open("ActiveStorage-", tmpdir)
  begin
    yield tempfile
  ensure
    tempfile.close!
  end
end

def preview(**options)

the underlying blob that is created.
anything accepted by ActiveStorage::Attached::One#attach). Pass the additional options to
Override this method in a concrete subclass. Have it yield an attachable preview image (i.e. 
def preview(**options)
  raise NotImplementedError
end

def tmpdir # :doc:

:doc: 
def tmpdir # :doc:
  Dir.tmpdir
end