class ActiveStorage::Previewer

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

Attributes

blob[R]

Public Class Methods

accept?(blob) click to toggle source

Implement this method in a concrete subclass. Have it return true when given a blob from which the previewer can generate an image.

# File lib/active_storage/previewer.rb, line 12
def self.accept?(blob)
  false
end
new(blob) click to toggle source
# File lib/active_storage/previewer.rb, line 16
def initialize(blob)
  @blob = blob
end

Public Instance Methods

preview(**options) click to toggle source

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

# File lib/active_storage/previewer.rb, line 23
def preview(**options)
  raise NotImplementedError
end

Private Instance Methods

capture(*argv, to:) click to toggle source
# File lib/active_storage/previewer.rb, line 71
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
download_blob_to_tempfile(&block) click to toggle source

Downloads the blob to a tempfile on disk. Yields the tempfile.

# File lib/active_storage/previewer.rb, line 29
def download_blob_to_tempfile(&block) # :doc:
  blob.open tmpdir: tmpdir, &block
end
draw(*argv) { |file| ... } click to toggle source

Executes a system command, capturing its binary output in a tempfile. Yields the tempfile.

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

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

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

# File lib/active_storage/previewer.rb, line 47
def draw(*argv) # :doc:
  open_tempfile do |file|
    instrument :preview, key: blob.key do
      capture(*argv, to: file)
    end

    yield file
  end
end
instrument(operation, payload = {}, &block) click to toggle source
# File lib/active_storage/previewer.rb, line 67
def instrument(operation, payload = {}, &block)
  ActiveSupport::Notifications.instrument "#{operation}.active_storage", payload, &block
end
logger() click to toggle source
# File lib/active_storage/previewer.rb, line 86
def logger # :doc:
  ActiveStorage.logger
end
open_tempfile() { |tempfile| ... } click to toggle source
# File lib/active_storage/previewer.rb, line 57
def open_tempfile
  tempfile = Tempfile.open("ActiveStorage-", tmpdir)

  begin
    yield tempfile
  ensure
    tempfile.close!
  end
end
tmpdir() click to toggle source
# File lib/active_storage/previewer.rb, line 90
def tmpdir # :doc:
  Dir.tmpdir
end