class ActiveStorage::Previewer

# File activestorage/lib/active_storage/previewer.rb, line 12
def self.accept?(blob)
  false
end

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

# File activestorage/lib/active_storage/previewer.rb, line 22
def preview
  raise NotImplementedError
end

Override this method in a concrete subclass. Have it yield an attachable preview image (i.e. anything accepted by ActiveStorage::Attached::One#attach).

# File activestorage/lib/active_storage/previewer.rb, line 28
def download_blob_to_tempfile(&block) #:doc:
  blob.open tmpdir: tmpdir, &block
end

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

# File activestorage/lib/active_storage/previewer.rb, line 46
def draw(*argv) #:doc:
  open_tempfile do |file|
    instrument :preview, key: blob.key do
      capture(*argv, to: file)
    end

    yield file
  end
end

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.