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.

Namespace

Class

Methods

Included Modules

Attributes

[R] blob

Class Public methods

accept?(blob)

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

📝 Source code 
# File activestorage/lib/active_storage/previewer.rb, line 16
    def self.accept?(blob)
      false
    end
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new(blob)

📝 Source code 
# File activestorage/lib/active_storage/previewer.rb, line 20
    def initialize(blob)
      @blob = blob
    end
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Instance Public methods

preview()

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

📝 Source code 
# File activestorage/lib/active_storage/previewer.rb, line 26
    def preview
      raise NotImplementedError
    end
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Instance Private methods

draw(*argv)

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 ActiveStorage::Downloading#tempdir.

📝 Source code 
# File activestorage/lib/active_storage/previewer.rb, line 45
      def draw(*argv) #:doc:
        ActiveSupport::Notifications.instrument("preview.active_storage") do
          open_tempfile_for_drawing do |file|
            capture(*argv, to: file)
            yield file
          end
        end
      end
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logger()

📝 Source code 
# File activestorage/lib/active_storage/previewer.rb, line 70
      def logger #:doc:
        ActiveStorage.logger
      end
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