Active Storage 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.

Namespace

Class

Methods

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 14
    def self.accept?(blob)
      false
    end
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new(blob)

📝 Source code 
# File activestorage/lib/active_storage/previewer.rb, line 18
    def initialize(blob)
      @blob = blob
    end
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Instance Public methods

preview(**options)

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.

📝 Source code 
# File activestorage/lib/active_storage/previewer.rb, line 25
    def preview(**options)
      raise NotImplementedError
    end
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Instance Private methods

download_blob_to_tempfile(&block)

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

📝 Source code 
# File activestorage/lib/active_storage/previewer.rb, line 31
      def download_blob_to_tempfile(&block) # :doc:
        blob.open tmpdir: tmpdir, &block
      end
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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 tmpdir.

📝 Source code 
# File activestorage/lib/active_storage/previewer.rb, line 49
      def draw(*argv) # :doc:
        open_tempfile do |file|
          instrument :preview, key: blob.key do
            capture(*argv, to: file)
          end

          yield file
        end
      end
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logger()

📝 Source code 
# File activestorage/lib/active_storage/previewer.rb, line 93
      def logger # :doc:
        ActiveStorage.logger
      end
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tmpdir()

📝 Source code 
# File activestorage/lib/active_storage/previewer.rb, line 97
      def tmpdir # :doc:
        Dir.tmpdir
      end
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