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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
Methods
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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.

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

# File activestorage/lib/active_storage/previewer.rb, line 22
def preview
  raise NotImplementedError
end
Instance Private methods
download_blob_to_tempfile(&block)

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

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

# 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
logger()
# File activestorage/lib/active_storage/previewer.rb, line 76
def logger #:doc:
  ActiveStorage.logger
end
tempdir()
# File activestorage/lib/active_storage/previewer.rb, line 80
def tempdir #:doc:
  Dir.tmpdir
end