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# frozen_string_literal: true
module ActiveStorage
# This is an abstract base class for previewers, which generate images from blobs. See
# ActiveStorage::Previewer::PDFPreviewer and ActiveStorage::Previewer::VideoPreviewer for examples of
# concrete subclasses.
class Previewer
attr_reader :blob
# Implement this method in a concrete subclass. Have it return true when given a blob from which
# the previewer can generate an image.
def self.accept?(blob)
false
end
def initialize(blob)
@blob = blob
end
# Override this method in a concrete subclass. Have it yield an attachable preview image (i.e.
# anything accepted by ActiveStorage::Attached::One#attach).
def preview
raise NotImplementedError
end
private
# Downloads the blob to a new tempfile. Yields the tempfile.
#
# Use this method to get a tempfile that you can provide to a drawing command.
def open # :doc:
Tempfile.open("input") do |file|
download_blob_to file
yield file
end
end
def download_blob_to(file)
file.binmode
blob.download { |chunk| file.write(chunk) }
file.rewind
end
# Executes a system command, capturing its binary output in a tempfile. Yields the tempfile.
#
# Use this method to shell out to system libraries (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
# open 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
def draw(*argv) # :doc:
Tempfile.open("output") do |file|
capture(*argv, to: file)
yield file
end
end
def capture(*argv, to:)
to.binmode
IO.popen(argv) { |out| IO.copy_stream(out, to) }
to.rewind
end
end
end
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