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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::MuPDFPreviewer 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 tempfile on disk. Yields the tempfile.
def download_blob_to_tempfile(&block) #:doc:
blob.open tmpdir: tmpdir, &block
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.
def draw(*argv) #:doc:
open_tempfile do |file|
instrument :preview, key: blob.key do
capture(*argv, to: file)
end
yield file
end
end
def open_tempfile
tempfile = Tempfile.open("ActiveStorage-", tmpdir)
begin
yield tempfile
ensure
tempfile.close!
end
end
def instrument(operation, payload = {}, &block)
ActiveSupport::Notifications.instrument "#{operation}.active_storage", payload, &block
end
def capture(*argv, to:)
to.binmode
IO.popen(argv, err: File::NULL) { |out| IO.copy_stream(out, to) }
to.rewind
end
def logger #:doc:
ActiveStorage.logger
end
def tmpdir #:doc:
Dir.tmpdir
end
end
end
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