95 lines
2.7 KiB
Ruby
95 lines
2.7 KiB
Ruby
# frozen_string_literal: true
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module ActiveStorage
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# This is an abstract base class for previewers, which generate images from blobs. See
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# ActiveStorage::Previewer::MuPDFPreviewer and ActiveStorage::Previewer::VideoPreviewer for
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# examples of concrete subclasses.
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class Previewer
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attr_reader :blob
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# Implement this method in a concrete subclass. Have it return true when given a blob from which
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# the previewer can generate an image.
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def self.accept?(blob)
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false
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end
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def initialize(blob)
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@blob = blob
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end
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# Override this method in a concrete subclass. Have it yield an attachable preview image (i.e.
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# anything accepted by ActiveStorage::Attached::One#attach). Pass the additional options to
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# the underlying blob that is created.
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def preview(**options)
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raise NotImplementedError
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end
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private
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# Downloads the blob to a tempfile on disk. Yields the tempfile.
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def download_blob_to_tempfile(&block) # :doc:
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blob.open tmpdir: tmpdir, &block
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end
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# Executes a system command, capturing its binary output in a tempfile. Yields the tempfile.
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#
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# Use this method to shell out to a system library (e.g. muPDF or FFmpeg) for preview image
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# generation. The resulting tempfile can be used as the +:io+ value in an attachable Hash:
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#
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# def preview
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# download_blob_to_tempfile do |input|
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# draw "my-drawing-command", input.path, "--format", "png", "-" do |output|
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# yield io: output, filename: "#{blob.filename.base}.png", content_type: "image/png"
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# end
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# end
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# end
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#
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# The output tempfile is opened in the directory returned by #tmpdir.
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def draw(*argv) # :doc:
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open_tempfile do |file|
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instrument :preview, key: blob.key do
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capture(*argv, to: file)
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end
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yield file
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end
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end
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def open_tempfile
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tempfile = Tempfile.open("ActiveStorage-", tmpdir)
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begin
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yield tempfile
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ensure
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tempfile.close!
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end
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end
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def instrument(operation, payload = {}, &block)
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ActiveSupport::Notifications.instrument "#{operation}.active_storage", payload, &block
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end
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def capture(*argv, to:)
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to.binmode
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open_tempfile do |err|
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IO.popen(argv, err: err) { |out| IO.copy_stream(out, to) }
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err.rewind
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unless $?.success?
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raise PreviewError, "#{argv.first} failed (status #{$?.exitstatus}): #{err.read.to_s.chomp}"
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end
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end
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to.rewind
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end
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def logger # :doc:
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ActiveStorage.logger
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end
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def tmpdir # :doc:
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Dir.tmpdir
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end
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end
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end
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