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-rw-r--r--activestorage/lib/active_storage/previewer.rb62
1 files changed, 62 insertions, 0 deletions
diff --git a/activestorage/lib/active_storage/previewer.rb b/activestorage/lib/active_storage/previewer.rb
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+++ b/activestorage/lib/active_storage/previewer.rb
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+# frozen_string_literal: true
+
+require "active_storage/downloading"
+
+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
+ include Downloading
+
+ 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
+ # 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 ActiveStorage::Downloading#tempdir.
+ def draw(*argv) #:doc:
+ Tempfile.open("ActiveStorage", tempdir) do |file|
+ capture(*argv, to: file)
+ yield file
+ end
+ 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
+ end
+end