1 files changed, 69 insertions, 0 deletions
diff --git a/activestorage/lib/active_storage/previewer.rb b/activestorage/lib/active_storage/previewer.rb
new file mode 100644
index 0000000000..c91f64ac65
--- /dev/null
+++ b/activestorage/lib/active_storage/previewer.rb
@@ -0,0 +1,69 @@
+# 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