1 files changed, 84 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..95a041fd16
--- /dev/null
+++ b/activestorage/lib/active_storage/previewer.rb
@@ -0,0 +1,84 @@
+# 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 tempdir: tempdir, &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 #tempdir.
+      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-", tempdir)
+
+        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 tempdir #:doc:
+        Dir.tmpdir
+      end
+  end
+end