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diff --git a/activestorage/app/models/active_storage/preview.rb b/activestorage/app/models/active_storage/preview.rb
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+# frozen_string_literal: true
+
+# Some non-image blobs can be previewed: that is, they can be presented as images. A video blob can be previewed by
+# extracting its first frame, and a PDF blob can be previewed by extracting its first page.
+#
+# A previewer extracts a preview image from a blob. Active Storage provides previewers for videos and PDFs:
+# ActiveStorage::Previewer::VideoPreviewer and ActiveStorage::Previewer::PDFPreviewer. Build custom previewers by
+# subclassing ActiveStorage::Previewer and implementing the requisite methods. Consult the ActiveStorage::Previewer
+# documentation for more details on what's required of previewers.
+#
+# To choose the previewer for a blob, Active Storage calls +accept?+ on each registered previewer in order. It uses the
+# first previewer for which +accept?+ returns true when given the blob. In a Rails application, add or remove previewers
+# by manipulating +Rails.application.config.active_storage.previewers+ in an initializer:
+#
+#   Rails.application.config.active_storage.previewers
+#   # => [ ActiveStorage::Previewer::PDFPreviewer, ActiveStorage::Previewer::VideoPreviewer ]
+#
+#   # Add a custom previewer for Microsoft Office documents:
+#   Rails.application.config.active_storage.previewers << DOCXPreviewer
+#   # => [ ActiveStorage::Previewer::PDFPreviewer, ActiveStorage::Previewer::VideoPreviewer, DOCXPreviewer ]
+#
+# Outside of a Rails application, modify +ActiveStorage.previewers+ instead.
+#
+# The built-in previewers rely on third-party system libraries:
+#
+# * {ffmpeg}[https://www.ffmpeg.org]
+# * {mupdf}[https://mupdf.com]
+#
+# These libraries are not provided by Rails. You must install them yourself to use the built-in previewers. Before you
+# install and use third-party software, make sure you understand the licensing implications of doing so.
+class ActiveStorage::Preview
+  class UnprocessedError < StandardError; end
+
+  attr_reader :blob, :variation
+
+  def initialize(blob, variation_or_variation_key)
+    @blob, @variation = blob, ActiveStorage::Variation.wrap(variation_or_variation_key)
+  end
+
+  # Processes the preview if it has not been processed yet. Returns the receiving Preview instance for convenience:
+  #
+  #   blob.preview(resize: "100x100").processed.service_url
+  #
+  # Processing a preview generates an image from its blob and attaches the preview image to the blob. Because the preview
+  # image is stored with the blob, it is only generated once.
+  def processed
+    process unless processed?
+    self
+  end
+
+  # Returns the blob's attached preview image.
+  def image
+    blob.preview_image
+  end
+
+  # Returns the URL of the preview's variant on the service. Raises ActiveStorage::Preview::UnprocessedError if the
+  # preview has not been processed yet.
+  #
+  # This method synchronously processes a variant of the preview image, so do not call it in views. Instead, generate
+  # a stable URL that redirects to the short-lived URL returned by this method.
+  def service_url(**options)
+    if processed?
+      variant.service_url(options)
+    else
+      raise UnprocessedError
+    end
+  end
+
+  private
+    def processed?
+      image.attached?
+    end
+
+    def process
+      previewer.preview { |attachable| image.attach(attachable) }
+    end
+
+    def variant
+      ActiveStorage::Variant.new(image, variation).processed
+    end
+
+
+    def previewer
+      previewer_class.new(blob)
+    end
+
+    def previewer_class
+      ActiveStorage.previewers.detect { |klass| klass.accept?(blob) }
+    end
+end