class ActiveStorage::Previewer
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.
Attributes
Public Class Methods
Implement this method in a concrete subclass. Have it return true when given a blob from which the previewer can generate an image.
# File lib/active_storage/previewer.rb, line 12 def self.accept?(blob) false end
# File lib/active_storage/previewer.rb, line 16 def initialize(blob) @blob = blob end
Public Instance Methods
Override this method in a concrete subclass. Have it yield an attachable preview image (i.e. anything accepted by ActiveStorage::Attached::One#attach
). Pass the additional options to the underlying blob that is created.
# File lib/active_storage/previewer.rb, line 23 def preview(**options) raise NotImplementedError end
Private Instance Methods
# File lib/active_storage/previewer.rb, line 71 def capture(*argv, to:) to.binmode open_tempfile do |err| IO.popen(argv, err: err) { |out| IO.copy_stream(out, to) } err.rewind unless $?.success? raise PreviewError, "#{argv.first} failed (status #{$?.exitstatus}): #{err.read.to_s.chomp}" end end to.rewind end
Downloads the blob to a tempfile on disk. Yields the tempfile.
# File lib/active_storage/previewer.rb, line 29 def download_blob_to_tempfile(&block) #:doc: blob.open tmpdir: tmpdir, &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 tmpdir
.
# File lib/active_storage/previewer.rb, line 47 def draw(*argv) #:doc: open_tempfile do |file| instrument :preview, key: blob.key do capture(*argv, to: file) end yield file end end
# File lib/active_storage/previewer.rb, line 67 def instrument(operation, payload = {}, &block) ActiveSupport::Notifications.instrument "#{operation}.active_storage", payload, &block end
# File lib/active_storage/previewer.rb, line 86 def logger #:doc: ActiveStorage.logger end
# File lib/active_storage/previewer.rb, line 57 def open_tempfile tempfile = Tempfile.open("ActiveStorage-", tmpdir) begin yield tempfile ensure tempfile.close! end end
# File lib/active_storage/previewer.rb, line 90 def tmpdir #:doc: Dir.tmpdir end