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Class: ActiveStorage::Previewer

Relationships & Source Files
Namespace Children
Classes:
MuPDFPreviewer, PopplerPDFPreviewer, VideoPreviewer
Inherits: Object
Defined in: activestorage/lib/active_storage/previewer.rb

Overview

=========================

This is an abstract base class for previewers, which generate images from blobs. See MuPDFPreviewer and VideoPreviewer for examples of concrete subclasses.

Class Method Summary

Instance Attribute Summary

Instance Method Summary

Constructor Details

.new(blob) ⇒ Previewer

[ GitHub ] 

  


# File 'activestorage/lib/active_storage/previewer.rb', line 20



def initialize(blob)
  @blob = blob
end


  







Class Method Details



  

    .accept?(blob)  ⇒ Boolean 
  



  

    
Implement this method in a concrete subclass. Have it return true when given a blob from which
the previewer can generate an image.

  





  


  [ GitHub ]


  

  


# File 'activestorage/lib/active_storage/previewer.rb', line 16



def self.accept?(blob)
  false
end


  







Instance Attribute Details



  

    #blob   (readonly)
  

  [ GitHub ]


  

  


# File 'activestorage/lib/active_storage/previewer.rb', line 12



attr_reader :blob


  







Instance Method Details



  

    #capture(*argv, to:)   (private)
  

  [ GitHub ]


  

  


# File 'activestorage/lib/active_storage/previewer.rb', line 82



def capture(*argv, to:)
  to.binmode

  open_tempfile do |err|
    IO.popen(argv, in: IO::NULL, 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


  








  

    #download_blob_to_tempfile(&block)   (private)
  



  

    
Downloads the blob to a tempfile on disk. See ActiveStorage::Blob#open for details.

  



  [ GitHub ]


  

  


# File 'activestorage/lib/active_storage/previewer.rb', line 33



def download_blob_to_tempfile(&block) # :doc:
  blob.open tmpdir: tmpdir, &block
end


  








  

    #draw(*argv)   (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 #tmpdir.

  



  [ GitHub ]


  

  


# File 'activestorage/lib/active_storage/previewer.rb', line 53



def draw(*argv) # :doc:
  open_tempfile do |file|
    instrument :preview, key: blob.key do
      capture(*argv, to: file)
    end

    yield file
  end
end


  








  

    #instrument(operation, payload = {}, &block)   (private)
  

  [ GitHub ]


  

  


# File 'activestorage/lib/active_storage/previewer.rb', line 73



def instrument(operation, payload = {}, &block)
  ActiveSupport::Notifications.instrument "#{operation}.active_storage", payload.merge(service: service_name), &block
end


  








  

    #logger   (private)
  

  [ GitHub ]


  

  


# File 'activestorage/lib/active_storage/previewer.rb', line 97



def logger # :doc:
  ActiveStorage.logger
end


  








  

    #open_tempfile   (private)
  

  [ GitHub ]


  

  


# File 'activestorage/lib/active_storage/previewer.rb', line 63



def open_tempfile
  tempfile = Tempfile.open("ActiveStorage-", tmpdir)

  begin
    yield tempfile
  ensure
    tempfile.close!
  end
end


  








  

    #preview(**options)  
  



  

    
Override this method in a concrete subclass. Have it yield an attachable preview image (i.e.
anything accepted by Attached::One#attach). Pass the additional options to
the underlying blob that is created.

  





  
Raises:


    
  
  • 
    (NotImplementedError)
  
    • 





  [ GitHub ]


  

  


# File 'activestorage/lib/active_storage/previewer.rb', line 27



def preview(**options)
  raise NotImplementedError
end


  








  

    #service_name   (private)
  

  [ GitHub ]


  

  


# File 'activestorage/lib/active_storage/previewer.rb', line 77



def service_name
  # ActiveStorage::Service::DiskService => Disk
  blob.service.class.to_s.split("::").third.remove("Service")
end


  








  

    #tmpdir   (private)
  

  [ GitHub ]


  

  


# File 'activestorage/lib/active_storage/previewer.rb', line 101



def tmpdir # :doc:
  Dir.tmpdir
end