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Class: ActiveStorage::Previewer

Relationships & Source Files
Namespace Children
Classes:
MuPDFPreviewer, PopplerPDFPreviewer, VideoPreviewer
Extension / Inclusion / Inheritance Descendants
Subclasses:
MuPDFPreviewer, PopplerPDFPreviewer, VideoPreviewer
Super Chains via Extension / Inclusion / Inheritance
Instance Chain:
self, Downloading
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

  • #preview

    Override this method in a concrete subclass.

  • #draw(*argv) private

    Executes a system command, capturing its binary output in a tempfile.

  • #logger private

Downloading - Included

#download_blob_to

Efficiently downloads blob data into the given file.
#download_blob_to_tempfile

Opens a new tempfile in #tempdir and copies blob data into it.
#tempdir

Returns the directory in which tempfiles should be opened.

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



  

    #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 Downloading#tempdir.


  



  [ GitHub ]


  

  


# File 'activestorage/lib/active_storage/previewer.rb', line 45



def draw(*argv) #:doc:
  ActiveSupport::Notifications.instrument("preview.active_storage") do
    open_tempfile_for_drawing do |file|
      capture(*argv, to: file)
      yield file
    end
  end
end


  








  

    #logger   (private)
  

  [ GitHub ]


  

  


# File 'activestorage/lib/active_storage/previewer.rb', line 70



def logger #:doc:
  ActiveStorage.logger
end


  








  

    #preview  
  



  

    

Override this method in a concrete subclass. Have it yield an attachable preview image (i.e. anything accepted by Attached::One#attach).


  





  
Raises:


    
  
  • 
    (NotImplementedError)
  
    • 





  [ GitHub ]


  

  


# File 'activestorage/lib/active_storage/previewer.rb', line 26



def preview
  raise NotImplementedError
end