Class: ActiveStorage::Preview

Inherits:
Object
  • Object
show all
Includes:
Blob::Servable
Defined in:
app/models/active_storage/preview.rb

Overview

Active Storage Preview

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 is used for videos whereas ActiveStorage::Previewer::PopplerPDFPreviewer and ActiveStorage::Previewer::MuPDFPreviewer are used for PDFs. 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::PopplerPDFPreviewer, ActiveStorage::Previewer::MuPDFPreviewer, ActiveStorage::Previewer::VideoPreviewer ]

# Add a custom previewer for Microsoft Office documents:
Rails.application.config.active_storage.previewers << DOCXPreviewer
# => [ ActiveStorage::Previewer::PopplerPDFPreviewer, ActiveStorage::Previewer::MuPDFPreviewer, ActiveStorage::Previewer::VideoPreviewer, DOCXPreviewer ]

Outside of a Rails application, modify ActiveStorage.previewers instead.

The built-in previewers rely on third-party system libraries. Specifically, the built-in video previewer requires FFmpeg. Two PDF previewers are provided: one requires Poppler, and the other requires muPDF (version 1.8 or newer). To preview PDFs, install either Poppler or muPDF.

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.

Defined Under Namespace

Classes: UnprocessedError

Instance Attribute Summary collapse

Instance Method Summary collapse

Methods included from Blob::Servable

#content_type_for_serving, #forced_disposition_for_serving

Constructor Details

#initialize(blob, variation_or_variation_key) ⇒ Preview

Returns a new instance of Preview.



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# File 'app/models/active_storage/preview.rb', line 42

def initialize(blob, variation_or_variation_key)
  @blob, @variation = blob, ActiveStorage::Variation.wrap(variation_or_variation_key)
end

Instance Attribute Details

#blob ⇒ Object (readonly)

Returns the value of attribute blob.



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# File 'app/models/active_storage/preview.rb', line 40

def blob
  @blob
end

#variation ⇒ Object (readonly)

Returns the value of attribute variation.



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# File 'app/models/active_storage/preview.rb', line 40

def variation
  @variation
end

Instance Method Details

#download(&block) ⇒ Object

Downloads the file associated with this preview’s variant. If no block is given, the entire file is read into memory and returned. That’ll use a lot of RAM for very large files. If a block is given, then the download is streamed and yielded in chunks. Raises ActiveStorage::Preview::UnprocessedError if the preview has not been processed yet.



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# File 'app/models/active_storage/preview.rb', line 90

def download(&block)
  if processed?
    presentation.download(&block)
  else
    raise UnprocessedError
  end
end

#image ⇒ Object

Returns the blob’s attached preview image.



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# File 'app/models/active_storage/preview.rb', line 59

def image
  blob.preview_image
end

#key ⇒ Object

Returns a combination key of the blob and the variation that together identifies a specific variant.



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# File 'app/models/active_storage/preview.rb', line 77

def key
  if processed?
    presentation.key
  else
    raise UnprocessedError
  end
end

#processed ⇒ Object

Processes the preview if it has not been processed yet. Returns the receiving ActiveStorage::Preview instance for convenience:

blob.preview(resize_to_limit: [100, 100]).processed.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.



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# File 'app/models/active_storage/preview.rb', line 52

def processed
  process unless processed?
  variant.processed if variant?
  self
end

#url(**options) ⇒ Object

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 URL returned by this method.



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# File 'app/models/active_storage/preview.rb', line 68

def url(**options)
  if processed?
    presentation.url(**options)
  else
    raise UnprocessedError
  end
end