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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.

Namespace
Methods
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Attributes

[R] blob
[R] variation

Class Public methods

new(blob, variation_or_variation_key)

Source: show | on GitHub 

# File activestorage/app/models/active_storage/preview.rb, line 38
def initialize(blob, variation_or_variation_key)
  @blob, @variation = blob, ActiveStorage::Variation.wrap(variation_or_variation_key)
end

Instance Public methods

download(&block)

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.

Source: show | on GitHub 

# File activestorage/app/models/active_storage/preview.rb, line 85
def download(&block)
  if processed?
    variant.download(&block)
  else
    raise UnprocessedError
  end
end

image()

Returns the blob’s attached preview image.

Source: show | on GitHub 

# File activestorage/app/models/active_storage/preview.rb, line 54
def image
  blob.preview_image
end

key()

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

Source: show | on GitHub 

# File activestorage/app/models/active_storage/preview.rb, line 72
def key
  if processed?
    variant.key
  else
    raise UnprocessedError
  end
end

processed()

Processes the preview if it has not been processed yet. Returns the receiving 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.

Source: show | on GitHub 

# File activestorage/app/models/active_storage/preview.rb, line 48
def processed
  process unless processed?
  self
end

url(**options)

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.

Source: show | on GitHub 

# File activestorage/app/models/active_storage/preview.rb, line 63
def url(**options)
  if processed?
    variant.url(**options)
  else
    raise UnprocessedError
  end
end