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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 and ActiveStorage::Previewer::PDFPreviewer. 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::PDFPreviewer, ActiveStorage::Previewer::VideoPreviewer ]

# Add a custom previewer for Microsoft Office documents:
Rails.application.config.active_storage.previewers << DOCXPreviewer
# => [ ActiveStorage::Previewer::PDFPreviewer, 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
I
N
P
S
Attributes
[R] blob
[R] variation
Class Public methods
new(blob, variation_or_variation_key)
# File activestorage/app/models/active_storage/preview.rb, line 35
def initialize(blob, variation_or_variation_key)
  @blob, @variation = blob, ActiveStorage::Variation.wrap(variation_or_variation_key)
end
Instance Public methods
image()

Returns the blob's attached preview image.

# File activestorage/app/models/active_storage/preview.rb, line 51
def image
  blob.preview_image
end
processed()

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

blob.preview(resize_to_fit: [100, 100]).processed.service_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.

# File activestorage/app/models/active_storage/preview.rb, line 45
def processed
  process unless processed?
  self
end
service_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 short-lived URL returned by this method.

# File activestorage/app/models/active_storage/preview.rb, line 60
def service_url(**options)
  if processed?
    variant.service_url(options)
  else
    raise UnprocessedError
  end
end