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

Class

Methods

Attributes

[R] blob
[R] variation

Class Public methods

new(blob, variation_or_variation_key)

📝 Source code 
# 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
🔎 See on GitHub

Instance Public methods

image()

Returns the blob's attached preview image.

📝 Source code 
# File activestorage/app/models/active_storage/preview.rb, line 51
  def image
    blob.preview_image
  end
🔎 See on GitHub

processed()

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

blob.preview(resize: "100x100").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.

📝 Source code 
# File activestorage/app/models/active_storage/preview.rb, line 45
  def processed
    process unless processed?
    self
  end
🔎 See on GitHub

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.

📝 Source code 
# 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
🔎 See on GitHub