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
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
The built-in previewers rely on third-party system libraries:
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.
# 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
Returns the blob's attached preview image.
# File activestorage/app/models/active_storage/preview.rb, line 51 def image blob.preview_image end
Processes the preview if it has not been processed yet. Returns the receiving Preview instance for convenience:
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
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