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
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.
# 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:
blob.preview(resize_to_limit: [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
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