Mac practice: Quicklook icons – how does the macOS file preview work? | News

Recognizing a file you are looking for at a glance – this saves a lot of time and makes using your own computer seem a little more effortless. Since the introduction of Mac OS This is shown as an icon in different Finder displays. In this way, you can visually track down not only images, but also layouts and texts – usually without having read the file name. To make this work quickly and smoothly, Apple has developed a sophisticated system, which Howard Oakley explains in a blog post.

The Finder offers four different display formats, which you can switch between via the “View” menu, the selection in the Finder toolbar or via keyboard shortcuts (+1 to +4) change. Each has its advantages and a differently detailed preview icon: It is the smallest in the list and column view, the largest in the gallery, and can be adjusted in the icon display – using the slider in the lower right corner if the status bar is displayed.

The Finder offers four different views, each with specific icon sizes.

Fast display
In order for detailed icons to be effective, they must appear as quickly as possible. For this purpose, macOS uses an algorithm: First, the system checks whether a current mini-preview already exists in the thumbnail cache; if so, this will be applied. If not, it continues depending on the type: For apps, macOS looks for the appropriate resource in the app bundle. You will find it there either in the ICNS file or in Assets.car under /Contents/Ressources/. The extended attributes are used for folders. macOS breaks down all other files and documents by type.

File preview via UTI
macOS uses the Uniform Type Identifier (UTI). This hierarchical type determination also decides which program opens a file type – and accordingly how the file thumbnail is created: Apps that include a Quicklook extension are assigned an icon display. This creates a current preview that reflects the actual content. If there is no suitable extension for a UTI, a generic icon is used instead. In any case, the result is also written to the cache in order to speed up the procedure the next time the file is displayed.

Expandable with apps
In order to get an up-to-date display of symbols and an overview for frequently used file types, you need an app equipped with the corresponding Quicklook extension. Many apps provide one for their own file formats; For others, such as Markdown or programming languages, there are universalists like the (unfortunately no longer developed) Peek. Other apps like Suspicious Package and Apparency use the Quicklook system to make software-specific information visible.