On Windows we have to create a COM server in a DLL to provide
thumbnails. The DLL must provide an
IThumbnailProvider
implementation. Our implementation is in
desktop::ThumbnailHandler class, the most
interesting member function is
ThumbnailHandler::GetThumbnail(),
which should return a HBITMAP of the thumbnail.
If you distribute your app in an installer remember to use
regsvr32.exe to register your DLL which will call your DLL's
DllRegisterServer(), a function that must
create registry keys to associate your file type extension with
your thumbnail handler.
More information in the MSDN.
On macOS we have to create a QuickLook plugin/extension to display
thumbnails and previews. The plugin is a .qlgenerator bundle which
can be installed in the ~/Library/QuickLook,
/System/Library/QuickLook, or /Library/QuickLook directories, or
included the in the same app bundle.
macOS pre-10.15,
has a COM-like functionality to load QuickLook plugins: the QuickLook
daemon will use the information in our bundle
Info.plist to know which function to load and call
from our library to create the plugin object. (The function generally
is called QuickLookGeneratorPluginFactory but can has any name, the
name is specified in a child element of CFPlugInFactories inside the
Info.plist file.) Then the created object is used abstractly as a
IUnknown, and the QLGeneratorInterface interface is queried to
generate then thumbnails and previews.
We can test our .qlgenerator without installing it using the
qlmanage utility.
We target to macOS 10.9, but we should migrate to the new macOS 10.15 API in a near future (or provide both).