Function QLThumbnailDispatchAsync 

pub unsafe extern "C-unwind" fn QLThumbnailDispatchAsync(
    thumbnail: &QLThumbnail,
    queue: Option<&DispatchQueue>,
    completion: dispatch_block_t,
)

👎Deprecated: renamed to QLThumbnail::dispatch_async

Available on crate features QLThumbnail and dispatch2 only.