Helper to generate previews of resources or files. This object is used to generate previews for resources of files. [b]Note:[/b] This class shouldn't be instantiated directly. Instead, access the singleton using [method EditorInterface.get_resource_previewer]. Create an own, custom preview generator. Check if the resource changed, if so, it will be invalidated and the corresponding signal emitted. Queue the [param resource] being edited for preview. Once the preview is ready, the [param receiver]'s [param receiver_func] will be called. The [param receiver_func] must take the following four arguments: [String] path, [Texture2D] preview, [Texture2D] thumbnail_preview, [Variant] userdata. [param userdata] can be anything, and will be returned when [param receiver_func] is called. [b]Note:[/b] If it was not possible to create the preview the [param receiver_func] will still be called, but the preview will be null. Queue a resource file located at [param path] for preview. Once the preview is ready, the [param receiver]'s [param receiver_func] will be called. The [param receiver_func] must take the following four arguments: [String] path, [Texture2D] preview, [Texture2D] thumbnail_preview, [Variant] userdata. [param userdata] can be anything, and will be returned when [param receiver_func] is called. [b]Note:[/b] If it was not possible to create the preview the [param receiver_func] will still be called, but the preview will be null. Removes a custom preview generator. Emitted if a preview was invalidated (changed). [param path] corresponds to the path of the preview.