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 [code]resource[/code] being edited for preview. Once the preview is ready, the [code]receiver[/code]'s [code]receiver_func[/code] will be called. The [code]receiver_func[/code] must take the following four arguments: [String] path, [Texture] preview, [Texture] thumbnail_preview, [Variant] userdata. [code]userdata[/code] can be anything, and will be returned when [code]receiver_func[/code] is called. [b]Note:[/b] If it was not possible to create the preview the [code]receiver_func[/code] will still be called, but the preview will be null. Queue a resource file located at [code]path[/code] for preview. Once the preview is ready, the [code]receiver[/code]'s [code]receiver_func[/code] will be called. The [code]receiver_func[/code] must take the following four arguments: [String] path, [Texture] preview, [Texture] thumbnail_preview, [Variant] userdata. [code]userdata[/code] can be anything, and will be returned when [code]receiver_func[/code] is called. [b]Note:[/b] If it was not possible to create the preview the [code]receiver_func[/code] will still be called, but the preview will be null. Removes a custom preview generator. Emitted if a preview was invalidated (changed). [code]path[/code] corresponds to the path of the preview.