in Diffusion-macOS/ContentView.swift [22:34]
func showSavePanel() -> URL? {
let savePanel = NSSavePanel()
savePanel.allowedContentTypes = [.png]
savePanel.canCreateDirectories = true
savePanel.isExtensionHidden = false
savePanel.title = "Save your image"
savePanel.message = "Choose a folder and a name to store the image."
savePanel.nameFieldLabel = "File name:"
savePanel.nameFieldStringValue = filename
let response = savePanel.runModal()
return response == .OK ? savePanel.url : nil
}