Implement message boxes using the dialog backend

This patch adds a first dialog box type, message boxes, and a first
backend, the dialog(1) tool.  It does not yet address the problems of
output handling and backend selection.

1 files changed, 22 insertions, 0 deletions

diff --git a/src/backends/mod.rs b/src/backends/mod.rs
new file mode 100644
index 0000000..c316307
--- /dev/null
+++ b/src/backends/mod.rs
@@ -0,0 +1,22 @@
+// Copyright (C) 2019 Robin Krahl <robin.krahl@ireas.org>
+// SPDX-License-Identifier: MIT
+
+mod dialog;
+
+pub use crate::backends::dialog::Dialog;
+
+use std::io::Result;
+
+/// A dialog backend.
+///
+/// A dialog backend is a program that can be used to display dialog boxes.  Use the
+/// [`default_backend`][] function to create a new instance of the default backend, or choose a
+/// backend and create an instance manually.  To use a backend, pass it to the [`show_with`][]
+/// method of a dialog box.
+///
+/// [`default_backend`]: ../function.default_backend.html
+/// [`show_with`]: ../trait.DialogBox.html#method.show_with
+pub trait Backend {
+    /// Shows the given message dialog.
+    fn show_message(&self, message: &super::Message) -> Result<()>;
+}