Add the stdio backend

This patch adds the stdio backend which acts as a fallback backend and
uses standard input and output.  For password queries, the rpassword
crate is used to suppress output.  Also, default_backend is changed to
return Stdio if Dialog is not available.

7 files changed, 121 insertions, 2 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index b11008c..ff1b4a1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,7 @@
 # Unreleased
 - Refactor `default_backend` to return a `Box<dyn Backend>`.
 - Check the `DIALOG` and `DISPLAY` environment variables in `default_backend`.
+- Add the `Stdio` backend.
 
 # v0.1.1 (2019-01-11)
 - Add the `Password` dialog box.
diff --git a/Cargo.toml b/Cargo.toml
index 1a6c816..a4087a8 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -12,3 +12,4 @@ readme = "README.md"
 license = "MIT"
 
 [dependencies]
+rpassword = "2"
diff --git a/examples/backend-stdio.rs b/examples/backend-stdio.rs
new file mode 100644
index 0000000..6dbeb8d
--- /dev/null
+++ b/examples/backend-stdio.rs
@@ -0,0 +1,13 @@
+// Copyright (C) 2019 Robin Krahl <robin.krahl@ireas.org>
+// SPDX-License-Identifier: MIT
+
+use dialog::backends;
+use dialog::DialogBox;
+
+fn main() -> dialog::Result<()> {
+    let backend = backends::Stdio::new();
+
+    dialog::Message::new("This is a message.")
+        .title("And this is a title:")
+        .show_with(&backend)
+}
diff --git a/src/backends/dialog.rs b/src/backends/dialog.rs
index 6f078e6..e681caf 100644
--- a/src/backends/dialog.rs
+++ b/src/backends/dialog.rs
@@ -49,6 +49,10 @@ impl Dialog {
         self.width = width.to_string();
     }
 
+    pub(crate) fn is_available() -> bool {
+        super::is_available("dialog")
+    }
+
     fn execute(
         &self,
         args: Vec<&str>,
diff --git a/src/backends/mod.rs b/src/backends/mod.rs
index f5af7ef..5ae3cef 100644
--- a/src/backends/mod.rs
+++ b/src/backends/mod.rs
@@ -2,9 +2,11 @@
 // SPDX-License-Identifier: MIT
 
 mod dialog;
+mod stdio;
 mod zenity;
 
 pub use crate::backends::dialog::Dialog;
+pub use crate::backends::stdio::Stdio;
 pub use crate::backends::zenity::Zenity;
 
 use std::env;
@@ -49,6 +51,7 @@ pub(crate) fn is_available(name: &str) -> bool {
 pub(crate) fn from_str(s: &str) -> Option<Box<dyn Backend>> {
     match s.to_lowercase().as_ref() {
         "dialog" => Some(Box::new(Dialog::new())),
+        "stdio" => Some(Box::new(Stdio::new())),
         "zenity" => Some(Box::new(Zenity::new())),
         _ => None,
     }
diff --git a/src/backends/stdio.rs b/src/backends/stdio.rs
new file mode 100644
index 0000000..6838ef1
--- /dev/null
+++ b/src/backends/stdio.rs
@@ -0,0 +1,89 @@
+// Copyright (C) 2019 Robin Krahl <robin.krahl@ireas.org>
+// SPDX-License-Identifier: MIT
+
+use std::io::{self, Write};
+
+use crate::{Choice, Input, Message, Password, Question, Result};
+
+/// The fallback backend using standard input and output.
+///
+/// This backend is intended as a fallback backend to use if no other backend is available.  The
+/// dialogs are printed to the standard output and user input is read from the standard input.
+#[derive(Debug)]
+pub struct Stdio {}
+
+impl Stdio {
+    /// Creates a new `Stdio` instance.
+    pub fn new() -> Stdio {
+        Stdio {}
+    }
+}
+
+impl AsRef<Stdio> for Stdio {
+    fn as_ref(&self) -> &Self {
+        self
+    }
+}
+
+fn print_title(title: &Option<String>) {
+    if let Some(ref title) = title {
+        println!("{}", title);
+        println!("{}", "=".repeat(title.len()));
+    }
+}
+
+fn read_input() -> Result<String> {
+    let mut input = String::new();
+    io::stdin().read_line(&mut input)?;
+    Ok(input.trim_end_matches("\n").to_string())
+}
+
+fn parse_choice(input: &str) -> Choice {
+    match input.to_lowercase().as_ref() {
+        "y" => Choice::Yes,
+        "yes" => Choice::Yes,
+        "n" => Choice::No,
+        "no" => Choice::No,
+        _ => Choice::Cancel,
+    }
+}
+
+impl super::Backend for Stdio {
+    fn show_input(&self, input: &Input) -> Result<Option<String>> {
+        print_title(&input.title);
+        if let Some(ref default) = input.default {
+            print!("{} [default: {}]: ", input.text, default);
+        } else {
+            print!("{}: ", input.text);
+        }
+        io::stdout().flush()?;
+
+        let user_input = read_input()?;
+        if user_input.is_empty() {
+            if let Some(ref default) = input.default {
+                return Ok(Some(default.to_string()));
+            }
+        }
+        Ok(Some(user_input))
+    }
+
+    fn show_message(&self, message: &Message) -> Result<()> {
+        print_title(&message.title);
+        println!("{}", message.text);
+        Ok(())
+    }
+
+    fn show_password(&self, password: &Password) -> Result<Option<String>> {
+        print_title(&password.title);
+        print!("{}: ", password.text);
+        io::stdout().flush()?;
+        Ok(Some(rpassword::read_password()?))
+    }
+
+    fn show_question(&self, question: &Question) -> Result<Choice> {
+        print_title(&question.title);
+        print!("{} [y/n]: ", question.text);
+        io::stdout().flush()?;
+        Ok(parse_choice(&read_input()?))
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index 80cabb5..da89e2f 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -15,6 +15,8 @@
 //! These dialog boxes can be displayed using various backends:
 //! - [`Dialog`][]: uses `dialog` to display ncurses-based dialog boxes (requires the external
 //!   `dialog` tool)
+//! - [`Stdio`][]: prints messages to the standard output and reads user input form standard input
+//!   (intended as a fallback backend)
 //! - [`Zenity`][]: uses `zenity` to display GTK-based dialog boxes (requires the external `zenity`
 //!   tool)
 //!
@@ -344,9 +346,11 @@ impl DialogBox for Question {
 /// - If the `DISPLAY` environment variable is set, the first available backend from this list is
 ///   used:
 ///   - [`Zenity`][]
-/// - Otherwise, a [`Dialog`][] instance is returned.
+/// - If the [`Dialog`][] backend is available, it is used.
+/// - Otherwise, a [`Stdio`][] instance is returned.
 ///
 /// [`Dialog`]: backends/struct.Dialog.html
+/// [`Stdio`]: backends/struct.Stdio.html
 /// [`Zenity`]: backends/struct.Zenity.html
 pub fn default_backend() -> Box<dyn backends::Backend> {
     if let Ok(backend) = env::var("DIALOG") {
@@ -363,5 +367,9 @@ pub fn default_backend() -> Box<dyn backends::Backend> {
         }
     }
 
-    Box::new(backends::Dialog::new())
+    if backends::Dialog::is_available() {
+        Box::new(backends::Dialog::new())
+    } else {
+        Box::new(backends::Stdio::new())
+    }
 }