Check the DIALOG environment variable in default_backend

This patch changes the logic in default_backend to respect the DIALOG
environment variable that may contain the name of the backend to use.

3 files changed, 23 insertions, 2 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 58a4919..695a4ad 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,6 @@
 # Unreleased
-- Refactor `default_backend()` to return a `Box<dyn Backend>`.
+- Refactor `default_backend` to return a `Box<dyn Backend>`.
+- Check the `DIALOG` environment variable in `default_backend`.
 
 # v0.1.1 (2019-01-11)
 - Add the `Password` dialog box.
diff --git a/src/backends/mod.rs b/src/backends/mod.rs
index b9cecd0..20cb8c1 100644
--- a/src/backends/mod.rs
+++ b/src/backends/mod.rs
@@ -31,3 +31,11 @@ pub trait Backend {
     /// Shows the given question dialog and returns the choice.
     fn show_question(&self, question: &super::Question) -> Result<super::Choice>;
 }
+
+pub(crate) fn from_str(s: &str) -> Option<Box<dyn Backend>> {
+    match s.to_lowercase().as_ref() {
+        "dialog" => Some(Box::new(Dialog::new())),
+        "zenity" => Some(Box::new(Zenity::new())),
+        _ => None,
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index 87234bd..3a6a090 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -85,6 +85,8 @@ mod error;
 /// [`Backend`]: trait.Backend.html
 pub mod backends;
 
+use std::env;
+
 pub use crate::error::{Error, Result};
 
 /// A dialog box that can be shown using a backend.
@@ -335,9 +337,19 @@ impl DialogBox for Question {
 
 /// Creates a new instance of the default backend.
 ///
-/// The current implementation always returns a [`Dialog`][] instance.
+/// The following steps are performed to determine the default backend:
+/// - If the `DIALOG` environment variable is set to a valid backend name, this backend is used.
+///   A valid backend name is the name of a struct in the `backends` module implementing the
+///   `Backend` trait in any case.
+/// - Otherwise, a [`Dialog`][] instance is returned.
 ///
 /// [`Dialog`]: backends/struct.Dialog.html
 pub fn default_backend() -> Box<dyn backends::Backend> {
+    if let Ok(backend) = env::var("DIALOG") {
+        if let Some(backend) = backends::from_str(&backend) {
+            return backend;
+        }
+    }
+
     Box::new(backends::Dialog::new())
 }