Check DISPLAY environment variable in default_backend

This patch changes the default_backend to also check the DISPLAY
environment variable.  If it is set, there probably is a X server
running, so we try to use the zenity backend.  Otherwise, the dialog
backend is used.

4 files changed, 31 insertions, 1 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 695a4ad..b11008c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,6 @@
 # Unreleased
 - Refactor `default_backend` to return a `Box<dyn Backend>`.
-- Check the `DIALOG` environment variable in `default_backend`.
+- Check the `DIALOG` and `DISPLAY` environment variables 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 20cb8c1..f5af7ef 100644
--- a/src/backends/mod.rs
+++ b/src/backends/mod.rs
@@ -7,6 +7,9 @@ mod zenity;
 pub use crate::backends::dialog::Dialog;
 pub use crate::backends::zenity::Zenity;
 
+use std::env;
+use std::path;
+
 use crate::Result;
 
 /// A dialog backend.
@@ -32,6 +35,17 @@ pub trait Backend {
     fn show_question(&self, question: &super::Question) -> Result<super::Choice>;
 }
 
+pub(crate) fn is_available(name: &str) -> bool {
+    if let Ok(path) = env::var("PATH") {
+        for part in path.split(":") {
+            if path::Path::new(part).join(name).exists() {
+                return true;
+            }
+        }
+    }
+    false
+}
+
 pub(crate) fn from_str(s: &str) -> Option<Box<dyn Backend>> {
     match s.to_lowercase().as_ref() {
         "dialog" => Some(Box::new(Dialog::new())),
diff --git a/src/backends/zenity.rs b/src/backends/zenity.rs
index 4bf862b..0730c64 100644
--- a/src/backends/zenity.rs
+++ b/src/backends/zenity.rs
@@ -59,6 +59,10 @@ impl Zenity {
         self.timeout = Some(timeout.to_string());
     }
 
+    pub(crate) fn is_available() -> bool {
+        super::is_available("zenity")
+    }
+
     fn execute(&self, args: Vec<&str>, title: &Option<String>) -> Result<process::Output> {
         let mut command = process::Command::new("zenity");
 
diff --git a/src/lib.rs b/src/lib.rs
index 3a6a090..80cabb5 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -341,9 +341,13 @@ impl DialogBox for Question {
 /// - 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.
+/// - If the `DISPLAY` environment variable is set, the first available backend from this list is
+///   used:
+///   - [`Zenity`][]
 /// - Otherwise, a [`Dialog`][] instance is returned.
 ///
 /// [`Dialog`]: backends/struct.Dialog.html
+/// [`Zenity`]: backends/struct.Zenity.html
 pub fn default_backend() -> Box<dyn backends::Backend> {
     if let Ok(backend) = env::var("DIALOG") {
         if let Some(backend) = backends::from_str(&backend) {
@@ -351,5 +355,13 @@ pub fn default_backend() -> Box<dyn backends::Backend> {
         }
     }
 
+    if let Ok(display) = env::var("DISPLAY") {
+        if !display.is_empty() {
+            if backends::Zenity::is_available() {
+                return Box::new(backends::Zenity::new());
+            }
+        }
+    }
+
     Box::new(backends::Dialog::new())
 }