Add FileSelection dialog type

This patch adds the FileSelection struct representing a file selection
dialog.  It can be displayed using the backend’s show_file_selection
function.

Currently, we only support file open dialogs (i. e.  choosing an
existing file).  Support for save dialogs should be added in the future.

7 files changed, 128 insertions, 4 deletions

diff --git a/Cargo.toml b/Cargo.toml
index 57bc68f..355b789 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -16,3 +16,4 @@ license = "MIT"
 
 [dependencies]
 rpassword = "2"
+dirs = "2.0"
diff --git a/examples/file_selection.rs b/examples/file_selection.rs
new file mode 100644
index 0000000..5436742
--- /dev/null
+++ b/examples/file_selection.rs
@@ -0,0 +1,13 @@
+// Copyright (C) 2019 Robin Krahl <robin.krahl@ireas.org>
+// SPDX-License-Identifier: MIT
+
+use dialog::DialogBox;
+
+fn main() -> dialog::Result<()> {
+    let choice = dialog::FileSelection::new("Please select a file")
+        .title("File Chooser Example")
+        .path("/etc")
+        .show()?;
+    println!("The user chose: {:?}", choice);
+    Ok(())
+}
diff --git a/src/backends/dialog.rs b/src/backends/dialog.rs
index 8061d98..668cf57 100644
--- a/src/backends/dialog.rs
+++ b/src/backends/dialog.rs
@@ -3,7 +3,7 @@
 
 use std::process;
 
-use crate::{Choice, Error, Input, Message, Password, Question, Result};
+use crate::{Choice, Error, FileSelection, Input, Message, Password, Question, Result};
 
 /// The `dialog` backend.
 ///
@@ -160,4 +160,11 @@ impl super::Backend for Dialog {
         self.execute(args, vec![], &question.title)
             .and_then(|output| get_choice(output.status))
     }
+
+    fn show_file_selection(&self, file_selection: &FileSelection) -> Result<Option<String>> {
+        let dir = file_selection.path_to_string().ok_or("path not valid")?;
+        let args = vec!["--fselect", &dir];
+        self.execute(args, vec![], &file_selection.title)
+            .and_then(get_stderr)
+    }
 }
diff --git a/src/backends/mod.rs b/src/backends/mod.rs
index 1331323..09013b1 100644
--- a/src/backends/mod.rs
+++ b/src/backends/mod.rs
@@ -37,6 +37,9 @@ pub trait Backend {
 
     /// Shows the given question dialog and returns the choice.
     fn show_question(&self, question: &super::Question) -> Result<super::Choice>;
+
+    /// Shows the given file selection dialog and returns the file name.
+    fn show_file_selection(&self, file_selection: &super::FileSelection) -> Result<Option<String>>;
 }
 
 pub(crate) fn is_available(name: &str) -> bool {
diff --git a/src/backends/stdio.rs b/src/backends/stdio.rs
index 9a153df..627714a 100644
--- a/src/backends/stdio.rs
+++ b/src/backends/stdio.rs
@@ -3,7 +3,7 @@
 
 use std::io::{self, Write};
 
-use crate::{Choice, Input, Message, Password, Question, Result};
+use crate::{Choice, FileSelection, Input, Message, Password, Question, Result};
 
 /// The fallback backend using standard input and output.
 ///
@@ -86,4 +86,17 @@ impl super::Backend for Stdio {
         io::stdout().flush()?;
         Ok(parse_choice(&read_input()?))
     }
+
+    fn show_file_selection(&self, file_selection: &FileSelection) -> Result<Option<String>> {
+        let dir = file_selection.path_to_string().ok_or("path not valid")?;
+        print_title(&file_selection.title);
+        print!("{} [{}]: ", file_selection.text, dir);
+        io::stdout().flush()?;
+        let result = read_input()?;
+        if result.starts_with('/') {
+            Ok(Some(result))
+        } else {
+            Ok(Some(dir + &result))
+        }
+    }
 }
diff --git a/src/backends/zenity.rs b/src/backends/zenity.rs
index 0deca0a..4ee7785 100644
--- a/src/backends/zenity.rs
+++ b/src/backends/zenity.rs
@@ -3,7 +3,7 @@
 
 use std::process;
 
-use crate::{Choice, Error, Input, Message, Password, Question, Result};
+use crate::{Choice, Error, FileSelection, Input, Message, Password, Question, Result};
 
 /// The `zenity` backend.
 ///
@@ -163,4 +163,11 @@ impl super::Backend for Zenity {
         self.execute(args, &question.title)
             .and_then(|output| get_choice(output.status))
     }
+
+    fn show_file_selection(&self, file_selection: &FileSelection) -> Result<Option<String>> {
+        let dir = file_selection.path_to_string().ok_or("path not valid")?;
+        let args = vec!["--file-selection", "--filename", &dir];
+        self.execute(args, &file_selection.title)
+            .and_then(get_stdout)
+    }
 }
diff --git a/src/lib.rs b/src/lib.rs
index 61be22b..da2a259 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -7,6 +7,7 @@
 //!
 //! The `dialog` crate can be used to display different types of dialog boxes.  The supported types
 //! are:
+//! - [`FileSelection`][]: a file chooser dialog box
 //! - [`Input`][]: a text input dialog
 //! - [`Message`][]: a simple message box
 //! - [`Password`][]: a password input dialog
@@ -70,6 +71,7 @@
 //! ```
 //!
 //! [`Dialog`]: backends/struct.Dialog.html
+//! [`FileSelection`]: struct.FileSelection.html
 //! [`Input`]: struct.Input.html
 //! [`Message`]: struct.Message.html
 //! [`Password`]: struct.Password.html
@@ -92,7 +94,11 @@ mod error;
 /// [`Backend`]: trait.Backend.html
 pub mod backends;
 
-use std::env;
+use dirs;
+use std::{
+    env,
+    path::{Path, PathBuf},
+};
 
 pub use crate::error::{Error, Result};
 
@@ -342,6 +348,80 @@ impl DialogBox for Question {
     }
 }
 
+/// A file chooser dialog box.
+///
+/// This dialog box opens a file chooser with an optional title in the specified path.  If the path
+/// is not specified, it defaults to the user’s home directory.
+///
+/// # Example
+///
+/// ```no_run
+/// use dialog::DialogBox;
+///
+/// let choice = dialog::FileSelection::new("Please select a file")
+///     .title("File Selection")
+///     .path("/home/user/Downloads")
+///     .show()
+///     .expect("Could not display dialog box");
+/// println!("The user chose: {:?}", choice);
+/// ```
+pub struct FileSelection {
+    text: String,
+    title: Option<String>,
+    path: Option<PathBuf>,
+}
+
+impl FileSelection {
+    /// Creates a new file chooser with the given path.
+    pub fn new(text: impl Into<String>) -> FileSelection {
+        FileSelection {
+            text: text.into(),
+            title: None,
+            path: dirs::home_dir(),
+        }
+    }
+
+    /// Sets the title of this file chooser dialog box.
+    ///
+    /// This method returns a reference to `self` to enable chaining.
+    pub fn title(&mut self, title: impl Into<String>) -> &mut FileSelection {
+        self.title = Some(title.into());
+        self
+    }
+
+    /// Sets the path of this file chooser dialog box.
+    ///
+    /// This method returns a reference to `self` to enable chaining.
+    pub fn path(&mut self, path: impl AsRef<Path>) -> &mut FileSelection {
+        self.path = Some(path.as_ref().to_path_buf());
+        self
+    }
+
+    /// Gets the path of this file chooser dialog box.
+    ///
+    /// This method returns the validated directory as a `String`.
+    pub fn path_to_string(&self) -> Option<String> {
+        match self.path {
+            Some(ref path) if path.is_dir() => {
+                // The backends expect a trailing / after the directory
+                path.to_str().map(|s| s.to_string() + "/")
+            }
+            _ => None,
+        }
+    }
+}
+
+impl DialogBox for FileSelection {
+    type Output = Option<String>;
+
+    fn show_with<B>(&self, backend: impl AsRef<B>) -> Result<Self::Output>
+    where
+        B: backends::Backend + ?Sized,
+    {
+        backend.as_ref().show_file_selection(self)
+    }
+}
+
 /// Creates a new instance of the default backend.
 ///
 /// The following steps are performed to determine the default backend: