Add Open/Save mode to the file selection dialogfile-selection

This patch adds the option to set a FileSelectionMode, either Open or
Save.  Not all backends might support this – currently, only zenity and
kdialog do.  Per default, the Open mode is used (as before).

4 files changed, 50 insertions, 6 deletions

diff --git a/examples/file_selection.rs b/examples/file_selection.rs
index 5436742..5d2d860 100644
--- a/examples/file_selection.rs
+++ b/examples/file_selection.rs
@@ -5,9 +5,17 @@ use dialog::DialogBox;
 
 fn main() -> dialog::Result<()> {
     let choice = dialog::FileSelection::new("Please select a file")
-        .title("File Chooser Example")
+        .title("File Chooser Example (Open)")
         .path("/etc")
         .show()?;
     println!("The user chose: {:?}", choice);
+
+    let choice = dialog::FileSelection::new("Please select a file")
+        .title("File Chooser Example (Save)")
+        .mode(dialog::FileSelectionMode::Save)
+        .path("/etc")
+        .show()?;
+    println!("The user chose: {:?}", choice);
+
     Ok(())
 }
diff --git a/src/backends/kdialog.rs b/src/backends/kdialog.rs
index e556ddf..21928f8 100644
--- a/src/backends/kdialog.rs
+++ b/src/backends/kdialog.rs
@@ -4,7 +4,9 @@
 
 use std::process;
 
-use crate::{Choice, Error, FileSelection, Input, Message, Password, Question, Result};
+use crate::{
+    Choice, Error, FileSelection, FileSelectionMode, Input, Message, Password, Question, Result,
+};
 
 /// Subprocess exit codes
 ///
@@ -139,7 +141,11 @@ impl super::Backend for KDialog {
 
     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!["--getopenfilename", &dir];
+        let option = match file_selection.mode {
+            FileSelectionMode::Open => "--getopenfilename",
+            FileSelectionMode::Save => "--getsavefilename",
+        };
+        let args = vec![option, &dir];
         self.execute(args, &file_selection.title)
             .and_then(get_stdout)
     }
diff --git a/src/backends/zenity.rs b/src/backends/zenity.rs
index 4ee7785..ac0f98f 100644
--- a/src/backends/zenity.rs
+++ b/src/backends/zenity.rs
@@ -3,7 +3,9 @@
 
 use std::process;
 
-use crate::{Choice, Error, FileSelection, Input, Message, Password, Question, Result};
+use crate::{
+    Choice, Error, FileSelection, FileSelectionMode, Input, Message, Password, Question, Result,
+};
 
 /// The `zenity` backend.
 ///
@@ -166,7 +168,10 @@ impl super::Backend for Zenity {
 
     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];
+        let mut args = vec!["--file-selection", "--filename", &dir];
+        if file_selection.mode == FileSelectionMode::Save {
+            args.push("--save");
+        }
         self.execute(args, &file_selection.title)
             .and_then(get_stdout)
     }
diff --git a/src/lib.rs b/src/lib.rs
index da2a259..aa19105 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -348,11 +348,24 @@ impl DialogBox for Question {
     }
 }
 
+/// The type of a file selection dialog.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum FileSelectionMode {
+    /// An Open File dialog, meaning that the user can only select an existing file.
+    Open,
+    /// A Save File dialog, meaning that the user is allowed to select a non-existing file.
+    Save,
+}
+
 /// A file chooser dialog box.
 ///
-/// This dialog box opens a file chooser with an optional title in the specified path.  If the path
+/// This dialog box opens a file choser with an optional title in the specified path.  If the path
 /// is not specified, it defaults to the user’s home directory.
 ///
+/// The backends might support multiple operation modes, for example open or save dialogs.  You can
+/// select a mode using the [`FileSelectionMode`][] enum, though the backend might ignore the mode
+/// and just display a simple file dialog.  Per default, the mode is set to `Open`.
+///
 /// # Example
 ///
 /// ```no_run
@@ -365,10 +378,13 @@ impl DialogBox for Question {
 ///     .expect("Could not display dialog box");
 /// println!("The user chose: {:?}", choice);
 /// ```
+///
+/// [`FileSelectionMode`]: enum.FileSelectionMode.html
 pub struct FileSelection {
     text: String,
     title: Option<String>,
     path: Option<PathBuf>,
+    mode: FileSelectionMode,
 }
 
 impl FileSelection {
@@ -378,6 +394,7 @@ impl FileSelection {
             text: text.into(),
             title: None,
             path: dirs::home_dir(),
+            mode: FileSelectionMode::Open,
         }
     }
 
@@ -409,6 +426,14 @@ impl FileSelection {
             _ => None,
         }
     }
+
+    /// Sets the operation mode of the file chooser.
+    ///
+    /// This method returns a reference to `self` to enable chaining.
+    pub fn mode(&mut self, mode: FileSelectionMode) -> &mut FileSelection {
+        self.mode = mode;
+        self
+    }
 }
 
 impl DialogBox for FileSelection {