Add required_if example

examples/README.md

@@ -69,6 +69,10 @@ How to completely remove version.
 
 How `#[structopt(rename_all)]` works.
 
+### [Required If](required_if.rs)
+
+How to use `#[structopt(required_if)]`.
+
 ### [Skip](skip.rs)
 
 How to use `#[structopt(skip)]`.

examples/required_if.rs

@@ -0,0 +1,43 @@
+//! How to use `required_if` with structopt.
+use structopt::StructOpt;
+
+#[derive(Debug, StructOpt, PartialEq)]
+struct Opt {
+    /// Where to write the output: to `stdout` or `file`
+    #[structopt(short)]
+    out_type: String,
+
+    /// File name: only required when `out-type` is set to `file`
+    #[structopt(name = "FILE", required_if("out-type", "file"))]
+    file_name: Option<String>,
+}
+
+fn main() {
+    let opt = Opt::from_args();
+    println!("{:?}", opt);
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_opt_out_type_file_without_file_name_returns_err() {
+        let opt = Opt::from_iter_safe(&["test", "-o", "file"]);
+        let err = opt.unwrap_err();
+        assert_eq!(err.kind, clap::ErrorKind::MissingRequiredArgument);
+    }
+
+    #[test]
+    fn test_opt_out_type_file_with_file_name_returns_ok() {
+        let opt = Opt::from_iter_safe(&["test", "-o", "file", "filename"]);
+        let opt = opt.unwrap();
+        assert_eq!(
+            opt,
+            Opt {
+                out_type: "file".into(),
+                file_name: Some("filename".into()),
+            }
+        );
+    }
+}