examples: add custom validator example

examples/15_CustomValidator.rs

@@ -0,0 +1,36 @@
+extern crate clap;
+
+use clap::{App, Arg};
+
+fn main() {
+    // You can define a function (or a closure) to use as a validator to argument values. The
+    // function must accept a String and return Result<(), String> where Err(String) is the message
+    // displayed to the user.
+
+    let matches = App::new("myapp")
+                        // Application logic goes here...
+                        .arg(Arg::with_name("input")
+                                     .help("the input file to use")
+                                     .index(1)
+                                     .required(true)
+
+                                     .validator(|val| {
+                                         // val is the argument value passed in by the user
+                                         // val has type of String.
+
+                                         if val.ends_with(".png") {
+                                             Ok(())
+                                         } else {
+                                             // clap automatically adds "error: " to the beginning
+                                             // of the message.
+                                             Err(String::from("the file format must be png."))
+                                         }
+
+                                         // Of course, you can do more complicated validation as
+                                         // well, but for the simplicity, this example only checks
+                                         // if the value passed in ends with ".png" or not.
+                                     }))
+                        .get_matches();
+
+    // Continued program logic here...
+}