docs(Examples): adds subcommand examples

Closes #766

examples/20_subcommands.rs

@@ -0,0 +1,144 @@
+// Working with subcommands is simple. There are a few key points to remember when working with
+// subcommands in clap. First, SubCommands are really just Apps. This means they can have their own
+// settings, version, authors, args, and even their own subcommands. The next thing to remember is
+// that subcommands are set up in a tree like heirachy.
+//
+// An ASCII art depiction may help explain this better. Using a fictional version of git as the demo
+// subject. Imagine the following are all subcommands of git (note, the author is aware these aren't
+// actually all subcommands in the real git interface, but it makes explanation easier)
+//
+//            Top Level App (git)                         TOP
+//                           |
+//    -----------------------------------------
+//   /             |                \          \
+// clone          push              add       commit      LEVEL 1
+//   |           /    \            /    \       |
+//  url      origin   remote    ref    name   message     LEVEL 2
+//           /                  /\
+//        path            remote  local                   LEVEL 3
+//
+// Given the above fictional subcommand hierarchy, valid runtime uses would be (not an all inclusive
+// list):
+//
+// $ git clone url
+// $ git push origin path
+// $ git add ref local
+// $ git commit message
+//
+// Notice only one command per "level" may be used. You could not, for example, do:
+//
+// $ git clone url push origin path
+//
+// It's also important to know that subcommands each have their own set of matches and may have args
+// with the same name as other subcommands in a different part of the tree heirachy (i.e. the arg
+// names aren't in a flat namespace).
+//
+// In order to use subcommands in clap, you only need to know which subcommand you're at in your
+// tree, and which args are defined on that subcommand.
+//
+// Let's make a quick program to illustrate. We'll be using the same example as above but for
+// brevity sake we won't implement all of the subcommands, only a few.
+
+extern crate clap;
+
+use clap::{App, Arg, SubCommand, AppSettings};
+
+fn main() {
+
+    let matches = App::new("git")
+        .about("A fictional versioning CLI")
+        .version("1.0")
+        .author("Me")
+        .subcommand(SubCommand::with_name("clone")
+            .about("clones repos")
+            .arg(Arg::with_name("repo")
+                .help("The repo to clone")
+                .required(true)))
+        .subcommand(SubCommand::with_name("push")
+            .about("pushes things")
+            .setting(AppSettings::SubcommandRequiredElseHelp)
+            .subcommand(SubCommand::with_name("remote")  // Subcommands can have thier own subcommands,
+                                                         // which in turn have their own subcommands
+                .about("pushes remote things")
+                .arg(Arg::with_name("repo")
+                    .required(true)
+                    .help("The remote repo to push things to")))
+            .subcommand(SubCommand::with_name("local")
+                .about("pushes local things")))
+        .subcommand(SubCommand::with_name("add")
+            .about("adds things")
+            .author("Someone Else")                     // Subcommands can list different authors
+            .version("v2.0 (I'm versioned differently") // or different version from their parents
+            .setting(AppSettings::ArgRequiredElseHelp)  // They can even have different settings
+            .arg(Arg::with_name("stuff")
+                .long("stuff")
+                .help("Stuff to add")
+                .takes_value(true)
+                .multiple(true)))
+        .get_matches();
+
+    // At this point, the matches we have point to git. Keep this in mind...
+
+    // You can check if one of git's subcommands was used
+    if matches.is_present("clone") {
+        println!("'git clone' was run.");
+    }
+
+    // You can see which subcommand was used
+    if let Some(subcommand) = matches.subcommand_name() {
+        println!("'git {}' was used", subcommand);
+
+        // It's important to note, this *only* check's git's DIRECT children, **NOT** it's
+        // grandchildren, great grandchildren, etc.
+        //
+        // i.e. if the command `git push remove --stuff foo` was run, the above will only print out,
+        // `git push` was used. We'd need to get push's matches to see futher into the tree
+    }
+
+    // An alternative to checking the name is matching on known names. Again notice that only the
+    // direct children are matched here.
+    match matches.subcommand_name() {
+        Some("clone") => println!("'git clone' was used"),
+        Some("push") => println!("'git push' was used"),
+        Some("add") => println!("'git add' was used"),
+        None        => println!("No subcommand was used"),
+        _           => unreachable!(), // Assuming you've listed all direct children above, this is unreachable
+    }
+
+    // You could get the independent subcommand matches, although this is less common
+    if let Some(clone_matches) = matches.subcommand_matches("clone") {
+        // Now we have a reference to clone's matches
+        println!("Cloning repo: {}", clone_matches.value_of("repo").unwrap());
+    }
+
+    // The most common way to handle subcommands is via a combined approach using
+    // `ArgMatches::subcommand` which returns a tuple of both the name and matches
+    match matches.subcommand() {
+        ("clone", Some(clone_matches)) =>{
+            // Now we have a reference to clone's matches
+            println!("Cloning {}", clone_matches.value_of("repo").unwrap());
+        },
+        ("push", Some(push_matches)) =>{
+            // Now we have a reference to push's matches
+            match push_matches.subcommand() {
+                ("remote", Some(remote_matches)) =>{
+                    // Now we have a reference to remote's matches
+                    println!("Pushing to {}", remote_matches.value_of("repo").unwrap());
+                },
+                ("local", Some(local_matches)) =>{
+                    // Now we have a reference to push's matches
+                    println!("'git push local' was used");
+                },
+                _            => unreachable!(),
+            }
+        },
+        ("add", Some(add_matches)) =>{
+            // Now we have a reference to add's matches
+            println!("Adding {}", add_matches.values_of("stuff").unwrap().collect::<Vec<_>>().join(", "));
+        },
+        ("", None)   => println!("No subcommand was used"), // If no subcommand was usd it'll match the tuple ("", None)
+        _            => unreachable!(), // If all subcommands are defined above, anything else is unreachabe!()
+    }
+
+    // Continued program logic goes here...
+}