Add SubcommandChooser

The SubcommandChooser can be used to customize the behavior when the
default map lookup cannot find the subcommand you want.

Example usage:

1. busybox-style subcommand invocation

```
// A symbolic link `ls` to binary `busybox` will invoke `busybox ls`
mySubcommandChooser = func(*c cli.CLI) (CommandFactory, error){
    subcommand = os.Args[0]
    if subcommand != c.Name() {
        if subcommandFunc, ok := c.Commands[subcommand]; ok {
            return subcommandFunc, nil
        }
    }
    return cli.DefaultSubcommandChooser(c)
}

c := cli.NewCLI("busybox", "1.0.0")
c.Args = os.Args[1:]
c.Commands = map[string]cli.CommandFactory{
    "ls": lsCommandFactory,
    "cat": catCommandFactory,
}
c.SubcommandChooser = mySubcommandChooser
```

2. default subcommand

```
// `myapp --help` == `myapp default --help`
mySubcommandChooser = func(*c cli.CLI) (CommandFactory, error) {
    if c.Subcommand() == "" {
        return c.Commands["default"], nil
    }
    return cli.DefaultSubcommandChooser(c)
}
c := cli.NewCLI("myapp", "1.0.0")
c.Args = os.Args[1:]
c.Commands = map[string]cli.CommandFactory{
    "default": defaultCommandFactory,
}
c.SubcommandChooser = mySubcommandChooser
```

cli.go

@@ -1,6 +1,7 @@
 package cli
 
 import (
+	"fmt"
 	"io"
 	"os"
 	"sync"
@@ -26,6 +27,8 @@ type CLI struct {
 	// Version of the CLI.
 	Version string
 
+	SubcommandChooser func(*CLI) (CommandFactory, error)
+
 	// HelpFunc and HelpWriter are used to output help information, if
 	// requested.
 	//
@@ -47,6 +50,28 @@ type CLI struct {
 	isVersion bool
 }
 
+// DefaultSubcommandChooser is the default SubcommandChooser. It will return
+// the proper subcommand CommandFactory, or if that cannot be found and
+// IsVersion() return true, return versionCommanFactory. The fallback of
+// helpCommandFactory will be returned when there's nothing matched. error will
+// be non-nil if a proper subcommand can't be found, so client user can wrap
+// this function according to error value.
+func DefaultSubcommandChooser(c *CLI) (CommandFactory, error) {
+	if commandFunc, ok := c.Commands[c.Subcommand()]; ok {
+		return commandFunc, nil
+	} else if c.IsVersion() {
+		versionCommandFactory := func() (Command, error) {
+			return OutputTextCommand{c.HelpWriter, c.Version}, nil
+		}
+		return versionCommandFactory, fmt.Errorf("Failed to find subcommand")
+	} else {
+		helpCommandFactory := func() (Command, error) {
+			return OutputTextCommand{c.HelpWriter, c.HelpFunc(c.Commands)}, nil
+		}
+		return helpCommandFactory, fmt.Errorf("Failed to find subcommand")
+	}
+}
+
 // NewClI returns a new CLI instance with sensible defaults.
 func NewCLI(app, version string) *CLI {
 	return &CLI{
@@ -75,12 +100,6 @@ func (c *CLI) IsVersion() bool {
 func (c *CLI) Run() (int, error) {
 	c.once.Do(c.init)
 
-	// Just show the version and exit if instructed.
-	if c.IsVersion() && c.Version != "" {
-		c.HelpWriter.Write([]byte(c.Version + "\n"))
-		return 1, nil
-	}
-
 	// If there is an invalid flag, then error
 	if len(c.topFlags) > 0 {
 		c.HelpWriter.Write([]byte(
@@ -90,25 +109,16 @@ func (c *CLI) Run() (int, error) {
 		return 1, nil
 	}
 
-	// Attempt to get the factory function for creating the command
-	// implementation. If the command is invalid or blank, it is an error.
-	commandFunc, ok := c.Commands[c.Subcommand()]
-	if !ok {
-		c.HelpWriter.Write([]byte(c.HelpFunc(c.Commands) + "\n"))
-		return 1, nil
-	}
+	commandFunc, _ := c.SubcommandChooser(c)
 
 	command, err := commandFunc()
+
 	if err != nil {
 		return 0, err
 	}
-
-	// If we've been instructed to just print the help, then print it
 	if c.IsHelp() {
-		c.HelpWriter.Write([]byte(command.Help() + "\n"))
-		return 1, nil
+		command = OutputTextCommand{c.HelpWriter, command.Help()}
 	}
-
 	return command.Run(c.SubcommandArgs()), nil
 }
 
@@ -140,6 +150,10 @@ func (c *CLI) init() {
 		c.HelpWriter = os.Stderr
 	}
 
+	if c.SubcommandChooser == nil {
+		c.SubcommandChooser = DefaultSubcommandChooser
+	}
+
 	c.processArgs()
 }
 

cli_test.go

@@ -158,8 +158,11 @@ func TestCLIRun_printHelp(t *testing.T) {
 			continue
 		}
 
-		if !strings.Contains(buf.String(), helpText) {
-			t.Errorf("Args: %#v. Text: %v", testCase, buf.String())
+		expect := strings.TrimSpace(buf.String())
+		got := strings.TrimSpace(helpText)
+
+		if !strings.Contains(expect, got) {
+			t.Errorf("Args: %#v, expect: %#v, got %#v", testCase, expect, got)
 		}
 	}
 }
@@ -195,8 +198,10 @@ func TestCLIRun_printCommandHelp(t *testing.T) {
 			t.Fatalf("bad exit code: %d", exitCode)
 		}
 
-		if buf.String() != (command.HelpText + "\n") {
-			t.Fatalf("bad: %#v", buf.String())
+		expect := strings.TrimSpace(command.HelpText)
+		got := strings.TrimSpace(buf.String())
+		if expect != got {
+			t.Fatalf("Expect %#v, got %#v.", expect, got)
 		}
 	}
 }

command.go

@@ -1,5 +1,7 @@
 package cli
 
+import "io"
+
 // A command is a runnable sub-command of a CLI.
 type Command interface {
 	// Help should return long-form help text that includes the command-line
@@ -21,3 +23,26 @@ type Command interface {
 // We need a factory because we may need to setup some state on the
 // struct that implements the command itself.
 type CommandFactory func() (Command, error)
+
+// OutputTextCommand implemented Command interface and is used to write text to
+// given writer
+type OutputTextCommand struct {
+	writer io.Writer
+	text   string
+}
+
+// Help is part of Command interface
+func (c OutputTextCommand) Help() string {
+	return c.text
+}
+
+// Synopsis is part of Command interface. Return Help()
+func (c OutputTextCommand) Synopsis() string {
+	return c.Help()
+}
+
+// Run is part of Command interface. Args will be ignored.
+func (c OutputTextCommand) Run(_ []string) int {
+	c.writer.Write([]byte(c.Help()))
+	return 1
+}