Add support for dynamically registered commands.

kong.go

@@ -66,6 +66,7 @@ type Kong struct {
 
 	// Set temporarily by Options. These are applied after build().
 	postBuildOptions []Option
+	dynamicCommands  []*dynamicCommand
 }
 
 // New creates a new Kong parser on grammar.
@@ -106,6 +107,20 @@ func New(grammar interface{}, options ...Option) (*Kong, error) {
 	k.Model = model
 	k.Model.HelpFlag = k.helpFlag
 
+	// Synthesise command nodes.
+	for _, dcmd := range k.dynamicCommands {
+		tag := newEmptyTag()
+		tag.Name = dcmd.name
+		tag.Help = dcmd.help
+		tag.Group = dcmd.group
+		tag.Cmd = true
+		v := reflect.Indirect(reflect.ValueOf(dcmd.cmd))
+		buildChild(k, k.Model.Node, CommandNode, reflect.Value{}, reflect.StructField{
+			Name: dcmd.name,
+			Type: v.Type(),
+		}, v, tag, dcmd.name, map[string]bool{})
+	}
+
 	for _, option := range k.postBuildOptions {
 		if err = option.Apply(k); err != nil {
 			return nil, err

kong_test.go

@@ -1002,3 +1002,30 @@ func TestPointers(t *testing.T) {
 	require.NotNil(t, cli.JSON)
 	require.Equal(t, "FOO", string(*cli.JSON))
 }
+
+type dynamicCommand struct {
+	Flag string
+
+	ran bool
+}
+
+func (d *dynamicCommand) Run() error {
+	d.ran = true
+	return nil
+}
+
+func TestDynamicCommands(t *testing.T) {
+	cli := struct {
+		One struct{} `cmd:"one"`
+	}{}
+	two := &dynamicCommand{}
+	var twoi interface{} = &two
+	p := mustNew(t, &cli, kong.DynamicCommand("two", "", "", twoi))
+	kctx, err := p.Parse([]string{"two", "--flag=flag"})
+	require.NoError(t, err)
+	require.Equal(t, "flag", two.Flag)
+	require.False(t, two.ran)
+	err = kctx.Run()
+	require.NoError(t, err)
+	require.True(t, two.ran)
+}

options.go

@@ -54,6 +54,28 @@ func Exit(exit func(int)) Option {
 	})
 }
 
+type dynamicCommand struct {
+	name  string
+	help  string
+	group string
+	cmd   interface{}
+}
+
+// DynamicCommand registers a dynamically constructed command with the root of the CLI.
+//
+// This is useful for command-line structures that are extensible via user-provided plugins.
+func DynamicCommand(name, help, group string, cmd interface{}) Option {
+	return OptionFunc(func(k *Kong) error {
+		k.dynamicCommands = append(k.dynamicCommands, &dynamicCommand{
+			name:  name,
+			help:  help,
+			group: group,
+			cmd:   cmd,
+		})
+		return nil
+	})
+}
+
 // NoDefaultHelp disables the default help flags.
 func NoDefaultHelp() Option {
 	return OptionFunc(func(k *Kong) error {