get fields from go template

templatefields.go

package page

import (
	"html/template"
	"sort"
	"text/template/parse"
)

func GetTemplateFields(tpl *template.Template) []string {
	return walker{fields: map[string]struct{}{}}.tree(tpl)
}

type walker struct {
	fields map[string]struct{}
}

func (c walker) tree(tpl *template.Template) []string {
	for _, node := range tpl.Tree.Root.Nodes {
		c.node(node)
	}

	var fields []string
	for field := range c.fields {
		fields = append(fields, field)
	}

	sort.Strings(fields)

	return fields
}

func (c *walker) node(node parse.Node) {
	switch v := node.(type) {
	case *parse.ActionNode:
		c.node(v.Pipe)
	case *parse.BoolNode:
	case *parse.BranchNode:
		c.node(v.Pipe)
		c.node(v.List)
		c.node(v.ElseList)
	case *parse.ChainNode:
		c.node(v.Node)
	case *parse.CommandNode:
		for _, n := range v.Args {
			c.node(n)
		}
	case *parse.CommentNode:
	case *parse.DotNode:
	case *parse.FieldNode:
		c.fields[v.String()] = struct{}{}
	case *parse.IdentifierNode:
	case *parse.IfNode:
		c.node(v.List)
		c.node(v.Pipe)
		c.node(v.ElseList)
	case *parse.ListNode:
		if v == nil {
			break
		}
		for _, n := range v.Nodes {
			c.node(n)
		}
	case *parse.NilNode:
	case *parse.NumberNode:
	case *parse.PipeNode:
		for _, cmd := range v.Cmds {
			c.node(cmd)
		}
		for _, variableNode := range v.Decl {
			c.node(variableNode)
		}
	case *parse.RangeNode:
		c.node(v.List)
		c.node(v.Pipe)
		c.node(v.ElseList)
	case *parse.StringNode:
	case *parse.TemplateNode:
		c.node(v.Pipe)
	case *parse.TextNode:
	case *parse.VariableNode:
	case *parse.WithNode:
		c.node(v.List)
		c.node(v.ElseList)
		c.node(v.Pipe)
	}
}