如何修复Rocket框架中Handlebars模板的‘Helper not defined’错误？

问题背景 #

访问网站时出现以下错误：

>> Matched: (index) GET /
>> Handlebars: Error rendering "index" line 9, col 1: Helper not defined: Path(Relative(([Named("parent")]), "parent"))
>> Template 'index' failed to render.
>> Outcome: Failure
>> No 500 catcher registered. Using Rocket default.
>> Response succeeded.

对应的代码与模板文件如下：

Rust代码 #

#[macro_use]
extern crate rocket;

use rocket::Request;
use rocket_dyn_templates::Template;

#[derive(serde::Serialize)]
struct BoardContext {
    parent: &'static str,
}

#[derive(serde::Serialize)]
struct AboutContext {
    parent: &'static str,
}

#[derive(serde::Serialize)]
struct LegalsContext {
    parent: &'static str,
}

#[derive(serde::Serialize)]
struct NotFoundContext {
    parent: &'static str,
    url: String,
}

#[get("/")]
fn index() -> Template {
    Template::render("index", &BoardContext { parent: "layout" })
}

#[get("/about")]
fn about() -> Template {
    Template::render("about", &AboutContext { parent: "layout" })
}

#[get("/legals")]
fn legals() -> Template {
    Template::render("legals", &AboutContext { parent: "layout" })
}

#[catch(404)]
fn not_found(req: &Request) -> Template {
    Template::render(
        "not_found",
        NotFoundContext {
            parent: "layout",
            url: req.uri().to_string(),
        },
    )
}

#[launch]
fn rocket() -> _ {
    rocket::build()
        .register("/", catchers![not_found])
        .mount("/", routes![index, about, legals])
        .attach(Template::fairing())
}

index.html.hbs #

{{#*inline "page"}}

<section id="message_board">
  <h1>Hi, there!</h1>
    Welcome to my meaningless Website!
</section>

{{/inline}}
{{~> (parent)~}}

layout.html.hbs #

<!doctype html>
<html>
  <head>
    <title>website</title>
  </head>
  <body>
    {{> header}}
    {{~> page}}
    {{> footer}}
  </body>
</html>

问题根源 #

你试图通过{{~> (parent)~}}直接引用上下文中的parent变量作为模板名称，但Handlebars默认不支持这种直接的路径解析写法，必须借助lookup helper从当前上下文获取变量值，再将其作为模板名称传递。

修复方案 #

1. 修改模板的动态引用写法 #

将index.html.hbs中的{{~> (parent)~}}替换为：

{{~> (lookup . "parent")~}}

lookup helper会从当前上下文（.代表当前上下文）中取出parent字段的值（即"layout"），然后将其作为模板名称加载layout.html.hbs，这是Handlebars动态引用模板的标准写法。

2. 验证模板文件位置 #

确保所有模板文件（layout.html.hbs、index.html.hbs、header.html.hbs、footer.html.hbs等）都放在Rocket默认的templates/目录下，目录结构应为：

your_project/
├── src/
│   └── main.rs
└── templates/
    ├── layout.html.hbs
    ├── index.html.hbs
    ├── about.html.hbs
    ├── legals.html.hbs
    ├── not_found.html.hbs
    ├── header.html.hbs
    └── footer.html.hbs

3. 优化上下文结构体（可选） #

当前定义了多个重复的上下文结构体，可以合并为一个通用结构体减少冗余：

#[derive(serde::Serialize)]
struct PageContext {
    parent: &'static str,
    url: Option<String>,
}

对应修改各个handler：

#[get("/")]
fn index() -> Template {
    Template::render("index", &PageContext { parent: "layout", url: None })
}

#[get("/about")]
fn about() -> Template {
    Template::render("about", &PageContext { parent: "layout", url: None })
}

#[get("/legals")]
fn legals() -> Template {
    Template::render("legals", &PageContext { parent: "layout", url: None })
}

#[catch(404)]
fn not_found(req: &Request) -> Template {
    Template::render(
        "not_found",
        PageContext {
            parent: "layout",
            url: Some(req.uri().to_string()),
        },
    )
}

验证修复效果 #

修改完成后重新编译运行项目，访问网站即可正常渲染页面，错误会被解决。

内容的提问来源于stack exchange，提问作者nilch