How to render attribute in Handlebars template from Rocket handler?

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05-10-2024

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Rendering Dynamic Attributes in Handlebars Templates with Rocket Handlers

Problem: You're using Rocket to build your web application and employing Handlebars for templating. You need to render a dynamic attribute, like a class name or data attribute, within your Handlebars template based on data retrieved from your Rocket handler.

Simplified Explanation: Imagine you want to apply a specific class to a button based on user permissions. You need a way to send the class name from your Rocket code to your Handlebars template and have it rendered dynamically.

Scenario:

Let's say we have a simple Rocket application with a route that displays a button. We want the button to have a class "primary" if the user is logged in and "secondary" otherwise. Here's how we might structure the code:

use rocket::response::content::Html;
use rocket::serde::json::Json;
use rocket::{get, routes};

#[derive(serde::Serialize)]
struct ButtonData {
    text: String,
    class: String,
}

#[get("/button")]
fn button_handler(user: Option<User>) -> Html<String> {
    let data = match user {
        Some(_) => ButtonData {
            text: "Log Out".to_string(),
            class: "primary".to_string(),
        },
        None => ButtonData {
            text: "Log In".to_string(),
            class: "secondary".to_string(),
        },
    };

    Html(handlebars::handlebars().render("button", &data).unwrap())
}

fn main() {
    rocket::ignite()
        .mount("/", routes![button_handler])
        .launch();
}

Analysis:

• We've defined a ButtonData struct containing the button text and the desired class.
• In the button_handler, we check if a user is logged in (represented by user: Option<User>).
• Based on the user's login status, we populate the class field of ButtonData.
• The handlebars::handlebars().render(...) line is where we render the "button" template with the data from ButtonData.

The Issue:

The above code won't work as intended. You can't directly pass a dynamic attribute like class to Handlebars using render(). Handlebars expects static template files.

Solution:

We can use a helper function to dynamically generate the attribute string within the Handlebars template itself.

1. Create a Handlebars Helper:

   use handlebars::{Helper, HelperDef, Context, RenderContext, Output, Error};
   
   fn dynamic_attr<'reg: 'rc, 'rc>(
       h: &Helper<'reg, 'rc>,
       rc: &'rc RenderContext<'reg>,
   ) -> Result<Output, Error> {
       let name = h.name().to_string();
       let value = h
           .param(0)
           .and_then(|v| v.value().as_str())
           .unwrap_or("");
       let attribute = format!("{}=\"{}\"", name, value);
       Ok(Output::write(&attribute))
   }
   
   fn register_helpers(handlebars: &mut handlebars::Handlebars) {
       handlebars.register_helper("dynamicAttr", Box::new(HelperDef::new(dynamic_attr)));
   }
   

2. Modify Rocket Handler:

   // ... (previous code) ...
   
   fn button_handler(user: Option<User>) -> Html<String> {
       let data = match user {
           Some(_) => ButtonData {
               text: "Log Out".to_string(),
               class: "primary".to_string(),
           },
           None => ButtonData {
               text: "Log In".to_string(),
               class: "secondary".to_string(),
           },
       };
   
       // Register the helper
       let mut handlebars = handlebars::handlebars();
       register_helpers(&mut handlebars); 
   
       Html(handlebars.render("button", &data).unwrap())
   }
   
   // ... (rest of the code) ...
   

3. Update Handlebars Template:

   <button {{dynamicAttr "class" this.class}}>{{this.text}}</button>
   

Explanation:

• The dynamic_attr helper takes the attribute name and its value as parameters.
• We format the attribute string in the helper and return it as an Output.
• In the template, we use {{dynamicAttr "class" this.class}} to dynamically generate the class attribute based on the class value from ButtonData.
• The this.class expression refers to the class field of the ButtonData object passed to the template.

Benefits of using Handlebars helpers:

• Encapsulation: Separates logic from the template, making it cleaner and easier to maintain.
• Reusability: Helpers can be used throughout your templates.
• Flexibility: Helps to create more dynamic and interactive templates.

Remember: This is a simplified example. You can tailor the dynamic_attr helper to your specific needs, handling different data types or adding more functionality.

Resources:

• Handlebars Documentation
• Rocket Documentation

By using a Handlebars helper, you can easily render dynamic attributes from Rocket handlers, adding another layer of flexibility to your application's templating.