Allow registration of handlebars helpers

contrib/src/lib.rs

@@ -48,3 +48,6 @@ pub use json::JSON;
 
 #[cfg(feature = "templates")]
 pub use templates::Template;
+
+#[cfg(feature = "handlebars_templates")]
+pub use templates::register_handlebars_helper;

contrib/src/templates/handlebars_templates.rs

@@ -6,6 +6,7 @@ use super::serde::Serialize;
 use super::TemplateInfo;
 
 use self::handlebars::Handlebars;
+use self::handlebars::HelperDef;
 
 lazy_static! {
     static ref HANDLEBARS: RwLock<Handlebars> = RwLock::new(Handlebars::new());
@@ -33,3 +34,39 @@ pub fn render<T>(name: &str, info: &TemplateInfo, context: &T) -> Option<String>
         }
     }
 }
+
+/// Registers a handlebars helper for use with `.hbs` templates
+///
+/// Define a helper function, exactly as you would for handlebars-rust:
+///
+/// ```rust,ignore
+/// # use handlebars::Context;
+/// # use handlebars::Helper;
+/// # use handlebars::Handlebars;
+/// # use handlebars::RenderContext;
+/// # use handlebars::RenderError;
+/// fn hex_helper (_: &Context, h: &Helper, _: &Handlebars, rc: &mut RenderContext) -> Result<(), RenderError> {
+///     // just for example, add error check for unwrap
+///     let param = h.param(0).unwrap().value();
+///     let rendered = format!("0x{:x}", param.as_u64().unwrap());
+///     try!(rc.writer.write(rendered.into_bytes().as_ref()));
+///     Ok(())
+/// }
+/// ```
+///
+/// Register it before you ignite (replaces handlebars.register_helper):
+///
+/// ```rust,ignore
+/// rocket_contrib::register_handlebars_helper("hex", Box::new(hex_helper));
+/// ```
+///
+/// Use the helper in your template, as usual:
+/// ```
+/// {{hex my_value}}
+/// ```
+pub fn register_handlebars_helper(name: &str,
+                       def: Box<HelperDef + 'static>)
+                       -> Option<Box<HelperDef + 'static>> {
+    let r = HANDLEBARS.write().unwrap().register_helper(name, def);
+    r
+}

contrib/src/templates/mod.rs

@@ -8,6 +8,9 @@ mod tera_templates;
 #[cfg(feature = "handlebars_templates")]
 mod handlebars_templates;
 
+#[cfg(feature = "handlebars_templates")]
+pub use self::handlebars_templates::register_handlebars_helper;
+
 #[macro_use] mod macros;
 
 use self::serde::Serialize;