Add "insert_fn2()", which provides inserted function with a renderer callback

src/builder.rs

@@ -149,6 +149,15 @@ impl MapBuilder {
         data.insert(key.to_string(), Data::Fun(RefCell::new(Box::new(f))));
         MapBuilder { data: data }
     }
+    #[inline]
+    pub fn insert_fn2<K: ToString, F>(self, key: K, f: F) -> MapBuilder
+    where
+        F: FnMut(String, &mut (FnMut(String) -> String)) -> String + Send + 'static,
+    {
+        let MapBuilder { mut data } = self;
+        data.insert(key.to_string(), Data::Fun2(RefCell::new(Box::new(f))));
+        MapBuilder { data: data }
+    }
 
     /// Return the built `Data`.
     #[inline]

src/data.rs

@@ -9,6 +9,7 @@ pub enum Data {
     Vec(Vec<Data>),
     Map(HashMap<String, Data>),
     Fun(RefCell<Box<FnMut(String) -> String + Send>>),
+    Fun2(RefCell<Box<FnMut(String, &mut (FnMut(String) -> String)) -> String + Send>>),
 }
 
 impl PartialEq for Data {
@@ -35,6 +36,7 @@ impl fmt::Debug for Data {
             Data::Vec(ref v) => write!(f, "VecVal({:?})", v),
             Data::Map(ref v) => write!(f, "Map({:?})", v),
             Data::Fun(_) => write!(f, "Fun(...)"),
+            Data::Fun2(_) => write!(f, "Fun2(...)"),
         }
     }
-}
+}

src/template.rs

@@ -65,6 +65,7 @@ struct RenderContext<'a> {
     template: &'a Template,
     indent: String,
     line_start: bool,
+    inner_fn2_rendered: bool,
 }
 
 impl<'a> RenderContext<'a> {
@@ -73,6 +74,7 @@ impl<'a> RenderContext<'a> {
             template: template,
             indent: "".to_string(),
             line_start: true,
+            inner_fn2_rendered: true,
         }
     }
 
@@ -291,6 +293,50 @@ impl<'a> RenderContext<'a> {
                         let tokens = try!(self.render_fun(src, otag, ctag, f));
                         try!(self.render(wr, stack, &tokens));
                     }
+                    Data::Fun2(ref fcell) => {
+                        let f = &mut *fcell.borrow_mut();
+                        let ctx2 = self.template.ctx.clone();
+                        let partials2 = self.template.partials.clone();
+                        self.inner_fn2_rendered = false;
+
+                        let mut f0 = {
+                            |src: String| {
+                                let compiler = Compiler::new_with(
+                                    ctx2.clone(),
+                                    src.chars(),
+                                    partials2.clone(),
+                                    otag.to_string(),
+                                    ctag.to_string(),
+                                );
+                                let mut vw: Vec<u8> = vec![];
+                                match compiler.compile() {
+                                    Ok((tokens, _)) => {
+                                        /* a trick to reset the flag is to pass an empty string */
+                                        self.inner_fn2_rendered = (src.len() > 0);
+                                        self.render(&mut vw, stack, &tokens).unwrap_or(())
+                                    }
+                                    _ => (),
+                                }
+                                String::from_utf8_lossy(&vw).to_string()
+                            }
+                        };
+                        let src = f(src.to_string(), &mut f0);
+                        if self.inner_fn2_rendered {
+                            self.inner_fn2_rendered = false;
+                            try!(self.render_text(wr, &src));
+                        } else {
+                            let compiler = Compiler::new_with(
+                                self.template.ctx.clone(),
+                                src.chars(),
+                                self.template.partials.clone(),
+                                otag.to_string(),
+                                ctag.to_string(),
+                            );
+
+                            let (tokens, _) = try!(compiler.compile());
+                            try!(self.render(wr, stack, &tokens));
+                        }
+                    }
                 }
             }
         };