Expanding explanation and example

src/demo_gen/custom_functions.md

@@ -1,45 +1,78 @@
 # Making Custom Functions
 
-Let's say you want to add your own custom demonstration functions for a given struct, without demo_gen automagically creating the content of JS functions for you.
+## What is a custom function?
 
-This is done with the `#[diplomat::demo(custom_func="filename.mjs")]` attribute, which can be added above any `struct` definition. demo_gen will search for files relative to `lib.rs`, and add the contents of `filename.mjs` to its output.
+demo_gen tends towards automagical configuration. demo_gen will do its best to take Rust functions and convert them into JS output.
 
-Then demo_gen will import the default export of `filename.mjs`, and append it to the list of [RenderInfo](https://github.com/rust-diplomat/diplomat/blob/main/docs/demo_gen.md#step-two-constructing-renderinfo) termini.
+But there arise situations where we want to create our own custom Javascript functions to demonstrate our library's capabilities to the user, then add them to demo_gen's output. This may be the case if you want to demonstrate functionality that is more involved than demo_gen's automagical work.
 
-Here's an example of what that looks like...
+### Example
+Let's look at the [quickstart](quickstart.md) repository for an example.
 
-In rust:
+We only have one function exposed: `get_add_str(left : u32, right: u32)`.
 
-```rs
-#[diplomat::bridge]
-mod ffi {
-    #[diplomat::demo(custom_func="classname_demo.mjs")]
-    struct ClassName;
-}
+What if we have variables `a`, `b`, and `c`, and we want to show the user the results of calling:
 
+`get_add_str(a, b)` and `get_add_str(b, c)`?
 
-```
+We can do this without adding a new binding, through the use of a custom Javascript function.
 
-Then in `classname_demo.mjs`, we write:
+## \#\[diplomat::demo(custom_func="...")\]
+
+`#[diplomat::demo(custom_func="filename.mjs")]` can be added above any `struct` definition. demo_gen will search for files relative to `lib.rs`, and add the contents of `filename.mjs` to its output.
+
+Then demo_gen will import the default export of `filename.mjs`, and append it to the list of [RenderInfo](https://github.com/rust-diplomat/diplomat/blob/main/docs/demo_gen.md#step-two-constructing-renderinfo) termini.
+
+### Example
+So, first we create a file called `adder_custom.mjs` in the same folder as `adder_bindings/src/lib.rs`:
 
 ```js
+// adder_bindings/src/adder_custom.mjs
+import { lib } from "./index.mjs";
 export default {
-    "ClassName.SampleFunctionName": {
-        func: () => { alert("Hello world!"); },
-        funcName: "ClassName.SampleFunctionName",
-        parameters: [
-
-        ]
-    },
-    "ClassName.OtherFunctionName": {
-        func: (a) => { prompt(`Testing!${a}`); },
-        funcName: "ClassName.OtherFunctionName",
+    "AddThreeVariables": {
+        func: (a, b, c) => { return lib.AddResult.getAddStr(a, b) + " and " + lib.AddResult.getAddStr(b, c); },
+        funcName: "Add a + b, b + c",
         parameters: [
-            name: "Prompt Question",
-            type: "string"
+            {
+                name: "a",
+                type: "number"
+            },
+            {
+                name: "b",
+                type: "number"
+            },
+            {
+                name: "c",
+                type: "number"
+            }
         ]
     }
-};
+}
+```
+
+Then we make sure to link `adder_custom.mjs` in `lib.rs`:
+
+```rs
+// adder_bindings/src/lib.rs
+#[diplomat::bridge]
+mod ffi {
+    use std::fmt::Write;
+
+	#[diplomat::opaque]
+	#[diplomat::rust_link(basic_adder, Mod)]
+    #[diplomat::demo(custom_func="adder_custom.mjs")]
+	pub struct AddResult;
+
+	impl AddResult {
+		pub fn get_add_str(left : u32, right : u32, write: &mut DiplomatWrite) {
+			write.write_str(&format!("{}", basic_adder::add(left, right))).unwrap();
+			write.flush();
+		}
+	}
+}
 ```
 
-And our exported object is then added to `RenderInfo`s list of render termini, and is evaluated by the renderer accordingly!
+And our exported object is then added to `RenderInfo`s list of render termini, and is evaluated by the renderer accordingly!
+
+If you [regenerate the bindings and start the web server](https://rust-diplomat.github.io/book/demo_gen/quickstart.html#getting-started), you should see `Add a + b, b + c` in the list of functions.