From 7a145f639ddbc18ba2f4810eeed2ea6e08edb29a Mon Sep 17 00:00:00 2001
From: Hayleigh Thompson <me@hayleigh.dev>
Date: Tue, 16 Apr 2024 22:19:43 +0100
Subject: :sparkles: Add a new 'ForceModel' debug action to experiment with
 time-travel debugging.

---
 src/client-runtime.ffi.mjs         | 32 +++++++++++++++++++++++++++++++-
 src/lustre/internals/runtime.gleam | 18 +++++++++++++++++-
 2 files changed, 48 insertions(+), 2 deletions(-)

diff --git a/src/client-runtime.ffi.mjs b/src/client-runtime.ffi.mjs
index 51c2eef..ecca48f 100644
--- a/src/client-runtime.ffi.mjs
+++ b/src/client-runtime.ffi.mjs
@@ -1,5 +1,10 @@
 import { ElementNotFound, NotABrowser } from "./lustre.mjs";
-import { Dispatch, Shutdown } from "./lustre/internals/runtime.mjs";
+import {
+  Dispatch,
+  Shutdown,
+  Debug,
+  ForceModel,
+} from "./lustre/internals/runtime.mjs";
 import { morph } from "./vdom.ffi.mjs";
 import { Ok, Error, isEqual } from "./gleam.mjs";
 
@@ -58,6 +63,11 @@ export class LustreClientApplication {
         return;
       }
 
+      case action instanceof Debug: {
+        this.#debug(action[0]);
+        return;
+      }
+
       default:
         return;
     }
@@ -114,6 +124,26 @@ export class LustreClientApplication {
     }
   }
 
+  #debug(action) {
+    switch (true) {
+      case action instanceof ForceModel: {
+        const vdom = this.#view(this.#model);
+        const dispatch = (handler) => (e) => {
+          const result = handler(e);
+
+          if (result instanceof Ok) {
+            this.send(new Dispatch(result[0]));
+          }
+        };
+
+        this.#queue = [];
+        this.#effects = [];
+        this.#didUpdate = false;
+        this.#root = morph(this.#root, vdom, dispatch, this.#isComponent);
+      }
+    }
+  }
+
   #shutdown() {
     this.#root.remove();
     this.#root = null;
diff --git a/src/lustre/internals/runtime.gleam b/src/lustre/internals/runtime.gleam
index a2b5365..b2374b4 100644
--- a/src/lustre/internals/runtime.gleam
+++ b/src/lustre/internals/runtime.gleam
@@ -3,8 +3,8 @@
 import gleam/dict.{type Dict}
 import gleam/dynamic.{type Decoder, type Dynamic}
 import gleam/erlang/process.{type Selector, type Subject}
-import gleam/list
 import gleam/json.{type Json}
+import gleam/list
 import gleam/option.{Some}
 import gleam/otp/actor.{type Next, type StartError, Spec}
 import gleam/result
@@ -46,6 +46,7 @@ pub type Action(msg, runtime) {
 }
 
 pub type DebugAction {
+  ForceModel(Dynamic)
   Model(reply: fn(Dynamic) -> Nil)
   View(reply: fn(Element(Dynamic)) -> Nil)
 }
@@ -133,6 +134,21 @@ fn loop(
       loop(Batch(rest, effect.batch([effects, other_effects])), next)
     }
 
+    Debug(ForceModel(model)) -> {
+      let model = dynamic.unsafe_coerce(model)
+      let html = state.view(model)
+      let diff = patch.elements(state.html, html)
+      let next =
+        State(..state, model: model, html: html, handlers: diff.handlers)
+
+      case patch.is_empty_element_diff(diff) {
+        True -> Nil
+        False -> run_renderers(state.renderers, Diff(diff))
+      }
+
+      actor.continue(next)
+    }
+
     Debug(Model(reply)) -> {
       reply(dynamic.from(state.model))
       actor.continue(state)
-- 
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