Update doc and tests with scripts state and onUndo

Author: panglesd

docs/syntax.rst

@@ -188,15 +188,48 @@ These attributes are actions that will be executed when a ``pause`` or ``step``
 Custom scripts
 --------------
 
-Use a slipscript code block to add a script, and ``exec-at-unpause`` to execute it. The script should return the function to undo the change.
+Use a slipscript code block to add a script, and ``exec-at-unpause`` to execute it. The script can return the function to undo the change.
 
 .. code-block:: markdown
 
-		{exec-at-unpause}
+		{exec-at-unpause pause}
 		```slip-script
                 let elem = document.querySelector("#id")
 		let old_value = elem.style.opacity;
                 elem.style.opacity = "1";
                 return {undo : () => { elem.style.opacity = old_value }}
 		```
 
+Alternatively, the script can use ``slip.onUndo`` to register a callback to run
+during undo. Multiple callback can be registered and will be called in the right
+order (that is, the reverse order in which they were registered).
+
+.. code-block:: markdown
+
+		{exec-at-unpause pause}
+		```slip-script
+                let set_class = (elem, name, value) => {
+                  let old_value = elem.style[name];
+                  elem.style[name] = value;
+                  slip.onUndo(() => {elem.style[name] = old_value})
+                }
+                let elem = document.querySelector("#id");
+                set_class(elem, "backgroundColor", "yellow");
+                set_class(elem, "color", "red");
+		```
+
+
+Scripts have access to the ``slip.state`` object, which will be persisted from
+scripts to scripts.
+
+.. code-block:: markdown
+
+		{exec-at-unpause pause}
+		```slip-script
+                slip.state.x = 1;
+                ```
+
+		{exec-at-unpause pause}
+		```slip-script
+                console.log(slip.state.x); // 1
+                ```

test/engine/basic.t/new_engine.md

@@ -1,5 +1,31 @@
 Salut !
 
+{#id}
+Hello
+
+{exec-at-unpause pause}
+```slip-script
+let set_class = (elem, name, value) => {
+  let old_value = elem.style[name];
+  elem.style[name] = value;
+  slip.onUndo(() => {elem.style[name] = old_value})
+}
+let elem = document.querySelector("#id");
+set_class(elem, "backgroundColor", "yellow");
+set_class(elem, "color", "red");
+```
+
+{exec-at-unpause pause}
+```slip-script
+console.log("setting state to 1");
+slip.state.one = 1
+```
+
+{exec-at-unpause pause}
+```slip-script
+console.log("state is", slip.state.one);
+```
+
 {#a .unstatic}
 a