3 files changed, 79 insertions, 0 deletions
diff --git a/ui/lib/components/swatch/EventLog.svelte b/ui/lib/components/swatch/EventLog.svelte
new file mode 100644
index 0000000..2d8c5c0
--- /dev/null
+++ b/ui/lib/components/swatch/EventLog.svelte
@@ -0,0 +1,37 @@
+<script>
+  /*
+   * The interface exposed by this component is designed to be compatible with the interface
+   * exposed by the `EventCapture` class, so that you can do this:
+   *
+   *    let capture = $state(new EventCapture());
+   *    const someEvent = capture.on('someEvent');
+   *
+   *    // …
+   *
+   *    <EventLog events={capture.events} clear={capture.clear.bind(capture)} />
+   */
+
+  let { events, clear = () => {} } = $props();
+
+  function onclick() {
+    clear();
+  }
+</script>
+
+<button {onclick}>clear</button>
+<table>
+  <thead>
+    <tr>
+      <th>event</th>
+      <th>arguments</th>
+    </tr>
+  </thead>
+  <tbody>
+    {#each events as { event, args }}
+      <tr>
+        <td>{event}</td>
+        <td><code>{JSON.stringify(args)}</code></td>
+      </tr>
+    {/each}
+  </tbody>
+</table>
diff --git a/ui/lib/swatch/derive.js b/ui/lib/swatch/derive.js
new file mode 100644
index 0000000..85547e8
--- /dev/null
+++ b/ui/lib/swatch/derive.js
@@ -0,0 +1,20 @@
+// A fun fact about $derive: if the deriving expression throws an error, then Svelte (at least as of
+// 5.20.2) _permanently_ stops evaluating it, even if the inputs change. To prevent this, you need
+// to ensure that derive expressions never raise an exception.
+function tryDerive(args, func, fallback) {
+  try {
+    return func(...args);
+  } catch (e) {
+    console.debug('deriver threw exception', e, func, args);
+    return fallback;
+  }
+}
+
+// A "deriver" is a function that never raises; if the underlying function would raise, the
+// corresponding deriver instead returns a fallback value (or `undefined`).
+export function makeDeriver(func, fallback) {
+  return (...args) => tryDerive(args, func, fallback);
+}
+
+// Some widely-used derivers, for convenience.
+export const json = makeDeriver(JSON.parse);
diff --git a/ui/lib/swatch/event-capture.svelte.js b/ui/lib/swatch/event-capture.svelte.js
new file mode 100644
index 0000000..32c0f39
--- /dev/null
+++ b/ui/lib/swatch/event-capture.svelte.js
@@ -0,0 +1,22 @@
+/*
+ * The interface exposed by this class is designed to closely match the interface expected by
+ * the `EventLog` component, so that you can do this:
+ *
+ *    let capture = $state(new EventCapture());
+ *    const someEvent = capture.on('someEvent');
+ *
+ *    // …
+ *
+ *    <EventLog events={capture.events} clear={capture.clear.bind(capture)} />
+ */
+export default class EventCapture {
+  events = $state([]);
+
+  on(event) {
+    return (...args) => this.events.push({ event, args });
+  }
+
+  clear() {
+    this.events = [];
+  }
+}