Step 5 Complete

src/connection.js

@@ -0,0 +1,49 @@
+import { ProtocolError } from './validation.js';
+
+export function exhibitURL(value, base) {
+  if (!value.trim()) throw new Error('Enter an exhibit URL or served path.');
+  const url = new URL(value.trim(), base);
+  if (!['http:', 'https:'].includes(url.protocol) || url.origin !== new URL(base).origin || url.username || url.password) {
+    throw new Error('Enter a same-origin HTTP exhibit URL or served path.');
+  }
+  return url.href;
+}
+
+// Frame lifecycle only; protocol negotiation remains in ExhibitHost.
+export class ExhibitConnection {
+  constructor({ host, createFrame, transport, base, changed = () => {}, loadTimeoutMs = 15000 }) {
+    Object.assign(this, { host, createFrame, transport, base, changed, loadTimeoutMs });
+    this.frame = null; this.url = ''; this.loading = false; this.generation = 0;
+  }
+  disconnect() {
+    this.generation++; clearTimeout(this.timer); this.loading = false;
+    this.frame?.remove(); this.frame = null; this.host.disconnect(); this.changed();
+  }
+  load(value) {
+    const url = exhibitURL(value, this.base); // Invalid input preserves the active exhibit.
+    this.disconnect(); this.url = url; this.loading = true;
+    const generation = this.generation;
+    const frame = this.createFrame(); this.frame = frame;
+    frame.addEventListener('load', () => {
+      if (generation !== this.generation) return;
+      clearTimeout(this.timer); this.loading = false; this.attach();
+    });
+    this.timer = setTimeout(() => {
+      if (generation !== this.generation) return;
+      this.disconnect();
+      this.host.report(new ProtocolError('LOAD_TIMEOUT', 'Exhibit did not finish loading. Check the served path and reconnect.'));
+    }, this.loadTimeoutMs);
+    frame.src = url; this.changed();
+  }
+  async attach() {
+    if (!this.frame || this.loading) return;
+    const generation = this.generation;
+    try { await this.host.connect(this.transport(this.frame)); }
+    catch (error) { if (generation === this.generation && !error.code) this.host.report(error); }
+    this.changed();
+  }
+  reconnect() {
+    if (this.frame && !this.loading) return this.attach();
+    if (this.url) this.load(this.url);
+  }
+}