Add document title handling

client/src/js/util/observe.js

@@ -0,0 +1,44 @@
+function subscribe(store, selector, handler) {
+  let prev = selector(store.getState());
+  handler(prev);
+
+  store.subscribe(() => {
+    const next = selector(store.getState());
+    if (next !== prev) {
+      handler(next);
+      prev = next;
+    }
+  });
+}
+
+function subscribeArray(store, selectors, handler) {
+  let state = store.getState();
+  let prev = selectors.map(selector => selector(state));
+  handler(...prev);
+
+  store.subscribe(() => {
+    state = store.getState();
+    const next = [];
+    let changed = false;
+
+    for (let i = 0; i < selectors.length; i++) {
+      next[i] = selectors[i](state);
+      if (next[i] !== prev[i]) {
+        changed = true;
+      }
+    }
+
+    if (changed) {
+      handler(...next);
+      prev = next;
+    }
+  });
+}
+
+export default function observe(store, selector, handler) {
+  if (Array.isArray(selector)) {
+    subscribeArray(store, selector, handler);
+  } else {
+    subscribe(store, selector, handler);
+  }
+}