[CCA] Add observer pattern to cca.state.

Bug: None
Test: By instrumented test code.
Change-Id: I7af7ada02e2a2bf09a7f174af03dd8c29e46c85e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1775686
Commit-Queue: Kuo Jen Wei <inker@chromium.org>
Reviewed-by: Shik Chen <shik@chromium.org>
Auto-Submit: Kuo Jen Wei <inker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691904}

chrome/browser/resources/chromeos/camera/src/js/state.js

@@ -14,6 +14,42 @@ var cca = cca || {};
  */
 cca.state = cca.state || {};
 
+/**
+ * @type {!Map<string, Set<!function(boolean)>>}
+ * @private
+ */
+cca.state.observers_ = new Map();
+
+/**
+ * Adds observer function to be called on any state change.
+ * @param {string} state State to be observed.
+ * @param {!function(boolean)} observer Observer function called with newly
+ *     changed value.
+ */
+cca.state.addObserver = function(state, observer) {
+  let observers = cca.state.observers_.get(state);
+  if (observers === undefined) {
+    observers = new Set();
+    cca.state.observers_.set(state, observers);
+  }
+  observers.add(observer);
+};
+
+/**
+ * Removes observer function to be called on state change.
+ * @param {string} state State to remove observer from.
+ * @param {!function(boolean)} observer Observer function to be removed.
+ * @return {boolean} Whether the observer is in the set and is removed
+ *     successfully or not.
+ */
+cca.state.removeObserver = function(state, observer) {
+  const observers = cca.state.observers_.get(state);
+  if (observers === undefined) {
+    return false;
+  }
+  return observers.delete(observer);
+};
+
 /**
  * Checks if the specified state exists.
  * @param {string} state State to be checked.
@@ -29,5 +65,11 @@ cca.state.get = function(state) {
  * @param {boolean} val True to set the state on, false otherwise.
  */
 cca.state.set = function(state, val) {
+  const oldVal = cca.state.get(state);
+  if (oldVal == val) {
+    return;
+  }
   document.body.classList.toggle(state, val);
+  const observers = cca.state.observers_.get(state) || [];
+  observers.forEach((f) => f(val));
 };