Add method to check if property is observed by given observer

js/src/observe.js

@@ -61,12 +61,23 @@ export default class uObserve {
     }
   }
 
-  static isObserved(obj, property) {
+  /**
+   * Check if object property is observed;
+   * Optionally check if it is observed by given observer
+   * @param {Object} obj
+   * @param {string} property
+   * @param {Function=} observer
+   * @return {boolean}
+   */
+  static isObserved(obj, property, observer) {
     if (typeof obj !== 'object' || obj === null || !obj.hasOwnProperty(property)) {
       return false;
     }
-    return obj.hasOwnProperty('_values') && obj._values.hasOwnProperty(property) &&
+    const isObserved = !!(obj._observers && obj._observers[property] && obj._observers[property].size > 0);
-      !!Object.getOwnPropertyDescriptor(obj, property)['set'];
+    if (isObserved && observer) {
+      return obj._observers[property].has(observer);
+    }
+    return isObserved;
   }
 
   /**
@@ -94,13 +105,16 @@ export default class uObserve {
   /**
    * Observe object's property. On change call observer
    * @param {Object} obj
-   * @param {?string} property
+   * @param {string} property
    * @param {ObserveCallback} observer
    */
   static observeProperty(obj, property, observer) {
     if (!obj.hasOwnProperty(property)) {
       throw new Error(`Invalid argument: object does not have property "${property}"`);
     }
+    if (this.isObserved(obj, property, observer)) {
+      throw new Error(`Observer already registered for property ${property}`);
+    }
     this.addObserver(obj, observer, property);
     if (!obj.hasOwnProperty('_values')) {
       Object.defineProperty(obj, '_values', { enumerable: false, configurable: false, value: {} });

js/test/observe.test.js

@@ -528,6 +528,53 @@ describe('Observe tests', () => {
       // then
       expect(uObserve.isObserved(object, 'observed')).toBe(true);
     });
+
+    it('should return false if property is not observed', () => {
+      // when
+      uObserve.observe(object, 'observed', (value) => {
+        result = true;
+        resultValue = value;
+      });
+      // then
+      expect(uObserve.isObserved(object, 'nonObserved')).toBe(false);
+    });
+
+    it('should return true if array property is observed', () => {
+      // when
+      const array = [ 1, 2 ];
+      object = { array: array };
+      uObserve.observe(object, 'array', (value) => {
+        result = true;
+        resultValue = value;
+      });
+      // then
+      expect(uObserve.isObserved(object, 'array')).toBe(true);
+    });
+
+    it('should return false if property is unobserved', () => {
+      // when
+      const observer = (value) => {
+        result = true;
+        resultValue = value;
+      };
+      uObserve.observe(object, 'observed', observer);
+      uObserve.unobserve(object, 'observed', observer);
+      // then
+      expect(uObserve.isObserved(object, 'observed')).toBe(false);
+    });
+
+    it('should return true if property is observed by given observer', () => {
+      // when
+      const observer = (value) => {
+        result = true;
+        resultValue = value;
+      };
+      const observer2 = () => {/* ignored */};
+      uObserve.observe(object, 'observed', observer);
+      // then
+      expect(uObserve.isObserved(object, 'observed', observer)).toBe(true);
+      expect(uObserve.isObserved(object, 'observed', observer2)).toBe(false);
+    });
   });
 
   describe('when notify is called directly', () => {