[add] Dimensions event listeners

Close #511

src/apis/Dimensions/__tests__/index-test.js

@@ -1,5 +1,23 @@
 /* eslint-env jasmine, jest */
 
+import Dimensions from '..';
+
 describe('apis/Dimensions', () => {
-  test.skip('NO TEST COVERAGE', () => {});
+  test('addEventListener', () => {
+    const handler = jest.fn();
+    Dimensions.addEventListener('change', handler);
+    Dimensions.set();
+    expect(handler).toHaveBeenCalledTimes(1);
+    expect(handler).toHaveBeenLastCalledWith({
+      window: Dimensions.get('window'),
+      screen: Dimensions.get('screen')
+    });
+  });
+
+  test('removeEventListener', () => {
+    const handler = jest.fn();
+    Dimensions.removeEventListener('change', handler);
+    Dimensions.set();
+    expect(handler).toHaveBeenCalledTimes(0);
+  });
 });

src/apis/Dimensions/index.js

@@ -27,6 +27,7 @@ const win = canUseDOM
     };
 
 const dimensions = {};
+const listeners = {};
 
 export default class Dimensions {
   static get(dimension: string): Object {
@@ -48,6 +49,21 @@ export default class Dimensions {
       scale: win.devicePixelRatio || 1,
       width: win.screen.width
     };
+
+    if (Array.isArray(listeners['change'])) {
+      listeners['change'].forEach(handler => handler(dimensions));
+    }
+  }
+
+  static addEventListener(type, handler): void {
+    listeners[type] = listeners[type] || [];
+    listeners[type].push(handler);
+  }
+
+  static removeEventListener(type, handler): void {
+    if (Array.isArray(listeners[type])) {
+      listeners[type] = listeners[type].filter(_handler => _handler !== handler);
+    }
   }
 }