feat(domUtil): add getPositionInParent function

Find an element's offset, then add it to the offset of the parent
until we are at the direct child of parentEl.
Use-case: find scroll offset of any element within a scroll container

js/utils/dom.js

@@ -12,6 +12,24 @@
 
   ionic.DomUtil = {
 
+    /*
+     * Find an element's offset, then add it to the offset of the parent
+     * until we are at the direct child of parentEl
+     * use-case: find scroll offset of any element within a scroll container
+     */
+    getPositionInParent: function(el, parentEl) {
+      var left = 0, top = 0;
+      while (el && el !== parentEl) {
+        left += el.offsetLeft;
+        top += el.offsetTop;
+        el = el.parentNode;
+      }
+      return {
+        left: left,
+        top: top
+      };
+    },
+
     ready: function(cb) {
       if(document.readyState === "complete") {
         window.rAF(cb);

test/js/utils/dom.js

@@ -0,0 +1,40 @@
+
+describe('js/utils/dom', function() {
+
+  describe('getPositionInParent', function() {
+    it('should return 0,0 if el===parentEl', function() {
+      var el = {};
+      expect(ionic.DomUtil.getPositionInParent(el, el)).toEqual({
+        left: 0,
+        top: 0
+      });
+    });
+    it('should return 0,0 if el is null', function() {
+      expect(ionic.DomUtil.getPositionInParent(null, null)).toEqual({
+        left: 0,
+        top: 0
+      });
+    });
+    it('should return element offset{Top,Left} of el if el is parent\'s child', function() {
+      var parent = {};
+      var el = {parentNode: parent, offsetLeft: 3, offsetTop: 2};
+      expect(ionic.DomUtil.getPositionInParent(el, parent)).toEqual({
+        left: 3,
+        top: 2
+      });
+    });
+    it('should return added element offset{Top,Left} of all children up to parent', function() {
+      var parent = {};
+      var child1 = {parentNode: parent, offsetLeft: 5, offsetTop: 6};
+      var child2 = {parentNode: child1, offsetLeft: 10, offsetTop: 11};
+      expect(ionic.DomUtil.getPositionInParent(child1, parent)).toEqual({
+        left: 5,
+        top: 6
+      });
+      expect(ionic.DomUtil.getPositionInParent(child2, parent)).toEqual({
+        left: 15,
+        top: 17
+      });
+    });
+  });
+});