WIP - feat($anchorScroll): add support for configurable scroll offset

Add support for a configurable scroll offset to $anchorScrollProvider.
The offset is expressed in pixels and can be specified either as a fixed
value or as a function that return the offset it pixels dynamically.
(This is a POC and a WIP: no docs, no tests)

Related to angular#9368

Closes angular#2070, angular#9360

Author: gkalpak

docs/app/src/app.js

@@ -17,7 +17,17 @@ angular.module('docsApp', [
   'ui.bootstrap.dropdown'
 ])
 
-
 .config(['$locationProvider', function($locationProvider) {
   $locationProvider.html5Mode(true).hashPrefix('!');
+}])
+
+.config(['$anchorScrollProvider', function($anchorScrollProvider) {
+  var header;
+  $anchorScrollProvider.setScrollOffset(function () {
+    header = header || window.document.querySelector('header');
+    if (!header) return 0;
+
+    var style = window.getComputedStyle(header);
+    return (style.position === 'fixed') ? header.offsetHeight : 0;
+  });
 }]);

src/ng/anchorScroll.js

@@ -53,12 +53,23 @@
  */
 function $AnchorScrollProvider() {
 
+  var DEFAULT_OFFSET = 0;
+
   var autoScrollingEnabled = true;
+  var scrollOffsetGetter = function() { return DEFAULT_OFFSET; };
 
   this.disableAutoScrolling = function() {
     autoScrollingEnabled = false;
   };
 
+  this.setScrollOffset = function(newScrollOffset) {
+    if (isFunction(newScrollOffset)) {
+      scrollOffsetGetter = function() { return newScrollOffset(); };
+    } else if (isNumber(newScrollOffset)) {
+      scrollOffsetGetter = function() { return newScrollOffset; };
+    }
+  };
+
   this.$get = ['$window', '$location', '$rootScope', function($window, $location, $rootScope) {
     var document = $window.document;
 
@@ -74,20 +85,33 @@ function $AnchorScrollProvider() {
       return result;
     }
 
+    function scrollTo(elem) {
+      if (elem) {
+        elem.scrollIntoView();
+      } else {
+        $window.scrollTo(0, 0);
+      }
+
+      var offset = scrollOffsetGetter();
+      if (offset) {
+        $window.scrollBy(0, -1 * offset);
+      }
+    }
+
     function scroll() {
       var hash = $location.hash(), elm;
 
       // empty hash, scroll to the top of the page
-      if (!hash) $window.scrollTo(0, 0);
+      if (!hash) scrollTo(null);
 
       // element with given id
-      else if ((elm = document.getElementById(hash))) elm.scrollIntoView();
+      else if ((elm = document.getElementById(hash))) scrollTo(elm);
 
       // first anchor with given name :-D
-      else if ((elm = getFirstAnchor(document.getElementsByName(hash)))) elm.scrollIntoView();
+      else if ((elm = getFirstAnchor(document.getElementsByName(hash)))) scrollTo(elm);
 
       // no element and hash == 'top', scroll to the top of the page
-      else if (hash === 'top') $window.scrollTo(0, 0);
+      else if (hash === 'top') scrollTo(null);
     }
 
     // does not scroll when user clicks on anchor link that is currently on
@@ -102,4 +126,3 @@ function $AnchorScrollProvider() {
     return scroll;
   }];
 }
-