function getViewById(viewId) {
    return (viewId ? viewHistory.views[ viewId ] : null);
  }
@ngdoc service
@name $ionicHistory
@module ionic
@description
$ionicHistory keeps track of views as the user navigates through an app. Similar to the way a
browser behaves, an Ionic app is able to keep track of the previous view, the current view, and
the forward view (if there is one).  However, a typical web browser only keeps track of one
history stack in a linear fashion.

Unlike a traditional browser environment, apps and webapps have parallel independent histories,
such as with tabs. Should a user navigate few pages deep on one tab, and then switch to a new
tab and back, the back button relates not to the previous tab, but to the previous pages
visited within _that_ tab.

`$ionicHistory` facilitates this parallel history architecture.