In short:
Everytime 'shareTwitterUrl' or 'shareTwitterText' changes, it will call the share function.
From another stackoverflow answer: (https://stackoverflow.com/a/14907826/2874153)
$observe() is a method on the Attributes object, and as such, it can only be used to observe/watch the value change of a DOM attribute. It is only used/called inside directives. Use $observe when you need to observe/watch a DOM attribute that contains interpolation (i.e., {{}}'s). E.g., attr1="Name: {{name}}", then in a directive: attrs.$observe('attr1', ...). (If you try scope.$watch(attrs.attr1, ...) it won't work because of the {{}}s -- you'll get undefined.) Use $watch for everything else.
From Angular docs: (http://docs.angularjs.org/api/ng/type/$compile.directive.Attributes)
$compile.directive.Attributes#$observe(key, fn);
Observes an interpolated attribute.
The observer function will be invoked once during the next $digest fol lowing compilation. The observer is then invoked whenever the interpolated value changes.