crmMailSubjectReview.js

(function(angular, $, _) {
	// Declare Angular module
	var crmMailSubjectReview = angular.module( 'crmMailSubjectReview', [] );

	// Hook into crmMailing module and append directive into crmMailingBlockReview directive
	angular.module( 'crmMailing' ).config( function( $provide ){
		$provide.decorator( 'crmMailingBlockReviewDirective', function( $delegate ){
			var directive = $delegate[0];
			
			var link = directive.link;

			directive.compile = function( Element, Attrs ){
				Element.children( ":first" ).children( ":first" ).children( ":first" ).children( ":nth-child(2)" ).after( '<div crm-subject-review></div>' );
				return function( scope, elem, attr ){
					link.apply( this, arguments );
				};
			};
			return $delegate;
		});
	});

	// crmSubjectReview directive
	angular.module( 'crmMailing' ).directive( 'crmSubjectReview', function(){
  		return { templateUrl: CRM.resourceUrls['uk.menas.subcustom'] + '/js/crmSubjectReview.html',
  			controller: 'crmSubjectReviewCtrl',
  		};
  	});

  	// crmSubjectReview factory
	angular.module('crmMailing').factory('crmSubjectReviewFactory', function(){
		return {
			editEnabled: false,
			enableEdit: function() {
    			this.editEnabled = true;
  			},
  			disableEdit: function() {
    			this.editEnabled = false;
  			}
		}
	});

	// crmSubjectReview controller
  	angular.module( 'crmMailing' ).controller( 'crmSubjectReviewCtrl', function( $scope, crmSubjectReviewFactory ){
  		$scope.crmSubjectReview = crmSubjectReviewFactory;
	});

})(angular, CRM.$, CRM._);

here's the directive

<div crm-ui-field="{title: ts('Subject')}">
    <div ng-hide="crmSubjectReview.editEnabled">
      	{{mailing.subject}}
      	<a class="crm-hover-button action-item" ng-click="crmSubjectReview.enableEdit()">{{ts('Edit')}}</a>
    </div>
    <div ng-show="crmSubjectReview.editEnabled">
      	<input calss="crm-form-text" name="subject" ng-model="mailing.subject" ng-show="crmSubjectReview.editEnabled">
      	<a class="crm-hover-button action-item" ng-click="crmSubjectReview.disableEdit()">{{ts('Done')}}</a>
    </div>
</div>

and here the result