#html
<p ng-controller="DemoCtrl">
    <textarea
            ng-model="content"
            placeholder="{{placeholder}}"
            ng-focus="focus()"
            ng-blur="blur()"></textarea>
</p>

#js
angular.module('app', [])

        .controller('DemoCtrl', function ($scope) {
            var defaultTitle = '#我是默认标题#';
            var defaultPlaceholder = '我是placeholder';
            $scope.placeholder = defaultTitle + ' ' + defaultPlaceholder;

            $scope.content = '';

            $scope.focus = function () {
                if (!$scope.content) {
                    $scope.content = defaultTitle;
                }
            };

            $scope.blur = function () {
                if ($scope.content === defaultTitle) {
                    $scope.content = '';
                }
            };
        })

I think it’s better to turn your question into a command. Here is my basic implementation.

html

<p ng-controller="main">
        <textarea my-textarea="#我是默认标题#" placeholder="我是placeholder" ng-model="myText" ng-change="log()"></textarea>
    </p>


    <script type="text/javascript">
    var app = angular.module('app', []);

    app.directive('myTextarea', function() {

        return {

            require: 'ngModel',

            link: function(scope, ele, attrs, modelController) {

                var text = attrs.myTextarea;
                var placeholder = attrs.placeholder;

                var alltext = text + ' ' + placeholder;

                ele.attr('placeholder', alltext);

                ele.on('focus', function() {
                    if (!modelController.$modelValue) {
                        setVal(text);
                    }

                });

                ele.on('blur', function() {
                    if (modelController.$modelValue === text) {
                        setVal('');
                    }
                });

                function setVal(v) {
                    modelController.$setViewValue(v);
                    modelController.$render();
                }

            }

        }

    });

    app.controller('main', ['$scope', function($scope){

        $scope.log = function() {
            console.log($scope.myText)
        }
    }]);

The basic idea is to control it through the model's controller.