为什么在 Angular 指令的链接函数中“this”为空？

我正在尝试使用 TypeScript 和 Angular 编写动态模板，但由于某种原因，“this”关键字始终为空，因此我无法访问我的私有成员 $compile。有任何想法吗？非常感谢！ :-)

指示：

namespace ROD.Features.Player {
    "use strict";

    export class VideoDirective implements ng.IDirective {
        public restrict: string = "E";
        public replace: boolean = true;
        public scope = {
            content: "="
        };

        constructor(private $compile: ng.ICompileService) {
        }

        public link(element: JQuery, scope: ng.IScope): any {
            const youtubeTemplate = "<p>Youtube</p>";
            const vimeoTemplate = "<p>Vimeo</p>";

            var linkFn = this.$compile(youtubeTemplate);
            const content: any = linkFn(scope);
            element.append(content);
        }
    }
}

App.ts:

namespace ROD {
    "use strict";
    angular.module("rodApp", [])
        .service("Settings", [() => new Settings.DevelopmentSettings()])
        .service("RedditService", [
            "$http", "Settings",
            ($http: ng.IHttpService, settings: Settings.ISettings) => new Services.Reddit.RedditService($http, settings.sourceUrl),
        ])
        .directive("videoItem", ["$compile",
            ($compile: ng.ICompileService) => new Features.Player.VideoDirective($compile)])
        .controller("PlayerController", [
            "$scope", "RedditService",
            ($scope: any, redditService: Services.Reddit.IRedditService) => new Features.Player.PlayerController($scope, redditService),
        ]);
}

看来我对链接函数使用了错误的语法。这是正确的实现：

        public link = (element: JQuery, scope: ng.IScope): any => {
            const youtubeTemplate = "<p>Youtube</p>";
            const vimeoTemplate = "<p>Vimeo</p>";

            var linkFn = this.$compile(youtubeTemplate);
            const content: any = linkFn(scope);
            element.append(content);
        }

谁能解释这是为什么吗？ :-)