ViewerService 类主要是处理图片浏览器的大部分业务逻辑，包括打开图片、关闭浏览器、放大图

片、缩小图片、浏览上一张图片、浏览下一张图片等功能，在这里需要再做一次说明，ViewerService

是有状态的Java 对象。-ViewerService principal is that most of the business logic processing image viewer, including open the picture, close your browser, Zoom, out pictures, browse the picture, the next picture browsing and other functions described here need to do it again, ViewerService stateful Java objects.