An application I'm building uses a local Service for downloading files from the web to the phone's SD card. In this app users can browse lists of books, and read them while online. A user can also download a pdf copy of a book for offline viewing.

To handle downloads I'm using a locally bound Service. I do not want this Service to run all the time, only when downloading files. So that the Service can shut itself down when its tasks are complete, I am not binding to the service, rather I'm sending an "enqueue for download" command through the Intent passed to Context.startService.

Books available for download are shown in a list. A user can choose to download a book by clicking on its row in the list. On download, I need to show download progress using a ProgressBar on the actual book list row. I need to also show, on the rows, if a book is enqueued for download, or if its download has completed or failed. The books can be shown in different activities throughout the application--in search, or in the user's list of favorite books, for example. When the books are shown in different places, these are not the same objects, but they are uniquely identified by their bookId.

Because I do not want to bind to the service from every Activity, my tentative plan was to use a public static final HashMap on the Service class itself to contain a mapping of bookId to download status, an enum of enqueued, downloading, cancelled, etc. Each book view, when displayed, would check this static HashMap, and if the bookId is in the map, retrieve and display its status. I don't particularly like this idea, but at the moment it's the only way I can think of to retrieve status from the Service without having to bind to it and start it.

Additionally I need to retrieve download progress percent from the Service, for a given bookId, if it is the active download. Again I'd rather not bind to the service from every activity, so I'm not sure how to go about retrieving current progress from the Service. My current plan is to use some sort of singleton mediator, that the Service will push updates to, and the views can read from. But I'm not terribly happy with this idea.

The reason I'd like to avoid binding to the Service from each Activity is 1.) I'm already running another Service and 2.) binding is verbose and I'd like to avoid needing to pass around a reference to the Service (but admittedly this isn't too much of a problem).

Perhaps binding to the local Service isn't expensive enough to warrant this other setup? Should I not be concerned about binding to it from each Activity? Maybe this is a non-issue?