Lobbing One Over the Fence

Classic IPC is one‑way: the client calls functions on the service. It is possible, through the creative use of AIDL, to allow the service to call back into an activity. However, this is a bit fragile, as the service may not know if the activity is still around or if it has been killed off to free up some memory.

An alternative approach, first mentioned in Chapter 23 which discusses filters, is to have the service send a broadcast Intent that can be picked up by the activity… assuming the activity is still around and is not paused. We will examine the client side of this exchange in Chapter 31; for now, let us examine how the service can send a broadcast.

The theory behind the implementation is that the service gets “tickled” when the device (or emulator) position changes. At that point, the service calls out to the Web service and generates a new forecast Web page for the activity to display. At the same time, though, the service also sends a broadcast, to alert the activity that there is a page update available if it wants it.

Here is the high‑level implementation of the aforementioned flow:

Private updateForecast

Format getLatitude

GetLongitude

New HttpGet

Try

New BasicResponseHandler

Execute

GeneratePage buildForecasts

Synchronized this