You may recall our discussion of watch faces in the previous chapter wherein a watch face starts out running in interactive mode. As the screen times out, the watch face continues to run as the device goes into its power-saving ambient mode.

While this feature, meaning the watch face's always-on capability, is inherent to watch faces—we don't want our watch to slack off when we want the time—it is not necessarily inherent to all wearable apps. For instance, if we had our todo app or Step counter app active, it would only be a matter of time before the screen timed out and rendered the watch face. If we then wanted to return to our app, we would have to interact with our wearable device to snap it out of ambient mode and bring up our last used activity or app. We can imagine scenarios where this can be a source of user frustration.

Fortunately, if our devices are running Android version 5.1 or higher, we can harness the power of the Android Wear APIs to conserve power...