App Inventor for Android: Build Your Own Apps — No Experience Required!

Chapter 5: AndroidDown: A Location-Aware Panic Button

In This Chapter

• Using LocationSensor for GPS location information

• Using TinyDB for persistent data

• Using SMS texting capabilities

• Using deferred processing to avoid force closures while in a wait state

The AndroidDown application gives you a solid introduction to utilizing the LocationSensor to pull GPS coordinates for your application. The LocationSensor component can use GPS, network, or Wi-Fi location systems. In this application, you use the GPS provider. The LocationSensor needs to have a “lock” on the signals from GPS satellites before it can provide coordinates. If you just told the application to wait until the LocationSensor had a strong enough signal, the Android operating system would very likely decide your application had crashed.

When an application does not accept input from the user, it assumes the application is dead and force-closes the application. To avoid this, you will learn how to use deferred processing. Deferred processing uses a Clock timer component and a procedure to check for the desired state (in this case, the GPS lock) on a timed basis. As your application attempts to get a signal lock for the LocationSensor, it bounces back and forth between a timer and a procedure. Each time the timer calls the procedure, you increment a counter. Incrementing a counter is an important programming concept that is used to count repeated events. You can use the incrementing steps found in this project ...