You parked somewhere near the stadium, but when the concert ends you don't have a clue where the car is. The friends you came with are equally as clueless.
Fortunately you haven't lost your phone that never forgets anything, and you remember you have the hot new app, Where's My Car?. With this app, you click a button when you park your car, and the phone uses its location sensor to record the car's GPS coordinates and address. Later, when you reopen the app, it shows you a map from where you are to the remembered location-- problem solved!
The app demonstrates how to communicate with the phone's location sensor, how to record data in the phone's long-term memory (database), and how you can open the Google Maps app from your app to show directions from one one location to another. It makes use of the following App Inventor components:
Here are the components for the Where's My Car? app, as shown in the Component Designer:
The user interface consists of labels to show location data and buttons to initiate events. Some labels just show static text, e.g., GPSLabel is the text "GPS:" that appears in the user interface. Others, such as CurrentLatLabel, will display dynamic data one the location sensor gets its readings. For these labels, a default value is set (0) here in the Component Designer.
The ActivityStarter1 component is used to launch the map when the user asks for directions. Its properties are only partially shown above. Here is how they should be specified:
Here are the blocks for the Where's My Car app (the yellow annotations will also appear when you load this app into App Inventor):
Let's examine the four different event-handlers of the app, starting in the top-left and working around in counter-clockwise order.
LocationSensor1.LocationChanged: This event occurs when the phone's location sensor first gets a reading, or when the phone is moved to produce a new reading. The event-handler just places the readings--latitude, longitude, and current (street) address-- into the corresponding "Current" labels so that they appear on the phone. The RememberButton is also enabled in this event-handler. Its enabled setting should be unchecked in the Component Designer because there is nothing for the user to remember until the sensor gets a reading.
RememberButton.Click: When the user clicks the RememberButton , the location sensor's current readings are put into the "remember" labels and stored to the database as well. The DirectionsButton is enabled as it now makes sense for the user click on it to see a map (though it will make more sense once the user changes location).
DirectionsButton.Click: When the user clicks the DirectionsButton , the event-handler builds a URL for a map and calls ActivityStarter to launch the Maps application and load the map. join is used to build the URL to send to the Maps application. The resulting URL consists of the Maps domain along with two crucial parameters, saddr and daddr , which specify the start and destination for the directions. For this app, the saddr is set to the latitude and longitude of the current location, and the daddr is set to the latitude and longitude of the location that was "remembered" (the location of your car!).
Screen1.Initialize: This event is always triggered when an app opens. To understand it, you have to envision the user recording the location of the car, then closing the app, then later re-opening the app. When the app re-opens, the user expects that the location remembered earlier should appear on the phone. To facilitate this, the event-handler queries the database (call TinyDB.GetValue). If there is indeed a remembered address stored in the database-- the length of the stored address is greater than zero--the remembered latitude, longitude, and street addres are placed in the corresponding labels.
Here are some of the ideas covered in this tutorial:
Scan the following barcode onto your phone to install and run the sample app.
If you'd like to work with this sample in App Inventor, download the source code to your computer, then open App Inventor, click Projects, choose Import project (.aia) from my computer..., and select the source code you just downloaded.Download Refined Version (Book Chapter PDF)