The purpose of this demonstration app is to show one example\u00a0of using the accelerometer.<\/p>\n
The user interface is simple – it displays a Start<\/em> button and a Stop<\/em> button, and the status of the accelerometer.<\/p>\n <\/p>\n <\/p>\n At the bottom of the Designer Viewer<\/em>, you can see that three non-visible components have been added to this app. The AccelerometerSensor1<\/em> is the component that detects phone movement. Find the Accelerometer in the Sensors section of the Palette.<\/p>\n The Player1<\/em> component is a media player – for our app, when the phone is shaken, it plays an mp3 audio file that is specified as the Source<\/em> in\u00a0the properties for the player. Find the Player<\/em> component in the Media section of the Palette.<\/p>\n To specify the mp3 file, click on source and select an mp3 file from your disk drive. I used a sound effects file from Soundbible.com<\/a>.<\/p>\n Regardless of the length of the mp3 audio clip, in our app we will\u00a0limit the mp3 file to playing for just\u00a0two seconds (because we feel like it!<\/em>)<\/p>\n How do we limit the playing of the file to two seconds? By using the App Inventor clock timer. Find the Clock<\/em> in the Sensors<\/em> section of the Palette and\u00a0drag a Clock<\/em> timer to the Viewer – this becomes a non-visible component. The clock acts as a timer – creating a time event at specified intervals. We write a clock event handler to process the timer event. \u00a0We set the timer to a two second time and then when the timer event occurs, the event handler turns off the media player.<\/p>\n How This Works – The Blocks<\/strong><\/p>\n The Start<\/em> button activates the accelerometer by setting its Enabled<\/em> property to true. The sensitivity of the accelerometer – that is, how sensitive it is to motion – is set by adjusting the value of the Sensitivity property. Setting the value to 3 makes the accelerometer more sensitive, while a value of 1 makes the accelerometer less\u00a0sensitive. \u00a0For our demonstration app, the Sensitivity<\/em> ought to be set to 1 (less sensitive) but because the app is intended to show some features of the accelerometer, we will detect the amount of shaking in a different way.<\/p>\n The remaining features, while simple, are explained in sort of reverse order – meaning we show how to stop the player before we see how to start it! \u00a0We start with the Stop<\/em> button. When the Stop<\/em> button is pressed on screen, we disable the accelerometer, disable the timer, and disable the media player.<\/p>\n The accelerometer is disabled by setting its Enabled<\/em> property to false. The Clock’s<\/em> timer is disabled be setting the TimerEnabled<\/em> property to false, and the media player is stopped by calling its Stop<\/em> method.<\/p>\n The media player is started by calling it’s Start<\/em> method, as seen below. Because this set of blocks is very wide, the blocks are displayed very narrow – click on the image to display in a larger size. The rest of the code blocks, here, will be explained in a moment.<\/p>\n When the Clock’s<\/em> time interval has elapsed, the Clock<\/em> throws a Timer<\/em> event. When this event occurs, we stop the player and stop the clock timer:<\/p>\n Now, let’s go back and look at the Accelerometer event handler:<\/p>\n The Accelerometer<\/em> has an event called Shaking<\/em>. Rather than detect each individual movement of the phone, we can let Android run some code behind the scenes that determines if the phone is being shaken – versus merely moved. \u00a0We do not need to know how this works – it just happens by magic and simplifies our programming.<\/p>\n Remember that the Accelerometer<\/em> indicates the device’s motion in X, Y and Z axes. Values corresponding to these directions of movement are\u00a0found in the XAccel<\/em>, YAccel<\/em> and ZAccel<\/em> properties and represent meters per second squared<\/em>. \u00a0Values may be either negative or positive.<\/p>\n For our app, we want to restrict the shake detection to fairly aggressive shaking. To do this, an if-then<\/em> block determines if any of the X, Y or Z values are fairly large – and if any of these comparisons is true, then the player is turned on and the two second timer is activated.<\/p>\n Note that we use the “absolute” value of the X, Y and Z acceleration. \u00a0The absolute value function converts negative values to positive values (the absolute value of -5 is 5 and the absolute value of 5 is just 5, unchanged). The absolute value of each acceleration component is then compared to the value we have stored in a global variable named SHAKELIMIT<\/em>. This is set with a global initialize variable and set to a value of 18 – but you can experiment with this to see if other values work better for you. With the value set at 18, you need to give the phone a very good shake to activate the media player.<\/em><\/strong><\/p>\n The accelerometer may be used in other ways too – for example, it can be used to detect any device motion. Then the values of the X, Y and Z axis motion can be converted, using math, into angles, magnitude of the movement, and other values.<\/p>\n Downloads<\/strong><\/p>\n This tutorial introduces the accelerometer – at a high level – as a tool to detect the phone being shaken. You can\u00a0use this feature as another kind of user input to your app – for example, make a game where shaking the phone resets the game play or starts a new game. An accelerometer is … Continue reading Sensors: Using the Accelerometer to detect motion<\/span> ![\"AccelUI\"](\"http:\/\/appinventor.pevest.com\/wp-content\/uploads\/2014\/11\/AccelUI.png\")
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