vb.net print barcode labels The accelerometers and movement in Objective-C

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10.2 The accelerometers and movement
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When you use orientation notification, the frameworks do the work for you: they take low-level acceleration reports and turn them into more meaningful events. It s similar to the concept of actions, which turn low-level touch events into high-level control events.
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WARNING Accelerometer programs can t be tested on the Simulator. Instead, you need to have a fully provisioned iPhone or iPad to test your code. See appendix C for information about provisioning your device.
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Notifications aren t sufficient if you want to program entire interfaces that effectively use the device s movement in three-dimensional space as a new user-input device. For that, you need to access two classes: UIAccelerometer and UIAcceleration. Let s look at accessing and parsing data from UIAccelerometer. Later in the section, you ll use the accelerometers to check for gravity and movement.
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10.2.1 Accessing the UIAccelerometer
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UIAccelerometer is a class you can use to receive acceleration-related data. It s a shared object, like UIApplication and UIDevice. The process of using it is as follows:
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- (void)viewDidLoad { UIAccelerometer *myAccel = [UIAccelerometer sharedAccelerometer]; myAccel.updateInterval = .1; myAccel.delegate = self; [super viewDidLoad]; }
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The accelerometers and movement
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The first step is to access the accelerometer, which you do with another call to a shared-object method. Having this step on its own line is probably unnecessary, because you could perform the other two steps as nested calls, but we find this a lot more readable. Next, you select your update interval, which specifies how often you receive information about acceleration. This is hardware limited, with a current default of 100 updates per second. That s most likely just right if you re creating a game using the accelerometer, but it s excessive for other purposes. We ve opted for 10 updates per second, which is an updateInterval of 0.1. You should always set the lowest acceptable input to preserve power on the device. Finally, you must set a delegate for the accelerometer, which is how you receive data on accelerometer changes. The delegate needs to respond to only one method, accelerometer:didAccelerate:, which sends a message containing a UIAcceleration object whenever acceleration occurs (to the limit of the updateInterval). Note that the class that utilizes this mechanism needs to declare the UIAccelerometerDelegate protocol in the interface.
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10.2.2 Parsing the UIAcceleration
+y You can use UIAcceleration information to accurately and easily measure two things: the device s relationship to gravity and its movement through three-z dimensional space. These are both done through a set of three properties x, y, and z, which refer to the three-x dimensional axes, as shown in figure 10.1. +x The x-axis measures along the short side of the iPhone or iPad, the y-axis measures along the long +z side, and the z-axis measures through the device. All values are measured in units of g, which is to say g-force. A value of 1 g represents the force of gravity on -y Earth at sea level. The thing to watch for when accessing the acceler- Figure 10.1 The accelerometers ometer is that it measures two types of force applied to measure acceleration in threethe device: both the force of movement in any direc- dimensional space. tion and the force of gravity, measured in units of g. That means an iPhone or iPad at rest always shows an acceleration of 1 g toward the Earth s core. This may require filtering if you re doing more sophisticated work.
10.2.3 Checking for gravity
When the accelerometers are at rest, they naturally detect gravity. You can use this feature to detect the precise orientation an iPhone or iPad is currently held in, going far beyond the four or six states supported by the orientation variables.
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