vb.net print barcode labels Positioning: accelerometers, location, and the compass in Objective-C

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Positioning: accelerometers, location, and the compass
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When we first introduced the iPhone and iPad, we highlighted a number of its unique features. Among them were three components that allow the device to figure out precisely where it is in space: a trio of accelerometers, which give it the ability to sense motion in three dimensions; a locational device (using either GPS or faux GPS), which lets it figure out where in the world it is; and a compass to figure out which direction it s facing. Other than accessing some basic orientation information, we haven t done much with these features. We ll now dive into these positioning technologies and examine how to use them in your programming. We ll start with some new ways to look at orientation data and then explain how to use the accelerometers, compass, and GPS in real applications.
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The accelerometers and orientation
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10.1 The accelerometers and orientation
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The easiest use of the accelerometers is to determine the device s current orientation. You already used the view controller s interfaceOrientation property, back in chapter 5. As we mentioned at the time, you can also access orientation information through the UIDevice object. It can provide more information and real-time access that isn t available using the view controller. You have two ways to access the UIDevice information: through properties and through a notification. Let s examine the orientation property first.
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10.1.1 The orientation property
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The easy way to access the UIDevice s orientation information is to look at its orientation property. You must first access the UIDevice itself, which you can do by calling a special UIDevice class method, pretty much the same way you access the UIApplication object:
UIDevice *thisDevice = [UIDevice currentDevice];
After you ve done this, you can get to the orientation property. It returns a constant drawn from UIDeviceOrientation. This looks exactly like the results from a view controller s orientation property except there are three additional values, shown in table 10.1. These three additional values are one reason you may want to access the UIDevice object rather than examine orientation using a view controller.
Table 10.1
UIDeviceOrientation lists seven types of the device orientation.
Constant Summary Device is vertical, right side up. Device is vertical, upside down. Device is horizontal, tilted left. Device is horizontal, tilted right. Device is lying on its back. Device is lying on its screen. Device is in an unknown state.
UIDeviceOrientationPortrait UIDeviceOrientationPortraitUpsideDown UIDeviceOrientationLandscapeLeft UIDeviceOrientationLandscapeRight UIDeviceOrientationFaceUp UIDeviceOrientationFaceDown UIDeviceOrientationUnknown
10.1.2 The orientation notification
The UIDevice class can also give you instant access to an orientation change when it occurs. This is done through a notification (a topic we introduced in chapter 6). The following code shows how to access this information:
[[UIDevice currentDevice] beginGeneratingDeviceOrientationNotifications]; [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(deviceDidRotate:)
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Positioning: accelerometers, location, and the compass
name:@"UIDeviceOrientationDidChangeNotification" object:nil];
This is a two-step process. First, you alert the device that you re ready to start listening for a notification about an orientation change. This is one of a pair of UIDevice instance methods, the other being endGeneratingDeviceOrientationNotifications. You generally should leave notifications on only when you need them, because they take up CPU cycles and increase your power consumption. Second, you register to receive the UIDeviceOrientationDidChangeNotification messages, the first live example of the notification methods we introduced in chapter 2. Then, whenever an orientation change notification occurs, the deviceDidRotate: method is called. Note that you don t receive notification of what the new orientation is; you only know that a change happened. For more details, you have to query the orientation property. You ve now seen the two ways in which orientation can be tracked with the UIDevice object, providing more information and more rapid notification than you receive when using the view controller. But that only touches the surface of what you can do with the device s accelerometers. It s the raw data about changes in threedimensional space that you ll really want to access.
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