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10.4 All about Core Location
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We have only one unique feature left to look at: the device s ability to detect a user s location.
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WARNING You can only minimally test Core Location using the Simulator. Longitude and latitude work, but they always report Apple s Cupertino headquarters. Altitude isn t displayed. For most realistic testing particularly including distance or altitude you must use a provisioned device.
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The original iPhone based its location information on the distance to cell phone towers and other set locations. Its accuracy could vary from a few blocks radius to a few miles, even in an urban area. The iPhone 3G and 3GS have a built-in GPS, but they still have limitations. The iPhone s antenna power is limited, which affects accuracy, and accuracy is further limited by concerns about power usage. As a result, even if you have an iPhone 3G or 3GS, the device makes preferential use of cell tower data and provides information about GPS locations using the minimal number of satellite contacts possible (although that minimum partially depends on an accuracy requirement that you set). This is also how the iPad 3G works. With all that said, the iPhone 3G and 3GS provide better location information than the original release, because cell tower triangulation is only one factor in its location information. But it may not be entirely accurate; in particular, altitude seems to be the least reliable information. The wi-fi only iPad can only determine your location based on its IP address, making it the least accurate.
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Positioning: accelerometers, location, and the compass
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We offer this preamble both to describe how the location information is created and to introduce a bit of skepticism about the results. What you get should be good enough for 99 percent of your programs, but you don t want to do anything mission critical unless you re careful. The good news is that you don t have to worry about which type of device a user owns. The Core Location API works identically whether they have a built-in GPS or not. In this section, we ll examine the location classes and how to use the compass. You ll also build two applications: one that finds the current location and distance traveled, and one that incorporates an altitude measurement.
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10.4.1 The location classes
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Location awareness is built into two SDK classes and one protocol. CLLocationManager gives you the ability to access location information in a variety of ways. It includes a delegate protocol, CLLocationManagerDelegate, which defines methods that can tell you when new location information arrives. Finally, the location information appears as CLLocation objects, each of which defines a specific location at a specific time. Table 10.2 describes the most important properties associated with each of these classes. For more details, you should, as usual, consult the Apple class references. You
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Table 10.2 The most important methods and properties for accessing location information Type Summary
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Method/Property Class: CLLocationManager delegate
Property
Defines the object that responds to CLLocationManagerDelegate Sets the desired accuracy of location as a CLLocationAccuracy object Specifies how much lateral movement must occur to cause a location update event Specifies the most recent location Starts generating update events Stops generating update events Starts generating heading update events Stops generating heading update events The minimum angle required to generate heading events
desiredAccuracy
Property
distanceFilter
Property
location startUpdatingLocation stopUpdatingLocation startUpdatingHeading
Property Method Method Method
stopUpdatingHeading
Method
headingFilter
Property
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All about Core Location
Table 10.2 The most important methods and properties for accessing location information (continued) Type Property Summary
Method/Property headingAvailable
Returns true if heading events can be generated
Class: CLLocationManagerDelegate locationManager:didUpdateToLocation: fromLocation: locationManager:didFailWithError: Method Delegate method that reports whenever an update event occurs Delegate method that reports whenever an update event fails to occur
Method
Class: CLLocation altitude Property Specifies the height of location in meters Returns the location s coordinates as a CLLocationCoordinate2D variable Specifies an NSDate of when the location was measured
coordinate
Property
timestamp
Property
should examine a number of additional properties and methods to aid with determining location (particularly for the CLLocation class), but we re staying with the basics here. Generally, location information is generated much like accelerometer information. You access a shared object (CLLocationManager) and set some standard properties for how you want it to work, including how often to update (distanceFilter). As with the accelerometer, you also have to explicitly turn on location updating (startUpdatingLocation). Afterward, you keep an eye on certain methods (as defined by CLLocationManagerDelegate). These methods generate an object (CLLocation) when location changes; you read the object to get the specifics. With those generalities out of the way, let s see how CLLocation works in a real example.
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