Книга: Beginning Android

Are We There Yet? Are We There Yet? Are We There Yet?

Are We There Yet? Are We There Yet? Are We There Yet?

Sometimes you want to know not where you are now, or even when you move, but when you get to where you’re going. This could be an end destination, or it could be getting to the next step on a set of directions so you can give the user the next turn.

To accomplish this, LocationManager offers addProximityAlert(). This registers a PendingIntent, which will be fired off when the device gets within a certain distance of a certain location. The addProximityAlert() method takes the following as parameters:

• The latitude and longitude of the position that you are interested in.

• A radius, specifying how close you should be to that position for the Intent to be raised.

• A duration for the registration, in milliseconds — after this period, the registration automatically lapses. A value of -1 means the registration lasts until you manually remove it via removeProximityAlert().

• The PendingIntent to be raised when the device is within the “target zone” expressed by the position and radius.

Note that it is not guaranteed that you will actually receive an Intent if there is an interruption in location services or if the device is not in the target zone during the period of time the proximity alert is active. For example, if the position is off by a bit and the radius is a little too tight, the device might only skirt the edge of the target zone, or go by so quickly that the device’s location isn’t sampled while in the target zone.

It is up to you to arrange for an activity or intent receiver to respond to the Intent you register with the proximity alert. What you then do when the Intent arrives is up to you: set up a notification (e.g., vibrate the device), log the information to a content provider, post a message to a Web site, etc. Note that you will receive the Intent whenever the position is sampled and you are within the target zone — not just upon entering the zone. Hence, you will get the Intent several times, perhaps quite a few times depending on the size of the target zone and the speed of the device’s movement.

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