Peter Lubbers - Pro HTML 5 Programming

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CHAPTER 4 ■ USING THE HTML5 GEOLOCATION API
■ Note Curiously, the high accuracy setting is only a toggle switch: true or false. The API was not
created to allow the accuracy to be set to various values or a numeric range. Perhaps this will be
addressed in future versions of the specification.
• timeout—This optional value, provided in milliseconds, tells the browser the
maximum amount of time it is allowed to calculate the current location. If the
calculation does not complete in this amount of time, the error handler is called
instead. This defaults the value of Infinity, or no limit.
• maximumAge—This value indicates how old a location value can be before the
browser must attempt to recalculate. Again, it is a value in milliseconds. This value
defaults to zero, meaning that the browser must attempt to recalculate a value
immediately.
■ Note You might be wondering what the difference is between the timeout and maximumAge options.
Although similarly named, they do have distinct uses. The timeout value deals with the duration needed
to calculate the location value, while maximumAge refers to the frequency of the location calculation. If
any single calculation takes longer than the timeout value, an error is triggered. However, if the browser
does not have an up-to-date location value that is younger than maximumAge, it must refetch another
value. Special values apply here: setting the maximumAge to “0” requires the value to always be refetched,
while setting it to Infinity means it should never be refetched.
However, note that the API does not allow us to tell the browser how often to recalculate the
position. This is left entirely up to the browser implementation. All we can do is tell the browser what the
maximumAge is of the value it returns. The actual frequency is a detail we cannot control.
Let’s update our location request to include an optional parameter using shorthand notation, as
shown in the following example:
navigator.geolocation.getCurrentPosition(updateLocation,handleLocationError,
{timeout:10000});
This new call tells HTML5 Geolocation that any request for location that takes longer than 10
seconds (10000 milliseconds) should trigger an error, in which case the handleLocationError function
will be called with the TIMEOUT error code.
Repeated Position Updates
Sometimes once is not enough. Thankfully, the designers of the Geolocation service made it trivial to
switch from an application that requests a user location one time to one that requests the location at
regular intervals. In fact, it’s largely as trivial as switching the request call, as shown in the following
examples: