Peter Lubbers - Pro HTML 5 Programming

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Download from Wow! eBook 112 CHAPTER 4 ■ USING THE HTML5 GEOLOCATION API Let’s break that example down a little. As before, we’ve got a function to update our status value on the page, as shown in the following example. function updateStatus(message) { document.getElementById("status").innerHTML = message; } Our status here will be a simple text display, although this approach applies equally well for more compelling graphical status displays (see Listing 4-16). Listing 4-16. Showing the status navigator.geolocation.getCurrentPosition(updateLocation, handleLocationError, {timeout:10000}); // 10 second timeout value updateStatus(“Requesting location data…”); Once again, we use the Geolocation API to get the user’s current position, but with a set timeout of ten seconds. Once ten seconds have elapsed, we should either have a success or failure due to the timeout option. We immediately update the status text display to indicate that a position request is in progress. Then, once the request completes or ten seconds elapses—whichever comes first—you use the callback method to reset the status text, as shown in Listing 4-17. Listing 4-17. Resetting the status text function endRequest() { updateStatus("Done."); } function updateLocation(position) { endRequest(); // handle the position data } A simple extra, but easy to extend. This technique works well for one-shot position lookups because it is easy for the developer to determine when a position lookup request starts. The request starts as soon as the developer calls getCurrentPosition(), of course. However, in the case of a repeated position lookup via watchPosition(), the developer is not in control of when each individual position request begins. Furthermore, the timeout does not begin until the user grants permission for the geolocation service to access position data. For this reason, it is impractical to implement a precise status display because the page is not informed during the instant when the user grants permission.
Show Me on a Google Map CHAPTER 4 ■ USING THE HTML5 GEOLOCATION API One very common request for geolocation data is to show a user’s position on a map, such as the popular Google Maps service. In fact, this is so popular that Google itself built support for HTML5 Geolocation into its user interface. Simply press the Show My Location button (see Figure 4-5); Google Maps will use the Geolocation API (if it is available) to determine and display your location on the map. Figure 4-5. Google Map’s Show My Location button However, it is also possible to do this yourself. Although the Google Map API is beyond the scope of this book, it has (not coincidentally) been designed to take decimal latitude and longitude locations. Therefore, you can easily pass the results of your position lookup to the Google Map API, as shown in Listing 4-18. You can read more on this subject in Beginning Google Maps Applications, Second Edition (Apress, 2010). 113
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THE EXPERT’S VOICE ® IN WEB DEVE
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■ ■ ■ Pro HTML5 Programming P
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For my beautiful wife, Vittoria, an
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Contents ■Foreword ..............
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■ CONTENTS Implementing Canvas Se
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■ CONTENTS Setting and Retrieving
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Foreword In June 2004, representati
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About the Technical Reviewer � Pa
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Introduction HTML5 is brand new. In
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C H A P T E R 1 ■ ■ ■ Overvie
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CHAPTER 1 ■ OVERVIEW OF HTML5 wit
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Interoperability Simplification CHA
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• MathML • Microdata • Server
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} } Summary incomingRow = docume
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Peter Lubbers - Pro HTML 5 Programming

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