Peter Lubbers - Pro HTML 5 Programming

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196 CHAPTER 8 ■ USING THE HTML5 WEB WORKERS API Let’s say that you want to build a simple Web Worker example that allows users to send a message to a worker, which in turn echoes back the message. This example may not be very useful in real life, but it’s useful enough to explain the concepts you need to build more complex examples. Figure 8-3 shows this example web page and its Web Worker in action. The code for this simple page is listed at the end of this section. Figure 8-3. A simple web page that uses HTML5 Web Workers To set up proper communication with your Web Worker, code has to be added to the main page (the page that calls the Web Worker) as well as the worker JavaScript file. Coding the Main Page To communicate from the page to the Web Worker, you call postMessage to pass in the required data. To listen to incoming messages and errors that are sent by the Web Worker to the page, you set up an event listener. To set up communication between the main page and the Web Worker, first add the call to postMessage to the main page, as follows:
document.getElementById("helloButton").onclick = function() { worker.postMessage("Here's a message for you"); } CHAPTER 8 ■ USING THE HTML5 WEB WORKERS API In the preceding example, a message is sent to the Web Worker when the user clicks the Post a Message button. Next, add an event listener to the page that listens for messages from the Web Worker: worker.addEventListener("message", messageHandler, true); function messageHandler(e) { // process message from worker } Coding the HTML5 Web Worker JavaScript File You must now add similar code to theWeb Worker JavaScript file—event handlers must be set up to process incoming messages and errors, and communication with the page is handled with a call to postMessage. To complete the communication between the page and the Web Worker, first, add the call to postMessage; for example, inside a messageHandler function: function messageHandler(e) { postMessage("worker says: " + e.data + " too"); } Next, add an event listener to the Web Worker JavaScript file that handles messages coming from the main page: addEventListener("message", messageHandler, true); In this example, the messageHandler function is called immediately when the message is received so that the message can be echoed back. Handling Errors Unhandled errors in an HTML5 Web Worker script fire error events on the Web Worker object. Listening for these error events is especially important when you are debugging scripts that make use of Web Workers. The following shows an example of an error handling function in a Web Worker JavaScript file that logs errors to the console: function errorHandler(e) { console.log(e.message, e); } To handle the errors, you must add an event listener to the main page: worker.addEventListener("error", errorHandler, true); 197
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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 Summary ..............
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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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Figure 1-1. A page rendered in stan
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Table 1-2. New sectioning HTML5 ele
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CHAPTER 1 ■ OVERVIEW OF HTML5 Wit
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article { background-color: #eee; d
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Figure 1-2. An HTML5 page with all
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CHAPTER 1 ■ OVERVIEW OF HTML5 The
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Figure 1-3. Developer Tools view in
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CHAPTER 1 ■ OVERVIEW OF HTML5 All
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26 CHAPTER 2 ■ USING THE HTML5 CA
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C H A P T E R 3 ■ ■ ■ Working
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Audio and Video Codecs CHAPTER 3
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Table 3-1. Browser support for HTML
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MEDIA FOR EVERYONE CHAPTER 3 ■ WO
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CHAPTER 3 ■ WORKING WITH HTML5 AU
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Table 3-4 shows a few of the read-o
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find the actual frame since the vid
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LOSING VIEWERS IN THE OF AN EYE CH
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C H A P T E R 4 ■ ■ ■ Using t
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A device can use any of the followi
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Table 4-4. Pros and cons of cell ph
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Figure 4-1. HTML5 Geolocation brows
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CHAPTER 4 ■ USING THE HTML5 GEOLO
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Listing 4-6. The Haversine formula
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Listing 4-9. Adding the loadDemo()
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THE EASIEST WAY TO TRAVEL CHAPTER 4
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var deltaLatitude = toRadians(latit
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Practical Extras CHAPTER 4 ■ USIN
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Show Me on a Google Map CHAPTER 4
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CHAPTER 5 ■ USING THE COMMUNICATI
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var defaultTitle = "Portal [http://
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Send Notification CHAPTER 5 ■ U
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Figure 5-4. Difference between clie
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“SEEMINGLY ARBITRARY” TIMES CHA
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crossOriginRequest.upload.onprogres
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Figure 6-1. The complexity of real-
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Listing 6-2. The WebSocket interfac
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Listing 6-4. HTTP response header H
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246 CHAPTER 10 ■ CREATING HTML5 O
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C H A P T E R 11 ■ ■ ■ The Fu
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Figure 11-1. Quake II 3D Shaders CH
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CHAPTER 11 ■ THE FUTURE OF HTML5
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270 ■ INDEX unsupported, 68 using
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274 ■ INDEX Web SQL database supp
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276 ■ INDEX HTML5 page CSS file f
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278 ■ INDEX media elements. See a
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280 ■ INDEX building application
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284 ■ INDEX stopping Web Workers,
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Peter Lubbers - Pro HTML 5 Programming

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