Peter Lubbers - Pro HTML 5 Programming

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220 CHAPTER 9 ■ USING THE HTML5 WEB STORAGE API Let’s look at the attributes and functions here in more detail. • The length attribute specifies how many key-value pairs are currently stored in the storage object. Remember that storage objects are specific to their origin, so that implies that the items (and length) of the storage object only reflect the items stored for the current origin. • The key(index) function allows retrieval of a given key. Generally, this is most useful when you wish to iterate across all the keys in a particular storage object. Keys are zero-based, meaning that the first key is at index (0) and the last key is at index (length – 1). Once a key is retrieved, it can be used to fetch its corresponding value. Keys will retain their indices over the life of a given storage object unless a key or one of its predecessors is removed. • As you’ve already seen, getItem(key) function is one way to retrieve the value based on a given key. The other is to reference the key as an array index to the storage object. In both cases, the value null will be returned if the key does not exist in storage. • Similarly, setItem(key, value) function will put a value into storage under the specified key name, or replace an existing value if one already exists under that key name. Note that it is possible to receive an error when setting an item value; if the user has storage turned off for that site, or if the storage is already filled to its maximum amount, a QUOTA_EXCEEDED_ERR error will be thrown during the attempt. Make sure to handle such an error should your application depend on proper storage behavior. Figure 9-3. Quota Exceeded Error in Chrome
CHAPTER 9 ■ USING THE HTML5 WEB STORAGE API • The removeItem(key) function does exactly as you might expect. If a value is currently in storage under the specified key, this call will remove it. If no item was stored under that key, no action is taken. ■ Note Unlike some collection and data frameworks, removing an item does not return the old value as a result of the call to remove it. Make sure you’ve stored any copy you need independent of the removal. • Finally, the clear() function removes all values from the storage list. It is safe to call this on an empty storage object; as such, a call will simply do nothing. DISK SPACE QUOTA Peter says: “The specification recommends that browsers allow five megabytes per origin. Browsers should prompt the user when the quota is reached in order to grant more space and may also provide ways for users to see how much space each origin is using. In reality, the behavior is still a bit inconsistent. Some browsers silently allow a larger quota or prompt for a space increase, while others simply throw the QUOTA_EXCEEDED_ERR error shown in Figure 9-3. The test file testQuota.html used in this example is located in the code/storage directory.” Communicating Web Storage Updates Sometimes, things get a little more complicated, and storage needs to be accessed by more than one page, browser tab, or worker. Perhaps your application needs to trigger many operations in succession whenever a storage value is changed. For just these cases, the HTML5 Web Storage API includes an event mechanism to allow notifications of data updates to be communicated to interested listeners. Web Storage events are fired on the window object for every window of the same origin as the storage operation, regardless of whether or not the listening window is doing any storage operations itself. ■ Note Web Storage events can be used to communicate between windows on the same origin. This will be explored a bit more thoroughly in the “Practical Extras” section. To register to receive the storage events of a window’s origin, simply register an event listener, for example: window.addEventListener("storage", displayStorageEvent, true); 221
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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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Figure 6-4. Latency comparison betw
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EXISTING WEBSOCKET SERVERS CHAPTER
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CHAPTER 6 ■ USING THE HTML5 WEBSO
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def terminate(self): self.ready_sta
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self.sessions = [] self.port = port
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Basic API Usage CHAPTER 6 ■ USING
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HTML5 WebSocket / Geolocation Track
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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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