HTML5 JavaScript API. What’s new?

Pweb

浏览: 53085 次
性别:
来自: 广州

最近访客更多访客>>

lianfengyimeng

博主相关

博客

微博

相册

留言

关于我

文章分类

社区版块

存档分类

博客分类：

Javascript

HTML5 JavaScript Chrome Web Cache

HTML5 has a lot of new features. There’s new HTML, CSS and, of course, JavaScript. Officially HTML5 specification and implementation won’t be ready until 2022. Personally I don’t believe in this. Majority of JavaScript features described further are already implemented in modern browsers (e.g. Sarafi, Chrome, Firefox, Opera). Even Internet Explorer gained capability to render canvas and other stuff (supposing we use ExplorerCanvas or Chrome Frame ). Take a closer look at what’s going to make your live better and happier :

New Selectors

How many times have you wondered why there’s getElementById , getElementsByTagName , but there is not getElementByClassName ? New JavaScript API solves this issue:

var elements = document.getElementsByClassName('entry');

Moreover there’s now possibility to fetch elements that match provided CSS syntax!

var elements = document.querySelectorAll("ul li:nth-child(odd)");
var first_td = document.querySelector("table.test > tr > td");

Web Storage

Cookie mechanism has some disadvantages. As W3C said :

A user could be buying plane tickets in two different windows, using the same site. If the site used cookies to keep track of which ticket the user was buying, then as the user clicked from page to page in both windows, the ticket currently being purchased would “leak” from one window to the other, potentially causing the user to buy two tickets for the same flight without really noticing.
Web applications may wish to store megabytes of user data, such as entire user-authored documents or a user’s mailbox, on the client side for performance reasons. Again, cookies do not handle this case well, because they are transmitted with every request.

Well, sessionStorage has been created to let developers cope with first of above troubles. It keeps data in per tab storage. To get along with second one, W3C has introduced localStorage – the persistent storage that never expires.

Look, how simple is saving draft every new character is pressed:

textarea.addEventListener('keyup', function () {
  window.localStorage['value'] = area.value;
  window.localStorage['timestamp'] = (new Date()).getTime();
}, false);
textarea.value = window.localStorage['value'];

Go to demo

Web SQL Database Storage

What about database accessible directly from JavaScript? Since now we no longer parse, sort, filter data using consuming-lot-of-memory-and-cpu JavaScript loops. We fetch data using well know SQL queries. Look at example:

var db = window.openDatabase("Database Name", "Database Version");
db.transaction(function(tx) {
  tx.executeSql("SELECT * FROM test", [], successCallback, errorCallback);
});

Database is stored on client’s computer so it’s secure.

Go to demo

Offline Application Cache API

Web SQL Storage is available even if client went offline. But if we want create fully-functional offline aplication, we must care about resources like images, CSS, JS and et caetera. It’s high time to familiarize with Application Cache API.

We create cache.manifest file and link to it from html element.

<html
 manifest="cache.manifest"

>

File must be served with text/cache-manifest mimetype and contain body like this:

CACHE MANIFEST

CACHE:
index.html

help.html

style
/default.css

images/logo.png

images/backgound.png


NETWORK:
server.cgi

Files listed below CACHE will be cached by browser and available even offline. Files listed after NETWORK are never going to be cached. The application cache automatically updates only if the manifest file changes. It does not automatically update if resources listed in the manifest file change. You may do it manually with JavaScript:

cache = window.applicationCache;
var cacheUpdatereadyListener = function() {
  // Swap cache with updated data
  cache.swapCache();
}
cache.addEventListener('updateready', cacheUpdatereadyListener, false);
if(cache.status == cache.UPDATEREADY) {
  // Update cached data and call updateready listener after
  cache.update();
}

You may find full list of Application Cache Event Listeners here .

There is also new window event handler: onOffline. It is fired when internet connection goes down and up . See how we may notice user if there’s change in user’s network connection:

window.addEventListener('offline', function() {
  if(navigator.onLine == false) {
    alert('We went offline');
  } else {
    alert('We are online again!');
  }
}, true);

Go to demo

Web workers

Workers are API for running scripts in the background independently of any user interface scripts. Generally, workers are expected to be long-lived, have a high start-up performance cost, and a high per-instance memory cost. Moreover they might be partially replaced by window.setTimeout() function. So have they any advantages? Yes, of course.

workers are separate JS processes () running in separate threads,
workers execute concurrently,
workers don’t block the UI,
workers allow you to extract up to the last drop of juice from a multicore CPU,
workers can be dedicated (single tab) or shared among tabs/windows,
workers can be persistent too (coming soon): they’ll keep running after the browser has quit.

If we call function by setTimeout, the execution of script and UI are suspended. When we call function in worker, it doesn’t affect UI and execution flow in any way. Enough said.

To create Worker, we put JavaScript in separate file and create new Worker instance:

var worker = new Worker(‘extra_work.js');

That’s it. We can communicate with worker using postMessage function and onmessage listener. Messages are sended to all threads in our application:

main.js:
  var worker = new Worker(‘extra_work.js');
  worker.onmessage = function (event) { alert(event.data); };

extra_work.js:
  // do some work; when done post message.
  // some_data could be string, array, object etc.
  postMessage(some_data);

There are also so called Shared Workers. Thay use slightly different APIs, since each worker can have multiple connections. An example purpose is logger or application manager. If you are intrested follow fakeworker-js from Google Code.

See demo using workers and similar one without them (will hang your browser).

Web sockets

Let me quote Chromium Blog :

———-

Web Sockets are “TCP for the Web,” a next-generation bidirectional communication technology for web applications. They allow a web server to push data to a browser (COMET). Developers have been using XMLHttpRequest (known as persistent Ajax connection) for such purposes, but XHR makes developing web applications that communicate back and forth to the server unnecessarily complex.

Web Sockets provide a real bidirectional communication channel in your browser. Once you get a Web Socket connection, you can send data from browser to server by calling a send() method, and receive data from server to browser by an onmessage event handler. A simple example is included below.

if ("WebSocket" in window) {
  var ws = new WebSocket("ws://example.com/service");
  ws.onopen = function() {
    // Web Socket is connected. You can send data by send() method.
    ws.send("message to send"); ....
  };
  ws.onmessage = function (evt) { var received_msg = evt.data; ... };
  ws.onclose = function() { // websocket is closed. };
} else {
  // the browser doesn't support WebSocket.
}

In addition to the new Web Sockets API, there is also a new protocol (the “web socket protocol “) that the browser uses to communicate with servers. We also developed pywebsocket , which can be used as an Apache extension module, or can even be run as standalone server.

———-

Personally, there’s long way for web sockets to be fully supported by browsers. At this time I would recommend NodeJS or APE Project .

Notifications

Google Chrome has introduced new way to show notifications. They are popping outside browser window and user could see them even if browser is minimalized. Before showing notifications you must ask user for permission to do so. Look at code below and try demo:

if (window.webkitNotifications.checkPermission() == 0) {
  // you can pass any url as a parameter
  window.webkitNotifications.createNotification(tweet.picture, tweet.title,
      tweet.text).show();
} else {
  window.webkitNotifications.requestPermission();
}

Show tweets Set notification permissions

Drag and Drop

Everyone knows what is it. Thanks to HTML5 we are able to drag and drop any element into any element, without heavy JavaScript frameworks. There’s also possibility to drag and drop text/images/files from other windows and desktop.

Go to demo

Drag and drop anything

Geolocation

Probably anything special, but since now we may recognize user’s location on frontend as well as backend. Nowadays geolocation is not always accurate and is supported only by Firefox and Safari Mobile.

if (navigator.geolocation) {
  navigator.geolocation.getCurrentPosition(function(position) {
    var lat = position.coords.latitude;
    var lng = position.coords.longitude;
    map.setCenter(new GLatLng(lat, lng), 13);
    map.addOverlay(new GMarker(new GLatLng(lat, lng)));
  });
}

Go to demo

Audio and Video manipulation

We can easily embed audio or video on page and super-duper-easily manipulate them with JavaScript.

<audio src="sound.mp3"

 controls
>
</audio>

document.getElementById("audio").muted=false;

<video src='movie.mp4'

 autoplay
 controls
>
</video>

document.getElementById("video").play();

For non-modern browsers you may include html5media . You may also be intrested in HTML5 video player .

Go to audio demo
Go to video demo

Canvas

The best of all! We are able to actually draw in browser. Even second Mac OS if we have patience. Canvas is supported by any browser (even IE thanks to ExplorerCanvas ). There’s huge set of canvas demos at ChromeExperiments.com . There’s CAKE (scenegraph library for the canvas tag). We could write games , physics engines , editors and even whole UI . Flash CS5 will export to HTML5 Canvas and even now we are able to run swf files using pure JavaScript and <canvas> tag (yes, now we can run flash on iPhone).

Useful links

RT: http://blog.frontendforce.com/2010/04/html5-javascript-api-whats-new/

分享到：

ie6下CSS存在的BUG | 15个 HTML5 教程和参考手册

2011-01-12 11:56
浏览 914
评论(0)
分类:Web前端
查看更多

发表评论

文章已被作者锁定，不允许评论。

最近访客更多访客>>

博主相关

文章分类

社区版块

存档分类

最新评论