HTML5 Server-Sent Events

Server-Sent Events allow a web page to get updates from a server.

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Server-Sent Events - One Way Messaging

A server-sent event is when a web page automatically gets updates from a server.

This was also possible before, but the web page would have to ask if any updates were available. With server-sent events, the updates come automatically.

Examples: Facebook/Twitter updates, stock price updates, news feeds, sport results, etc.

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Browser Support

The numbers in the table specify the first browser version that fully support server-sent events.

API

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Receive Server-Sent Event Notifications

The EventSource object is used to receive server-sent event notifications:

Example

var source = new EventSource("demo_sse.php");
source.onmessage = function(event) {
    document.getElementById("result").innerHTML += event.data + "<br>";
};

Example explained:

• Create a new EventSource object, and specify the URL of the page sending the updates (in this example "demo_sse.php")
• Each time an update is received, the onmessage event occurs
• When an onmessage event occurs, put the received data into the element with id="result"

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Check Server-Sent Events Support

In the tryit example above there were some extra lines of code to check browser support for server-sent events:

if(typeof(EventSource) !== "undefined") {
    // Yes! Server-sent events support!
    // Some code.....
} else {
    // Sorry! No server-sent events support..
}

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Server-Side Code Example

For the example above to work, you need a server capable of sending data updates (like PHP or ASP).

The server-side event stream syntax is simple. Set the "Content-Type" header to "text/event-stream". Now you can start sending event streams.

Code in PHP (demo_sse.php):

<?php
header('Content-Type: text/event-stream');
header('Cache-Control: no-cache');

$time = date('r');
echo "data: The server time is: {$time}\n\n";
flush();
?>

Code in ASP (VB) (demo_sse.asp):

<%
Response.ContentType = "text/event-stream"
Response.Expires = -1
Response.Write("data: The server time is: " & now())
Response.Flush()
%>

Code explained:

• Set the "Content-Type" header to "text/event-stream"
• Specify that the page should not cache
• Output the data to send (Always start with "data: ")
• Flush the output data back to the web page

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The EventSource Object

In the examples above we used the onmessage event to get messages. But other events are also available:

Events	Description
onopen	When a connection to the server is opened
onmessage	When a message is received
onerror	When an error occurs

HTML5 Web Workers

A web worker is a JavaScript running in the background, without affecting the performance of the page.

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What is a Web Worker?

When executing scripts in an HTML page, the page becomes unresponsive until the script is finished.

A web worker is a JavaScript that runs in the background, independently of other scripts, without affecting the performance of the page. You can continue to do whatever you want: clicking, selecting things, etc., while the web worker runs in the background.

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Browser Support

The numbers in the table specify the first browser version that fully support Web Workers.

API

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HTML Web Workers Example

The example below creates a simple web worker that count numbers in the background:

Example

Count numbers:

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Check Web Worker Support

Before creating a web worker, check whether the user's browser supports it:

if(typeof(Worker) !== "undefined") {
    // Yes! Web worker support!
    // Some code.....
} else {
    // Sorry! No Web Worker support..
}

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Create a Web Worker File

Now, let's create our web worker in an external JavaScript.

Here, we create a script that counts. The script is stored in the "demo_workers.js" file:

var i = 0;
function timedCount() {
    i = i + 1;
    postMessage(i);
    setTimeout("timedCount()",500);
}
timedCount();

The important part of the code above is the postMessage() method - which is used to post a message back to the HTML page.

Note: Normally web workers are not used for such simple scripts, but for more CPU intensive tasks.

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Create a Web Worker Object

Now that we have the web worker file, we need to call it from an HTML page.

The following lines checks if the worker already exists, if not - it creates a new web worker object and runs the code in "demo_workers.js":

if(typeof(w) == "undefined") {
    w = new Worker("demo_workers.js");
}

Then we can send and receive messages from the web worker.

Add an "onmessage" event listener to the web worker.

w.onmessage = function(event){
    document.getElementById("result").innerHTML = event.data;
};

When the web worker posts a message, the code within the event listener is executed. The data from the web worker is stored in event.data.

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Terminate a Web Worker

When a web worker object is created, it will continue to listen for messages (even after the external script is finished) until it is terminated.

To terminate a web worker, and free browser/computer resources, use the terminate() method:

w.terminate();

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Reuse the Web Worker

If you set the worker variable to undefined, after it has been terminated, you can reuse the code:

w = undefined;

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Full Web Worker Example Code

We have already seen the Worker code in the .js file. Below is the code for the HTML page:

Example

<!DOCTYPE html> <html> <body>
<p>Count numbers: <output id="result"></output></p> <button onclick="startWorker()">Start Worker</button> <button onclick="stopWorker()">Stop Worker</button> <br><br>
<script>
var w;
function startWorker() {
    if(typeof(Worker) !== "undefined") {
        if(typeof(w) == "undefined") {
            w = new Worker("demo_workers.js");
        }
        w.onmessage = function(event) {
            document.getElementById("result").innerHTML = event.data;
        };
    } else {
        document.getElementById("result").innerHTML = "Sorry! No Web Worker support.";
    }
}
function stopWorker() {
    w.terminate();
    w = undefined;
} </script>
</body> </html>

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Web Workers and the DOM

Since web workers are in external files, they do not have access to the following JavaScript objects:

• The window object
• The document object
• The parent object

HTML5 Application Cache

With application cache it is easy to make an offline version of a web application, by creating a cache manifest file.

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What is Application Cache?

HTML5 introduces application cache, which means that a web application is cached, and accessible without an internet connection.

Application cache gives an application three advantages:

1. Offline browsing - users can use the application when they're offline
2. Speed - cached resources load faster
3. Reduced server load - the browser will only download updated/changed resources from the server

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Browser Support

The numbers in the table specify the first browser version that fully supports Application Cache.

API

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HTML Cache Manifest Example

The example below shows an HTML document with a cache manifest (for offline browsing):

Example

<!DOCTYPE HTML> <html manifest="demo.appcache">
<body>
The content of the document...... </body>
</html>

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Cache Manifest Basics

To enable application cache, include the manifest attribute in the document's <html> tag:

<!DOCTYPE HTML>
<html manifest="demo.appcache">
...
</html>

Every page with the manifest attribute specified will be cached when the user visits it. If the manifest attribute is not specified, the page will not be cached (unless the page is specified directly in the manifest file).

The recommended file extension for manifest files is: ".appcache"

	A manifest file needs to be served with the correct media type, which is "text/cache-manifest". Must be configured on the web server.

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The Manifest File

The manifest file is a simple text file, which tells the browser what to cache (and what to never cache).

The manifest file has three sections:

• CACHE MANIFEST - Files listed under this header will be cached after they are downloaded for the first time
• NETWORK - Files listed under this header require a connection to the server, and will never be cached
• FALLBACK - Files listed under this header specifies fallback pages if a page is inaccessible

CACHE MANIFEST

The first line, CACHE MANIFEST, is required:

CACHE MANIFEST
/theme.css
/logo.gif
/main.js

The manifest file above lists three resources: a CSS file, a GIF image, and a JavaScript file. When the manifest file is loaded, the browser will download the three files from the root directory of the web site. Then, whenever the user is not connected to the internet, the resources will still be available.

NETWORK

The NETWORK section below specifies that the file "login.asp" should never be cached, and will not be available offline:

NETWORK:
login.asp

An asterisk can be used to indicate that all other resources/files require an internet connection:

NETWORK:
*

FALLBACK

The FALLBACK section below specifies that "offline.html" will be served in place of all files in the /html/ catalog, in case an internet connection cannot be established:

FALLBACK:
/html/ /offline.html

Note: The first URI is the resource, the second is the fallback.

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Updating the Cache

Once an application is cached, it remains cached until one of the following happens:

• The user clears the browser's cache
• The manifest file is modified (see tip below)
• The application cache is programmatically updated

Example - Complete Cache Manifest File

CACHE MANIFEST
# 2012-02-21 v1.0.0
/theme.css
/logo.gif
/main.js
NETWORK:
login.asp
FALLBACK:
/html/ /offline.html

	Tip: Lines starting with a "#" are comment lines, but can also serve another purpose. An application's cache is only updated when its manifest file changes. If you edit an image or change a JavaScript function, those changes will not be re-cached. Updating the date and version in a comment line is one way to make the browser re-cache your files.

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Notes on Application Cache

Be careful with what you cache.

Once a file is cached, the browser will continue to show the cached version, even if you change the file on the server. To ensure the browser updates the cache, you need to change the manifest file.

Note: Browsers may have different size limits for cached data (some browsers have a 5MB limit per site).

HTML5 Local Storage

HTML local storage, better than cookies.

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What is HTML Local Storage?

With local storage, web applications can store data locally within the user's browser.

Before HTML5, application data had to be stored in cookies, included in every server request. Local storage is more secure, and large amounts of data can be stored locally, without affecting website performance.

Unlike cookies, the storage limit is far larger (at least 5MB) and information is never transferred to the server.

Local storage is per domain. All pages, from one domain, can store and access the same data.

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Browser Support

The numbers in the table specify the first browser version that fully supports Local Storage.

 

HTML Local Storage Objects

HTML local storage provides two objects for storing data on the client:

• window.localStorage - stores data with no expiration date
• window.sessionStorage - stores data for one session (data is lost when the browser tab is closed)

Before using local storage, check browser support for localStorage and sessionStorage:

if(typeof(Storage) !== "undefined") {
    // Code for localStorage/sessionStorage.
} else {
    // Sorry! No Web Storage support..
}

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The localStorage Object

The localStorage object stores the data with no expiration date. The data will not be deleted when the browser is closed, and will be available the next day, week, or year.

Example

// Store
localStorage.setItem("lastname", "Smith");
// Retrieve
document.getElementById("result").innerHTML = localStorage.getItem("lastname");

Example explained:

• Create a localStorage name/value pair with name="lastname" and value="Smith"
• Retrieve the value of "lastname" and insert it into the element with id="result"

The example above could also be written like this:

// Store
localStorage.lastname = "Smith";
// Retrieve
document.getElementById("result").innerHTML = localStorage.lastname;

The syntax for removing the "lastname" localStorage item is as follows:

localStorage.removeItem("lastname");

Note: Name/value pairs are always stored as strings. Remember to convert them to another format when needed!

The following example counts the number of times a user has clicked a button. In this code the value string is converted to a number to be able to increase the counter:

Example

if (localStorage.clickcount) {
    localStorage.clickcount = Number(localStorage.clickcount) + 1;
} else {
    localStorage.clickcount = 1;
}
document.getElementById("result").innerHTML = "You have clicked the button " +
localStorage.clickcount + " time(s).";

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The sessionStorage Object

The sessionStorage object is equal to the localStorage object, except that it stores the data for only one session. The data is deleted when the user closes the specific browser tab.

The following example counts the number of times a user has clicked a button, in the current session:

Example

if (sessionStorage.clickcount) {
    sessionStorage.clickcount = Number(sessionStorage.clickcount) + 1;
} else {
    sessionStorage.clickcount = 1;
}
document.getElementById("result").innerHTML = "You have clicked the button " +
sessionStorage.clickcount + " time(s) in this session.";

HTML5 Drag and Drop

Drag and drop is a part of the HTML5 standard.

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Drag and Drop

Drag and drop is a very common feature. It is when you "grab" an object and drag it to a different location.

In HTML5, drag and drop is part of the standard, and any element can be draggable.

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Browser Support

The numbers in the table specify the first browser version that fully supports Drag and Drop.

API
Drag and Drop	4.0	9.0	3.5	6.0	12.0

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HTML Drag and Drop Example

The example below is a simple drag and drop example:

Example

<!DOCTYPE HTML> <html> <head> <script>
function allowDrop(ev) {
    ev.preventDefault();
}
function drag(ev) {
    ev.dataTransfer.setData("text", ev.target.id);
}
function drop(ev) {
    ev.preventDefault();
    var data = ev.dataTransfer.getData("text");
    ev.target.appendChild(document.getElementById(data));
} </script> </head> <body>
<div id="div1" ondrop="drop(event)" ondragover="allowDrop(event)"></div>
<img id="drag1" src="img_logo.gif" draggable="true"
ondragstart="drag(event)" width="336" height="69">
</body> </html>

It might seem complicated, but lets go through all the different parts of a drag and drop event.

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Make an Element Draggable

First of all: To make an element draggable, set the draggable attribute to true:

<img draggable="true">

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What to Drag - ondragstart and setData()

Then, specify what should happen when the element is dragged.

In the example above, the ondragstart attribute calls a function, drag(event), that specifies what data to be dragged.

The dataTransfer.setData() method sets the data type and the value of the dragged data:

function drag(ev) {
    ev.dataTransfer.setData("text", ev.target.id);
}

In this case, the data type is "text" and the value is the id of the draggable element ("drag1").

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Where to Drop - ondragover

The ondragover event specifies where the dragged data can be dropped.

By default, data/elements cannot be dropped in other elements. To allow a drop, we must prevent the default handling of the element.

This is done by calling the event.preventDefault() method for the ondragover event:

event.preventDefault()

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Do the Drop - ondrop

When the dragged data is dropped, a drop event occurs.

In the example above, the ondrop attribute calls a function, drop(event):

function drop(ev) {
    ev.preventDefault();
    var data = ev.dataTransfer.getData("text");
    ev.target.appendChild(document.getElementById(data));
}

Code explained:

• Call preventDefault() to prevent the browser default handling of the data (default is open as link on drop)
• Get the dragged data with the dataTransfer.getData() method. This method will return any data that was set to the same type in the setData() method
• The dragged data is the id of the dragged element ("drag1")
• Append the dragged element into the drop element

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HTML5 Geolocation

HTML Geolocation is used to locate a user's position.

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Locate the User's Position

The HTML Geolocation API is used to get the geographical position of a user.

Since this can compromise user privacy, the position is not available unless the user approves it.

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Browser Support

The numbers in the table specify the first browser version that fully supports Geolocation.

API
Geolocation	5.0	9.0	3.5	5.0	16.0

Note: Geolocation is much more accurate for devices with GPS, like iPhone.

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Using HTML Geolocation

Use the getCurrentPosition() method to get the user's position.

The example below is a simple Geolocation example returning the latitude and longitude of the user's position:

Example

<script>
var x = document.getElementById("demo");
function getLocation() {
    if (navigator.geolocation) {
        navigator.geolocation.getCurrentPosition(showPosition);
    } else {
        x.innerHTML = "Geolocation is not supported by this browser.";
    }
}
function showPosition(position) {
    x.innerHTML = "Latitude: " + position.coords.latitude +
    "<br>Longitude: " + position.coords.longitude;
}
</script>

Example explained:

• Check if Geolocation is supported
• If supported, run the getCurrentPosition() method. If not, display a message to the user
• If the getCurrentPosition() method is successful, it returns a coordinates object to the function specified in the parameter ( showPosition )
• The showPosition() function gets the displays the Latitude and Longitude

The example above is a very basic Geolocation script, with no error handling.

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Handling Errors and Rejections

The second parameter of the getCurrentPosition() method is used to handle errors. It specifies a function to run if it fails to get the user's location:

Example

function showError(error) {
    switch(error.code) {
        case error.PERMISSION_DENIED:
            x.innerHTML = "User denied the request for Geolocation."
            break;
        case error.POSITION_UNAVAILABLE:
            x.innerHTML = "Location information is unavailable."
            break;
        case error.TIMEOUT:
            x.innerHTML = "The request to get user location timed out."
            break;
        case error.UNKNOWN_ERROR:
            x.innerHTML = "An unknown error occurred."
            break;
    }
}

Error Codes:

• Permission denied - The user did not allow Geolocation
• Position unavailable - It is not possible to get the current location
• Timeout - The operation timed out

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Displaying the Result in a Map

To display the result in a map, you need access to a map service that can use latitude and longitude, like Google Maps:

Example

function showPosition(position) {
    var latlon = position.coords.latitude + "," + position.coords.longitude;

    var img_url = "http://maps.googleapis.com/maps/api/staticmap?center=
    "+latlon+"&zoom=14&size=400x300&sensor=false";

    document.getElementById("mapholder").innerHTML = "<img src='"+img_url+"'>";
}

In the example above we use the returned latitude and longitude data to show the location in a Google map (using a static image).

Google Map Script
How to use a script to show an interactive map with a marker, zoom and drag options.

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Location-specific Information

This page demonstrated how to show a user's position on a map. However, Geolocation is also very useful for location-specific information.

Examples:

• Up-to-date local information
• Showing Points-of-interest near the user
• Turn-by-turn navigation (GPS)

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The getCurrentPosition() Method - Return Data

The getCurrentPosition() method returns an object if it is successful. The latitude, longitude and accuracy properties are always returned. The other properties below are returned if available.

Property	Description
coords.latitude	The latitude as a decimal number
coords.longitude	The longitude as a decimal number
coords.accuracy	The accuracy of position
coords.altitude	The altitude in meters above the mean sea level
coords.altitudeAccuracy	The altitude accuracy of position
coords.heading	The heading as degrees clockwise from North
coords.speed	The speed in meters per second
timestamp	The date/time of the response

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Geolocation object - Other interesting Methods

watchPosition() - Returns the current position of the user and continues to return updated position as the user moves (like the GPS in a car).

clearWatch() - Stops the watchPosition() method.

The example below shows the watchPosition() method. You need an accurate GPS device to test this (like iPhone):

Example

<script>
var x = document.getElementById("demo");
function getLocation() {
    if (navigator.geolocation) {
        navigator.geolocation.watchPosition(showPosition);
    } else {
        x.innerHTML = "Geolocation is not supported by this browser.";
    }
}
function showPosition(position) {
    x.innerHTML = "Latitude: " + position.coords.latitude +
    "<br>Longitude: " + position.coords.longitude;
}
</script>