<<High Performance JavaScript>>读书笔记-7.Ajax

Data Transmission
Requesting Data
There are five general techniques for requesting data from a server:
• XMLHttpRequest (XHR)
var url = '/data.php';
var params = [
'id=934875',
'limit=20'
];
var req = new XMLHttpRequest();
req.onreadystatechange = function() {
if (req.readyState === 4) {
var responseHeaders =req.getAllResponseHeaders(); // Get the response
headers.
var data = req.responseText; // Get the data.
// Process the data here...
}
}
req.open('GET', url + '?' + params.join('&'), true);
req.setRequestHeader('X-Requested-With','XMLHttpRequest'); // Set a request
header.
req.send(null); // Send the request.

• Dynamic script tag insertion
var scriptElement = document.createElement('script');
scriptElement.src = 'http://any-domain.com/javascript/lib.js';
document.getElementsByTagName('head')[0].appendChild(scriptElement);

var scriptElement = document.createElement('script');
scriptElement.src = 'http://any-domain.com/javascript/lib.js';
document.getElementsByTagName('head')[0].appendChild(scriptElement);
function jsonCallback(jsonString) {
var data = eval('(' + jsonString + ')');
// Process the data here...
}

• iframes
• Comet
• Multipart XHR
var req = new XMLHttpRequest();
req.open('GET', 'rollup_images.php', true);
req.onreadystatechange = function() {
if (req.readyState == 4) {
splitImages(req.responseText);
}
};
req.send(null);

// Read the images and convert them into base64 encoded strings.
$images = array('kitten.jpg', 'sunset.jpg', 'baby.jpg');
foreach ($images as $image) {
$image_fh = fopen($image, 'r');
$image_data = fread($image_fh, filesize($image));
fclose($image_fh);
$payloads[] = base64_encode($image_data);
}
// Roll up those strings into one long string and output it.
$newline = chr(1); // This character won't appear naturally in anybase64 string.
echo implode($newline, $payloads);

function splitImages(imageString) {
var imageData =imageString.split("\u0001");
var imageElement;
for (var i = 0, len = imageData.length; i < len;i++) {
imageElement = document.createElement('img');
imageElement.src = 'data:image/jpeg;base64,' +imageData[i];
document.getElementById('container').appendChild(imageElement);
}
}

function handleImageData(data, mimeType) {
var img = document.createElement('img');
img.src = 'data:' + mimeType + ';base64,' + data;
return img;
}

function handleCss(data) {
var style = document.createElement('style');
style.type = 'text/css';
var node = document.createTextNode(data);
style.appendChild(node);
document.getElementsByTagName('head')[0].appendChild(style);
}

function handleJavaScript(data) {
eval(data);
}

var req = new XMLHttpRequest();
var getLatestPacketInterval, lastLength = 0;
req.open('GET', 'rollup_images.php', true);
req.onreadystatechange = readyStateHandler;
req.send(null);
function readyStateHandler{
if (req.readyState === 3 &&getLatestPacketInterval === null) {
// Start polling.
getLatestPacketInterval =window.setInterval(function() {
getLatestPacket();
}, 15);
}
if (req.readyState === 4) {
// Stop polling.
clearInterval(getLatestPacketInterval);
// Get the last packet.
getLatestPacket();
}
}

function getLatestPacket() {
var length = req.responseText.length;
var packet = req.responseText.substring(lastLength,length);
processPacket(packet);
lastLength = length;
}

Sending Data
1.XMLHttpRequest
var url = '/data.php';
var params = [
'id=934875',
'limit=20'
];
function xhrPost(url, params, callback) {
var req = new XMLHttpRequest();
req.onerror = function() {
setTimeout(function() {
xhrPost(url, params, callback);
}, 1000);
};
req.onreadystatechange = function() {
if (req.readyState == 4) {
if (callback && typeof callback ==='function') {
callback();
}
}
};
req.open('POST', url, true);
req.setRequestHeader('Content-Type','application/x-www-form-urlencoded');
req.setRequestHeader('Content-Length',params.length);
req.send(params.join('&'));
}

2.Beacons
var url = '/status_tracker.php';
var params = [
'step=2',
'time=1248027314'
];
(new Image()).src = url + '?' + params.join('&');

Data Formats
1.XML
2.JSON
3.HTML
4.Custom Formatting

Format	Size	Download time	Parse time	Total load time
Verbose XML	582,960 bytes	999.4 ms	343.1 ms	1342.5 ms
Verbose JSON-P	487,913 bytes	598.2 ms	0.0 ms	598.2 ms
Simple XML	437,960 bytes	475.1 ms	83.1 ms	558.2 ms
Verbose JSON	487,895 bytes	527.7 ms	26.7 ms	554.4 ms
Simple JSON	392,895 bytes	498.7 ms	29.0 ms	527.7 ms
Simple JSON-P	392,913 bytes	454.0 ms	3.1 ms	457.1 ms
Array JSON	292,895 bytes	305.4 ms	18.6 ms	324.0 ms
Array JSON-P	292,912 bytes	316.0 ms	3.4 ms	319.4 ms
Custom Format (script insertion)	222,912 bytes	66.3 ms	11.7 ms	78.0 ms
Custom Format (XHR)	222,892 bytes	63.1 ms	14.5 ms	77.6 ms

http://techfoolery.com/formats/