Création des Logiciels de gestion d'Entreprise, Création et référencement des sites web, Réseaux et Maintenance, Conception
Création des Logiciels de gestion d'Entreprise, Création et référencement des sites web, Réseaux et Maintenance, Conception
Last week Jason Grigsby visited Google as part of our Web Exponents speaker series which highlights innovations in web technology. Jason is a tech leader in mobile web development. In addition to spotting trends in the mobile space, Jason is at the front lines building mobile apps at Cloud Four. His humorous and informative talk includes technology recommendations and insightful examples from the world of mobile. Check out the video of the talk below. You can also download the slides.
Jason’s mobile strategy counterexamples include Chanel (they have an iPhone app but their website is unusable on the iPhone) and the difficulties of finding an Apple Store on the iPhone. His DOs and DON’Ts are:
DOs:
DON’Ts:
These all ring true for anyone with experience building for mobile. The hard part is figuring out the right solution for providing the right experience for desktop as well as diverse mobile devices. Jason gives a glimpse into the key features of a solution:
The challenge in my opinion is in the steps of breaking out content from markup and determining which content is appropriate for a given device. We need frameworks that better support these ideas, but there’s still a lot of heavy design work on the developer’s shoulders. Jason points to NPR as an example of a large site that has successfully implemented this architecture. Check out Jason’s talk to find out more, and also check out some of the other videos in the Web Exponents playlist.
By Steve Souders, Performance Evangelist2013, By: Seo Master.CSS_FLOATY_BAR {
...
top: -50px; /* start off the screen, so it slides in nicely */
-webkit-transition: top 0.2s ease-out;
...
}
In JavaScript:// Constructor for the floaty bar
gmail.FloatyBar = function() {
this.menuDiv = document.createElement('div');
this.menuDiv.className = CSS_FLOATY_BAR;
...
};
// Called when it's time for the floaty bar to move
gmail.FloatyBar.prototype.setTop = function() {
this.menuDiv.style.top = window.scrollY + 'px';
};
// Called when the floaty bar menu is dismissed
gmail.FloatyBar.prototype.hideOffScreen = function() {
this.menuDiv.style.top = '-50px';
};
gmail.floatyBar = new gmail.FloatyBar();
// Listen for scroll events on the top level window
window.onscroll = function() {
...
gmail.floatyBar.setTop();
...
};
The essence here is that when the viewport scrolls, the floaty bar 'top' is set to the new viewport offset. The -webkit-transition rule specifies the animation parameters. (The 'top' property is to be animated, over 0.2s, using the ease-out timing function.) This is the animation behavior we had at launch, and it works just fine on Android and mobile Safari browsers..CSS_FLOATY_BAR {
...
top: -50px; /* start off the screen, so it slides in nicely */
-webkit-transition: -webkit-transform 0.2s ease-out;
...
}
In JavaScript:// Called when it's time for the floaty bar to move
gmail.FloatyBar.prototype.setTop = function() {
var translate = window.scrollY - (-50);
this.menuDiv.style['-webkit-transform'] = 'translateY(' + translate + 'px)';
};
// Called when the floaty bar menu is dismissed
gmail.FloatyBar.prototype.hideOffScreen = function() {
this.menuDiv.style['-webkit-transform'] = 'translateY(0px)';
};
Upon every scroll event, the floaty bar is translated vertically to the new viewport offset (modulo the offscreen offset which is important to the floaty bar's initial appearance). And, why exactly is this such an improvement? Even though the logic is equivalent, iPhone OS's implementation of CSS transforms is "performance enhanced", whilst our first iteration (animating the 'top' property) is performed by the OS in software. That's why the experience was unfortunately somewhat chunky at times, depending on the speed of the iPhone hardware.<script type="text/JavaScript">
function loadFile(url) {
var script = document.createElement('SCRIPT');
script.src = url;
document.getElementsByTagName('HEAD')[0].appendChild(script);
}
</script>
Option 2: XmlHttpRequest (XHR)<script type="text/JavaScript">
function loadFile(url) {
function callback() {
if (req.readyState == 4) { // 4 = Loaded
if (req.status == 200) {
eval(req.responseText);
} else {
// Error
}
}
};
var req = new XMLHttpRequest();
req.onreadystatechange = callback;
req.open("GET", url, true);
req.send("");
}
</script>
The next question is, when to lazy load the modules? One strategy is to lazy load the modules in the background once the home page has been loaded. This approach has some drawbacks. First, JavaScript execution in the browser is single threaded. So while you are loading the modules in the background, the rest of your app becomes non-responsive to user actions while the modules load. Second, it's very difficult to decide when, and in what order, to load the modules. What if a user tries to access a feature/page you have yet to lazy load in the background? A better strategy is to associate the loading of a module with a user's action. Typically, user actions are associated with an invocation of an asynchronous function (for example, an onclick handler). This is the perfect time for you to lazy load the module since the code will have to be fetched over the network. If mobile networks are slow, you can adopt a strategy where you prefetch the code of the modules in advance and keep them stored in the javascript heap. Only then parse and load the corresponding module on user action. One word of caution is that you should make sure your prefetching strategy doesn't impact the user's experience - for example, don't prefetch all the modules while you are fetching user data. Remember, dividing up the latency has far better for users than bunching it all together during startup. <html>
...
<script id="lazy">
// Make sure you strip out (or replace) comment blocks in your JavaScript first.
/*
JavaScript of lazy module
*/
</script>
<script>
function lazyLoad() {
var lazyElement = document.getElementById('lazy');
var lazyElementBody = lazyElement.innerHTML;
var jsCode = stripOutCommentBlock(lazyElementBody);
eval(jsCode);
}
</script>
<div onclick=lazyLoad()> Lazy Load </div>
</html>
In the future, we hope that the HTML5 standard will allow more control over when the application cache should download resources in the manifest, since using comments to pass along code is not elegant but worked nicely for us. In addition, the snippets of code are not meant to be a reference implementation and one should consider many additional optimizations such as stripping white space and compiling the JavaScript to make its parsing and execution faster. To learn more about web performance, get tips and tricks to improve the speed of your web applications and to download tools, please visit http://code.google.com/speed.function grow() {
var textarea = document.getElementById('growingTextarea');
var newHeight = textarea.scrollHeight;
var currentHeight = textarea.clientHeight;
…
}
One limitation of using scrollHeight and clientHeight is that we aren't able to shrink the textarea when content is deleted. When all the content of a textarea is visible, the scrollHeight is equal to the clientHeight. Therefore we aren't able to detect that our textarea is actually larger than the minimum size required to fit all the content (please do leave a comment if you think of a solution that doesn't require re-rendering the page).if (newHeight > currentHeight) {
textarea.style.height = newHeight + 5 * TEXTAREA_LINE_HEIGHT + 'px';
}
Notice how we only change the height if newHeight > currentHeight. Depending on the browser, changing the height (even if it's to the same value) will cause the page to re-render. On a mobile device, we want to try our best to minimize the number of operations.<script>
// Value of the line-height CSS property for the textarea.
var TEXTAREA_LINE_HEIGHT = 13;
function grow() {
var textarea = document.getElementById('growingTextarea');
var newHeight = textarea.scrollHeight;
var currentHeight = textarea.clientHeight;
if (newHeight > currentHeight) {
textarea.style.height = newHeight + 5 * TEXTAREA_LINE_HEIGHT + 'px';
}
}
</script>
<textarea id="growingTextarea"
onkeyup="grow();">
</textarea>
qr
, with attributes to tell the service what to produce:cht=qr<text> is text for the QR code. This must be url-encoded in UTF8. Note the space between
chl=<text>
choe=<output>
hello
and world
is written as %20
in the following example.UTF-8
is used. Available options are: Shift_JIS
, UTF-8
, or ISO-8859-1
.Mozilla/5.0 (Linux; U; Android 3.0; en-us; Xoom Build/HRI39) AppleWebKit/534.13 (KHTML, like Gecko) Version/4.0 Safari/534.13
Mozilla/5.0 (Linux; U; Android 2.2.1; en-us; Nexus One Build/FRG83) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1
I recently gave a talk (preserved YouTube here) about the cache pattern and the Web Storage Portability Layer (WSPL) at Google I/O. It was exciting getting to give a talk at the Moscone Center as previously I had only ever been one of the audience members. The conference seemed to go by in a blur for me as I was sleep-deprived from getting the WSPL to "just good enough" to actually be released. (And some ofyou have already pointed out that I missed several bugs.) In my talk, I provided a general overview of the cache pattern and this post expands on the handling of hit determination and merging server and local changes.
The cache pattern is a design pattern for building an offline-capable web application. We implemented the cache pattern to make Gmail for Mobile tolerant of flaky wireless connections but the approach is generally applicable. Here's how it works. Consider a typical AJAX application. As shown in the diagram, we have a web application with a local model, view and controllers. The user interacts with theapplication and the controller dispatches XmlHttpRequests (XHRs for short) to the server. The server sends asynchronous requests to the application which it inserts into the model.
As shown in this next diagram, in the cache pattern, we insert a cache between the application and the server. Having done so, many requests that would otherwise require a round-trip to the network.
A software cache like this one shares a great deal conceptually with hardware caches. When designing the cache used in Gmail for mobile, we used this similarity to guide our design. For example, to keep our cache as simple as possible, we implemented a software equivalent to a write-through cache with early forwarding and LRU eviction. The cache pattern in general (and consequently our implementation) has four important data flows as shown in the diagram.