传统的AJAX轮询方式，客服端以用户定义的时间间隔去服务器上查询最新的数据。种这种拉取数据的方式需要很短的时间间隔才能保证数据的精确度，但太短的时间间隔客服端会对服务器在短时间内发送出多个请求。
反转AJAX，就是所谓的长轮询或者COMET。服务器与客服端需要保持一条长时间的请求，它使得服务器在有数据时可以返回消息给客户端。
XHTML
<ol class="dp-xml"> <li class="alt"><span><span class="tag"><span class="tag-name">div</span><span> </span><span class="attribute">id</span><span>=</span><span class="attribute-value">"msg"</span><span class="tag">></span><span class="tag"></span><span class="tag-name">div</span><span class="tag">></span><span> </span></span></span></li> <li><span class="tag"><span class="tag-name">input</span><span> </span><span class="attribute">id</span><span>=</span><span class="attribute-value">"btn"</span><span> </span><span class="attribute">type</span><span>=</span><span class="attribute-value">"button"</span><span> </span><span class="attribute">value</span><span>=</span><span class="attribute-value">"测试"</span><span> </span><span class="tag">/></span><span> </span></span></li> </ol>
jQuery
这里使用AJAX请求data.php页面获得‘success’的值,请求的时间达到80秒。在这80秒中若没有从服务端返回‘success’则一直保持连接状态，直到有数据返回或‘success’的值为0才关闭连接。在关闭连接后在继续下一次的请求。
<ol class="dp-c">
<li class="alt"><span><span>$(</span><span class="keyword">function</span><span>(){ </span></span></li>
<li>
<span> $(</span><span class="string">"#btn"</span><span>).bind(</span><span class="string">"click"</span><span>,{btn:$(</span><span class="string">"#btn"</span><span>)},</span><span class="keyword">function</span><span>(evdata){ </span>
</li>
<li class="alt"><span> $.ajax({ </span></li>
<li>
<span> type:</span><span class="string">"POST"</span><span>, </span>
</li>
<li class="alt">
<span> dataType:</span><span class="string">"json"</span><span>, </span>
</li>
<li>
<span> url:</span><span class="string">"data.php"</span><span>, </span>
</li>
<li class="alt">
<span> timeout:80000, </span><span class="comment">//ajax请求超时时间80秒 </span><span> </span>
</li>
<li>
<span> data:{time:</span><span class="string">"80"</span><span>}, </span><span class="comment">//40秒后无论结果服务器都返回数据 </span><span> </span>
</li>
<li class="alt">
<span> success:</span><span class="keyword">function</span><span>(data,textStatus){ </span>
</li>
<li>
<span> </span><span class="comment">//从服务器得到数据，显示数据并继续查询 </span><span> </span>
</li>
<li class="alt">
<span> </span><span class="keyword">if</span><span>(data.success==</span><span class="string">"1"</span><span>){ </span>
</li>
<li>
<span> $(</span><span class="string">"#msg"</span><span>).append(</span><span class="string">"<br>[有数据]"</span><span>+data.text); </span>
</li>
<li class="alt"><span> evdata.data.btn.click(); </span></li>
<li><span> } </span></li>
<li class="alt">
<span> </span><span class="comment">//未从服务器得到数据，继续查询 </span><span> </span>
</li>
<li>
<span> </span><span class="keyword">if</span><span>(data.success==</span><span class="string">"0"</span><span>){ </span>
</li>
<li class="alt">
<span> $(</span><span class="string">"#msg"</span><span>).append(</span><span class="string">"<br>[无数据]"</span><span>); </span>
</li>
<li><span> evdata.data.btn.click(); </span></li>
<li class="alt"><span> } </span></li>
<li><span> }, </span></li>
<li class="alt">
<span> </span><span class="comment">//Ajax请求超时，继续查询 </span><span> </span>
</li>
<li>
<span> error:</span><span class="keyword">function</span><span>(XMLHttpRequest,textStatus,errorThrown){ </span>
</li>
<li class="alt">
<span> </span><span class="keyword">if</span><span>(textStatus==</span><span class="string">"timeout"</span><span>){ </span>
</li>
<li>
<span> $(</span><span class="string">"#msg"</span><span>).append(</span><span class="string">"<br>[超时]"</span><span>); </span>
</li>
<li class="alt"><span> evdata.data.btn.click(); </span></li>
<li><span> } </span></li>
<li class="alt"><span> } </span></li>
<li><span> </span></li>
<li class="alt"><span> }); </span></li>
<li><span> }); </span></li>
<li class="alt"><span> </span></li>
<li><span>}); </span></li>
</ol>PHP
在这里是无限的循环，循环的结束条件就是获取到了返回结果返回Json数据。
并且接受$_POST['time']参数来限制循环的超时时间，避免资源的过度浪费。(浏览器关闭不会发消息给服务器，使用可能一直循环下去)
<ol class="dp-c">
<li class="alt"><span><span class="keyword">if</span><span>(</span><span class="func">empty</span><span class="keyword">empty</span><span>(</span><span class="vars">$_POST</span><span>[</span><span class="string">'time'</span><span>]))</span><span class="func">exit</span><span>(); </span></span></li>
<li>
<span>set_time_limit(0);</span><span class="comment">//无限请求超时时间 </span><span> </span>
</li>
<li class="alt">
<span class="vars">$i</span><span>=0; </span>
</li>
<li>
<span class="keyword">while</span><span> (true){ </span>
</li>
<li class="alt">
<span> </span><span class="comment">//sleep(1); </span><span> </span>
</li>
<li>
<span> usleep(500000);</span><span class="comment">//0.5秒 </span><span> </span>
</li>
<li class="alt">
<span> </span><span class="vars">$i</span><span>++; </span>
</li>
<li><span> </span></li>
<li class="alt">
<span> </span><span class="comment">//若得到数据则马上返回数据给客服端，并结束本次请求 </span><span> </span>
</li>
<li>
<span> </span><span class="vars">$rand</span><span>=rand(1,999); </span>
</li>
<li class="alt">
<span> </span><span class="keyword">if</span><span>(</span><span class="vars">$rand</span><span></span>
</li>
<li>
<span> </span><span class="vars">$arr</span><span>=</span><span class="keyword">array</span><span>(</span><span class="string">'success'</span><span>=></span><span class="string">"1"</span><span>,</span><span class="string">'name'</span><span>=></span><span class="string">'xiaocai'</span><span>,</span><span class="string">'text'</span><span>=></span><span class="vars">$rand</span><span>); </span>
</li>
<li class="alt">
<span> </span><span class="func">echo</span><span> json_encode(</span><span class="vars">$arr</span><span>); </span>
</li>
<li>
<span> </span><span class="func">exit</span><span>(); </span>
</li>
<li class="alt"><span> } </span></li>
<li><span> </span></li>
<li class="alt">
<span> </span><span class="comment">//服务器($_POST['time']*0.5)秒后告诉客服端无数据 </span><span> </span>
</li>
<li>
<span> </span><span class="keyword">if</span><span>(</span><span class="vars">$i</span><span>==</span><span class="vars">$_POST</span><span>[</span><span class="string">'time'</span><span>]){ </span>
</li>
<li class="alt">
<span> </span><span class="vars">$arr</span><span>=</span><span class="keyword">array</span><span>(</span><span class="string">'success'</span><span>=></span><span class="string">"0"</span><span>,</span><span class="string">'name'</span><span>=></span><span class="string">'xiaocai'</span><span>,</span><span class="string">'text'</span><span>=></span><span class="vars">$rand</span><span>); </span>
</li>
<li>
<span> </span><span class="func">echo</span><span> json_encode(</span><span class="vars">$arr</span><span>); </span>
</li>
<li class="alt">
<span> </span><span class="func">exit</span><span>(); </span>
</li>
<li><span> } </span></li>
<li class="alt"><span>} </span></li>
</ol>运行效果:在图中可以看到无数据的请求时间达到了40S，在40S的请求中若获得数据则请求关闭。
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