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Amit ChaudharyAmit Chaudhary 

I want to check which statement execute after another one ,?

1.<apex:page controller="exampleCon4">
2.    <apex:form >
3.        <apex:actionPoller action="{!incrementCounter}" rerender="counter" status="counterStatus" interval="500"/>
4.        <apex:outputText value="Watch this counter: {!count}" id="counter"/>
5.        <apex:actionStatus startText=" (incrementing...)" stopText=" (done)" id="counterStatus"/>
6.    </apex:form>
7.     </apex:page>

     after execution of 3rd statement ,execution control where goes , on 4th statement or on 5th statement or on controller class.
and is there any other way to check this flow ?  
once you loaded the page as you gave time interval as 500 seconds it will be in the queue to execute the action method. But 4 and 5 will execute mean while.
After the 500 seconds (it may take more than this as this is in a queue) the action will fire and rerender 4 along with 5. 

unfortunatly we dont have any flow of debugging scope line by line at vf page level. you can use just the script alerts to know what is happening at any level to check the same.