Friday, July 26, 2013

Maxwell Smart push counter

To illustrate the push functionality the previous example is confusing since it seems to be a request response interaction with a web browser, but it is not, since the text box send to the server a string, it is stored in a variable, the push widget detect the update of the variable and send it back in a separate message, different that the reception message. In fact various unrelated push widgets can be updated using this asynchronous mechanism   simultaneously in the same page.

So I created another push example theoretically more simple.

It is a countdown after which the page will navigate back to the main menu. In this case the widget generates its own output by decreasing the variable. It also illustrates the use of Hamlet.

Note that

 - push has a new parameter that is the delay for a new ajax request when the previous connection has been lost.

- The last push after 0, send a script that forces a navigation to the menu. And then kill itself.

pushDecrease= do
 tv ←  liftIO $ newTVarIO 10
 page $ 
       <h2> Maxwell Smart push counter
       <p> This example shows a reverse counter
       <p> To avoid unnecessary load, the push process will be killed when reaching 0
       <p> The last push message will be an script that will redirect to the menu"
       <h3> This message will be autodestroyed within ‥

  |] ++>  counter tv <++  b << "seconds" 

 counter tv = push Html 0 $ do
      setTimeouts 100 0     -- kill  the thread if the user navigate away
      n ←  atomic $ readTVar tv
      if (n≡ -1) 
        then  do
          script << "window.location='/'" ++> noWidget
          liftIO $ myThreadId ↠ killThread 
        else do
          atomic $ writeTVar tv $ n - 1
          liftIO $ threadDelay 1000000
          h1 << (show n) ++> noWidget

atomic= liftIO . atomically


Ops.. No.

killThread is never called since noWidget return an invalid formlet value, so the next statements are not executed in the View monad (See: The promising land of monadic formlets). 

That kill is in order to drop the process immediately, but otherwise, the timeout of setTimeouts would do the same job. Since the delay between push sends is one second, by shortening the timeout from 100 to two seconds will do the same job.  So the killthread line can be eliminated.

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