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# Advanced Control Flow - Tutorial

## Tutorial: A Gentle Introduction to Cocoon Control Flow

In this tutorial, we will create a simple number guessing game using Cocoon's Control Flow engine.

After you have Cocoon 2.1 deployed and running, go to where you have Cocoon deployed and create a new subdirectory named `game`. Cocoon's default main sitemap will automatically mount the sitemap in the subdirectory.

Create the following `sitemap.xmap` in the new subdirectory:

    <?xml version="1.0" encoding="UTF-8"?>
    <map:sitemap xmlns:map="http://apache.org/cocoon/sitemap/1.0">

    <map:components>
      <map:generators default="file">
        <!-- in this example we use JXTemplateGenerator to insert
             Flow variables in page content -->
        <map:generator label="content,data" logger="sitemap.generator.jx"
                       name="jx" src="org.apache.cocoon.generation.JXTemplateGenerator"/>
      </map:generators>
      <map:transformers default="xslt"/>
      <map:serializers default="html"/>
      <map:matchers default="wildcard"/>
      <map:selectors default="browser">
        <map:selector name="exception" src="org.apache.cocoon.selection.XPathExceptionSelector">
          <exception name="invalid-continuation"
                     class="org.apache.cocoon.components.flow.InvalidContinuationException"/>
          <exception class="java.lang.Throwable" unroll="true"/>
        </map:selector>
      </map:selectors>
      <map:actions/>
      <map:pipes default="caching"/>
    </map:components>

    <map:views/>
    <map:resources/>
    <map:action-sets/>

    <map:flow language="javascript">
      <!-- Flow will use the javascript functions defined in game.js -->
      <map:script src="flow/game.js"/>
    </map:flow>

    <map:pipelines>
      <map:component-configurations>
        <global-variables/>
      </map:component-configurations>

      <map:pipeline>
        <!-- no filename: call main() in game.js -->
        <map:match pattern="">
          <map:call function="main"/>
        </map:match>

        <!-- use JXtemplate to generate page content -->
        <map:match pattern="*.jx">
          <map:generate type="jx" src="documents/{1}.jx"/>
          <map:serialize type="xhtml"/>
        </map:match>

        <!-- .kont URLs are generated by the Flow system for continuations -->
        <map:match pattern="*.kont">
          <map:call continuation="{1}"/>
        </map:match>

        <!-- handle invalid continuations -->

        <!-- this style of handling invalidContinuation is now deprecated: -->
        <!-- this URI will never be called automatically anymore. -->
        <!-- see handle-errors below -->
        <map:match pattern="invalidContinuation">
          <map:generate src="documents/invalidContinuation.xml"/>
          <map:serialize type="xml"/>
        </map:match>

        <!-- the new non-hardcoded way of handling invalidContinuation -->
        <map:handle-errors>
          <map:select type="exception">
            <map:when test="invalid-continuation">
              <map:generate src="documents/invalidContinuation.html"/>
              <map:serialize type="xhtml"/>
            </map:when>
          </map:select>
        </map:handle-errors>
      </map:pipeline>

    </map:pipelines>

    </map:sitemap>

Inside the new subdirectory, create two more directories, `documents/` and `flow/`.

Inside `documents/`, you will store the "views" -- pages to send to the player. Create the file `guess.jx`, which will be the page the player will enter their guess:

    <?xml version="1.0"?>
    <html xmlns:jx="http://apache.org/cocoon/templates/jx/1.0">
    <head>
      <title>cocoon flow number guessing game</title>
    </head>
    <body>
      <h1>Guess the Number Between 1 and 10</h1>
      <h2>${hint}</h2>
      <h3>You've guessed ${guesses} times.</h3>
      <form method="post" action="${cocoon.continuation.id}.kont">
        <input type="text" name="guess"/>
        <input type="submit"/>
      </form>
    </body>
    </html>

You'll also need a page to display when the person chooses the correct number. Name it `success.jx` (Again in `documents/`):

    <?xml version="1.0"?>
    <html xmlns:jx="http://apache.org/cocoon/templates/jx/1.0">
    <head>
      <title>cocoon flow number guessing game</title>
    </head>
    <body>
      <h1>Success!</h1>
      <h2>The number was: ${random}</h2>
      <h3>It took you ${guesses} tries.</h3>
      <p><a href="./">Play again</a></p>
    </body>
    </html>

You may notice some strange codes inside the files -- namely things like `${random}` and `${guesses}`. They look like variables and they will be replaced with values when the pages are sent to the client. This is where the JXTemplateGenerator comes in.

Inside `flow/` you will store the code that actually controls how this application runs. In the "MVC" pattern the Flow is the "Controller" and it is very powerful.

Create the following file named `game.js`:

    function main() {

      var random =  Math.round( Math.random() * 9 ) + 1;

      var hint = "No hint for you!"
      var guesses = 0;

      while (true) {

        cocoon.sendPageAndWait("guess.jxt", { "random" : random, "hint" : hint,
                                              "guesses" : guesses} );

        var guess = parseInt( cocoon.request.get("guess") );
        guesses++;

        if (guess) {
          if (guess > random) {
            hint = "Nope, lower!"
          } else if (guess < random) {
            hint = "Nope, higher!"
          } else {
            break;
          }
        }
      }

      cocoon.sendPage("success.jx", {"random" : random, "guess" : guess,
                                      "guesses" : guesses} );
    }

Alright, now let's follow the execution of this Flow and pipeline: The player accesses the URL `http://host/cocoon/game/` and the <map:match pattern=""> matches, and starts the pipeline.

The function `main()` which is referenced in `flow/game.js` is called and a new Continuation object is created. Without getting into too much detail the state of the Javascript code is saved and can be recalled any number of times.

We now enter the code in `game.js`:

A random number between 1 and 10 is chosen.

Variables containing a hint for the player and the player's current number of guesses are initialized. The Flow now enters the `while(true)` loop which basically keeps the game going until the player guesses the correct number.

We now get to the following line, where things start to get interesting:

    cocoon.sendPageAndWait("guess.jx", { "random" : random, "hint" : hint, "guesses" : guesses} );

The Flow layer sends the contents of the URI "guess.jx" which is matched in the sitemap (see above). We also pass an inline Javascript object, containing three key/value pairs, one named "random" which contains the value of the variable random as initialized above, and so on for hint and guesses. The keys are substituted later down the line, when the `JXTemplateGenerator` comes into play.

We could also do the following:

    cocoon.sendPageAndWait("guess.jx", { "foo" : random } );

In this case, the value of random would be able to be substituted in our JXTemplate, but under the name "foo" instead -- we'd just have to make sure we have the correct keyname in our template.

The Flow Layer also does another interesting thing: it halts the execution of the Javascript! Through the magic of continuations the Flow Layer is able to resume execution of the script at the exact line in which it left off. This creates some very powerful situations with respect to web programming, and forces the reader to think very differently about how web applications are designed.

Picking back up in the script execution, the client is sent through the pipeline matching "guess.jx". Referring back to the sitemap, we match *.jx, and run the file through the JXTemplateGenerator, which substitutes the keynames for the values sent from the cocoon.sendPageAndWait() function.

One thing to note is in the form which is sent back to Cocoon when the player submits the guess:

    <form method="post" action="${cocoon.continuation.id}.kont">

Here, ${cocoon.continuation.id} is resolved to a unique identifier which points to the current continuation. One can think of this somewhat of a session ID.

When the player submits the form, it is submitted to a unique URL which contains the continuation ID, plus ".kont", which we end up matching in the sitemap:

    <map:match pattern="*.kont">
      <map:call continuation="{1}"/>
    </map:match>

When Cocoon sees a URL like this, it attempts to restart the continuation with the specified ID and we re-enter the Javascript code where we left off previously.

We are now back in the Javascript at the line after sendPageAndWait(). We create a new variable (an int), which we get from the POST request that was sent by the form. Notice in the form we had `<input type="text" name="guess"/>` and in the Javascript we get the request parameter by using `cocoon.request.get("guess");`.

Now we increment the player's guess count and we test to see if they guessed the correct number. If the guess was too high, we set the hint variable telling them to guess lower, we fall through the bottom of the while loop and we send the guess form back to the player.

If the guess was too low, we tell them to guess higher, we fall through the loop as well sending the player the form again.

If the guess was correct, we break out of the main loop and send the player to a different view, this time to "success.jx", and we give the template not only their number and the random number (pointless, yes, because they were the same), but also the number of guesses to tell the player how good or bad at guessing numbers they are.

The main point of interest in the Flow script at this point is the use of `sendPage()` instead of `sendPageAndWait()`. `sendPage()` works exactly the same, except, yes, you guessed it, we don't halt execution of code and keep processing.

At this point there's no more code left and the game is over and the Flow stops.

Another thing to note is the <map:handle-errors> tag in the sitemap. Previously, when a continuation which did not exist was called, the Flow layer would automatically redirect to the URI "invalidContinuation". Now, the Flow layer throws an `InvalidContinuationException` and you can now handle it as described in the handle-errors tag.

And that's it! You have now just made your very first application using the Flow layer.

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