Building Blocks

Building Software Better

A Taste of Flex (Part 2)

Posted by Mark on September 2, 2008

This is a multi-part series. The first in the series is here.

Tower of Hanoi — Procedural With Input

The second example from “A Taste of Smalltalk” is in Chapter 3, page 36 and does basic IO. Except that Smalltalk-80 includes a Windowing graphics system so it can automatically open Dialogs and other things. Fortunately Flex includes a Windowing system too, and can easily open up dialogs. Opening up a Dialog might match the Smalltalk example the best, but given Flex is already running within a Window, opening up an extra dialog is a bit artificial. Instead we can just add an input field for the value and a button to kick off the Hanoi.

Tower of Hanoi — Procedural With Input — Flex

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="" layout="vertical"
        function moveTower(height : int, fromPin : *, toPin : *, usingPin : *) : void {
            if (height > 0) {
                moveTower(height-1, fromPin, usingPin, toPin);
                moveDisk(fromPin, toPin);
                moveTower(height-1, usingPin, toPin, fromPin);
        function moveDisk(fromPin : *, toPin : *) : void {
            vConsole.text = vConsole.text+fromPin + "->" + toPin+"\n";
        function handleCreationComplete() : void {
            //Don't need to do anything anymore
        function handleDoItClicked() : void {
            vConsole.text = vConsole.text+"\n=== Doing a Hanoi tower "+vTowerInput.text+" tall\n";
            moveTower(new Number(vTowerInput.text), 1, 3, 2);        
    <mx:Text text="How tall a tower?" />
    <mx:TextInput id="vTowerInput" />
    <mx:Button label="Do It" click="handleDoItClicked()" />
<mx:TextArea id="vConsole" width="100%" height="100%" />

To replace the dialog (in Smalltalk) or the command line request (in Ruby), we just create a horizontal row of a question label, an input field, and a Do It button. Whenever a user clicks Do It, it takes the current value of the field and runs the Hanoi algorithm.

This is both pleasingly simply and also has the added benefit that we produced an interactive tool: you can just keep typing new numbers into it and it will give you new results.


Flex is highly capable with user input and interactivity. It has a cleaner / simpler UI model than Smalltalk (IMO), which is good given we have had about 30 years of working with UIs to take things forward. Compared to Ruby, Flex is as easy as RubyShoes for simple things like this example.

Where Next?

Next we should go to Chapter 4 — Page 44. The Third Example (with Real Hanoi Classes)


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: