10.1 Your Personality and a Little Maths
This tutorial is based on some code that we ported from our iPhone App Personality Profile. In this App you use a dial to answer a number of questions and a profile is provided based on your selection. We used a dial because we wanted a touch friendly solution and this seemed like an intuitive approach. The App is FREE on iTunes so download it and compare the functionality of the Objective C version to our Lua implementation. The embedded video above gives you an idea of how it works but the effect is better understood when used on your iDevice.
The trick to working out how to move the dial is a mathematical one. In essence you need to convert the cartesian co-ordinates of your finger on the screen to equivalent polar co-ordinates.
It is about now that you realise that those maths lessons way back weren't completely useless and perhaps you should have paid more attention. Don't panic, all you need to know is that it is possible to convert from a point (x, y) to polar co-ordinates which consist of an angle and a distance. The angle tells you how much to rotate the dial and you can use the distance portion to work out whether a point is within a circle which is handy for detecting if a tap was on the dial.
To make life easy we created a function which does all the hard work. We have reproduced it below but note that you also need the math.hypot() function which isn't available in the Codea version of Lua. Both of these functions (and more) are contained in the Math tab of RSLibrary.
We have made use of the new Codea dependency functionality so if you download the entire tutorial code in one file, then you also need to download RSLibrary and link it as a dependency to DialDemo. See Tutorial 9 if you want to understand dependencies better or see what is in RSLibrary. For this tute you only need the Math and Button tabs but you might as well grab the whole thing.
As an aside, a handy trick which we use in the math.polar function is to assign a default value to a parameter using "or". This works because nil evaluates to false in Lua. See the code below to understand how you can use this to overload functions.
10.2 The Main Class
In this instance the Main class is just there to demonstrate the Dial Class. Both Landscape and Portrait orientations are supported using the orientationChanged() function.
10.3 The Dial Class
The Dial Class is where the magic happens. We only have a licence to use the knob and dial images in our App, so you are going to have to draw or obtain your own version. Strictly speaking you only need a knob image (the bit that moves). The class should handle any differences in size to our sprites.