Delphi: Reading joysticks

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Delphi: Reading joysticks (And making a stopwatch to illustrate the concepts)

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This tutorial needs editing! Sorry! But I thought you'd rather have the information in it, however badly presented, than wait (probably forever) for the cosmetic surgery it needs? What happened is this: I wrote a first attempt at explaining the material in this. I then forgot I'd done that, and wrote a second attempt! You can read the first attempt just by reading the rest of this page. The second attempt is better, and covers everything that appears below and more... but you have to download it... but that means you get the sourcecode without having to type!

The downloadable version is not presented in the unusual format of these tutorials. The main text of the tutorial is included, as a text file, in the self extracting zip (SEZ) archive which also contains the application's source code. Click here to download that archive. Save the SEZ on your disc. You can go offline at that point. Double click on the SEZ. It was created with WinZip, and will allow you to select a destination folder for the material to be unzipped.

Confused yet? No? Well, let me mention that there is yet another joysticks tutorial available for you here. It shows you a better way to read them, at least for some purposes, but it does involve more advanced concepts. I.e. responding to Windows messages.

Here begins the first version of the tutorial....

This tutorial is based on a newsgroups post from Mark Wilkinson... thank you, Mark! This tutorial will be short on explanation and not much longer on program... because Delphi makes things so easy!!

I have a switch that goes off-on-off-on... in response to rainfall. (It can come to rest in either state.) The more rain, the more cycles. I wanted to monitor that switch, and the "fire button" input of a joystick seemed the obvious, easy way. Don't worry, I have another computer for important things like Interactive Magic's Apache and Novalogic's F-16, so losing the joystick port was acceptable.

The bad news: This information is helpful only if you have gone beyond Delphi 1.

The good news: Though I couldn't find it in the Delphi 2 helpfile, Delphi 2 has built in joystick support.

Thanks to a newsgroups post from Mark Wilkinson (homepage: Mark Wilkinson), I discovered enough to write my program for my rainfall monitor.

The essential bits of reading the joystick port are all in DD18. A zip archive of it's source files source can be downloaded by clicking here.

It puts up a window showing you the current x and y positions of the joystick, and the state of Fire buttons 1 and 2.

A few things you shouldn't overlook:

Set the timer interval to something small, say 20. (This determines how often the joystick is checked.)

Add MMSYSTEM to the "USES" clause. The unit is a standard, Borland unit, don't sorry. But be sure to add the reference.

A few things you don't need to worry about:

The variable MyJoy is declared to be of type TJoyInfo. This class is set up by Borland; you don't need to worry about the internal details.

The call of JoyGetPos passes a parameter by means of a pointer... I think!! the "@" sign isn't a mistake, it is to do with pointers, I should probably go learn something about why Borland did JoyGetPos the way they did... but in the meantime, I'm just going to use it and be happy.

The following procedure declaration header involves "var", which I don't often use, so I'll say a bit about it here if you haven't come across it. Note it appears before li1 AND before bo1. (If it isn't repeated, the boolean parameters bo1 and bo2 will not behave the way li1 and li2 behave.)

procedure ReadJoystick(var li1,li2:longint;var bo1,bo2:boolean);

The following is messy, and I wouldn't do things like this... but it illustrates what var is doing...

Imagine I have a global variable called Answer, and a procedure as follows:

procedure DoubleAndAdd(b1,b2:byte);
begin
b1:=b1+b1;
b2:=b2+b2;
Answer:=b1+b2;
end;

If, in my program, I said

DoubleAndAdd(3,4);

...then Answer would be filled with 14. Okay so far... simple stuff, so far.

Now suppose....

b1:=3;
b2:=4;
DoubleAndAdd(b1,b2);

.... Same result, I hope you agree. Answer is still filled with 14. Also, I were to check what was in b1 and b2 after the above, they would still hold 3 and 4. The b1 and b2 of the main program are separate from the b1 and b2 of the procedure. The fact that they have the same names is mere coincidence. This is all about scope... an important concept, one of the things Pascal Got Right, thus making it easy to write robust programs.

Now we start the tricky stuff....

If the procedure said:

procedure DoubleAndAdd(var b1,b2:byte);
begin
b1:=b1+b1;
b2:=b2+b2;
Answer:=b1+b2;
end;

(same as before, except for the "var")

and the main part of the program said the same as before....

b1:=3;
b2:=4;
DoubleAndAdd(b1,b2);

.... Answer would still be filled with 14... BUT!....now if I were to check what was in b1 and b2 after the above, they would hold 6 and 8. Even more "magic": if I did.....

FirstVariable:=3;
SecondVariable:=4;
DoubleAndAdd(FirstVariable,SecondVariable);

... Answer would hold 14, FirstVariable would hold 6, and SecondVariable would hold 8. 3 went into FirstVariable, in that it went into the procedure. In the procedure, it was moved to b1. As the procedure was finishing up, it passed back to FirstVariable whatever was in b1 at the end of executing the procedure.

There are other solutions. (user defined types.) Using var in the procedure declaration might be seen as a clumsy solution... but it works! Be sure to put var in front of each group of variables you want to pass that way.

Enjoy!

That's the simple approach to joysticks.... remember there are other tutorials on the topic in these pages, especially the downloadable self extracting zip with the improved version of this material!

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