A quick breakdown on datablocks, objects, %this and %obj

So, today, whilst rewriting my AI code in order to make it more modular, I had to get a much better grasp on the difference between datablocks and objects when it comes to calling functions. In this context, I was getting confused between AIPlayer and, we'll say, DemoPlayer, and why calling functions between them wasn't working as expected.

Well, the way I now remember it, when writing functions that I want to be attached to a datablock, and thus more easily overloaded, is that a datablock function, typically uses %obj, a reference variable to the object the datablock is assigned to. In my AI example, my AIPlayer object has a datablock, DemoPlayer, assigned to it, but if I wanted a different type of AI, I'd assign it a different datablock, say TestPlayer.

So, here's the situation broken down: AIPlayer has functions that are used by all my AI types. One AI type will use a datablock named DemoPlayer and another AI type will use a datablock named TestPlayer. Both of these AIs will be able to make a melee attack, but one of them has twice the range. Below is how I would set up the script:

pickData(%this, %num)
{
   switch(%num)
   {
      case 1:
         AIPlayer::spawnAI("DemoPlayer");
      case 2:
         AIPlayer::spawnAI("TestPlayer");
   }
}

AIPlayer::spawnAI(%this, %datablock)
{
   %player= new AIPlayer()     //create a new AIPlayer object and give it the DemoPlayer datablock
   {
       datablock = %datablock;
   }
   %player.mainloop();
}

AIPlayer::mainloop(%this)
{
   %this.getDatablock().doMelee(%this);
   %this.mainloop();
}

DemoPlayer::doMelee(%this, %obj)
{
   echo(%obj @ " does short range melee attack");
}

TestPlayer::doMelee(%this, %obj)
{
   echo(%obj @ " does long range melee attack");
}

The above code makes a new AIPlayer with either the DemoPlayer datablock or the TestPlayer datablock, depending on whether you call the pickData function with either a 1 or a 2. Then, the mainloop is called and the doMelee function is called depending on whether the AI uses a DemoPlayer or TestPlayer datablock. Then the echo function prints a string that starts with the handle of the object that that datablock is assigned to, which is passed as the %obj variable.

While I'm at it, I'll try to describe the %this variable. It's pretty simple, really. When a function gets called, the handle of the calling object gets sent along with the function call. In this case, when we call %player.mainloop(), the AIPlayer object's handle is passed along with the call for the mainloop function. In the mainloop function, when we call %this.mainloop(), what we're really saying is (AIPlayer object handle).mainloop().

And that's about it. I don't know if this was very clear or not, but I know I regularly get confused about it, so I figured I'd write a blog about it.