Quote from a fan

In the last post I set an outline for a tower defense game based on Plants vs Zombies. My child says: “Plants vs Zombies is awesome, and I defeated Zombies until I got the hypnoshroom!”. So it must be pretty fun.

Theme

Plants vs Zombies has a great theme, and many interesting features and characters. The game play is simple, but wrapped in great art and concept. This is the key to any successful game.

Destroy Enemies

The next step is create enemies that we can destroy. In plants vs zombies, enemies can be hit multiple times before they are destroyed. This is an interesting game idea. It allows us to create enemies that can take more or fewer hits than other enemies. It also allows us to create weapons that deal more or less damage to enemies. These types of options allow players more interesting game play. It’s all about choice. It’s also about balance, but we’re getting ahead of ourselves.

At this stage we need to assign each enemy a value to keep track of how much damage it has taken. We will also need to monitor this value and remove the enemy when it reaches 0. To keep things simple at this stage in this example touching a zombie will apply damage to a zombie.

Dynamic objects

In Corona all objects are Dynamic. In this context Dynamic is a programming term. Dynamic objects allow you to add properties to them. Essentially this means we can create a new variables that objects carry around. In the case of the enemies in our game we’ll add a property called life. This will be a number that sets the number of times the enemy can be tapped before it is removed from from the screen.

When each object is added to the screen we need to add the life property to it. Inside the make_alien() function, after alien is defined add:

alien.life = 5

Now each alien created will also have a variable attached to it named life, and each alien will keep track of it’s own value for life. In this way each alien can have different values for life.

Next add a touch event to the alien:

alien:addEventListener( "touch", touch_alien )

I’m using a touch event here because it will react to a touch down on the screen. A tap event would not be fired until after your finger leaves the screen. This will make our game more responsive.

The last step is to define our handler touch_alien(). Remember, in Lua you need to define this function before we call on it, so it will have to be defined above make_alien(), or we need to add a forward declaration above.

local function touch_alien( event ) -- This handles touches on aliens.
	local phase = event.phase
	if phase == "began" then 			-- Check the phase.
		local alien = event.target		-- Get the target
		alien.life = alien.life - 1		-- reduce alien life total by 1
		if alien.life <= 0 then 		-- if 0 or less
			transition.cancel( alien.transition ) -- cancel transition
			remove_alien( alien )		-- remove alien
		end
	end
end

This function first checks the event phase. If the phase is “began” then it gets a reference to the alien that initiated the event via event.target. I used the variable name alien here for consistency. Next subtract 1 from the alien’s life. Last check the life total. If it 0 or less, remove the transition and remove the alien.

Testing the game. Touching an alien 5 times should remove it from the screen.