Tag: game loop

Corona – Game Loops part 3

Following up on my previous two posts, this discussion will continue with the game loop topic. Here I’ll create a couple game objects, or actor objects, that will work with the game loop.

As previously discussed, actors work need to interface with the game loop by implementing the update(deltaTime) method. This interface is just a function that each actor object must posses. How the actors implement this function is up to the actor.

In this example I’ll create two types of objects. One object will start at the top of the screen and move down the screen. I’ll call these objects rocks.

The second object will move to the location of a touch on the screen. I’ll call this a ship.

Object when created will need to add themselves to the game_loop. To make this easy we’ll use the singleton nature of Lua modules to help. Every file that uses require(“game_loop”) will be getting a reference to the same programming object. Any module that require game loop will be talking to the same game loop.

The rock objects will remove themselves from the display when they reach the bottom of the screen. This means, rocks will also have to remove themselves from the game loop at the time they remove themselves from the display. To make this easy we’ll make use of the singleton nature of the game_loop module again.

rocks.lua –
I made a file named rocks.lua. This file contains the code that will produce “rock” objects. The rocks will fall down the and remove themselves when they get past the bottom edge of the screen.

The rocks module have the following methods.

make() rock – This function returns a rock.

This module requires game_loop.

The rocks module acts as factory that produces rock objects. Each rock is is a Corona shape object, a rectangle. Corona display objects are essentially tables. Each rock has two methods.

rock:remove() – removes the rock from game_loop, and then from display.
rock:update( deltaTime ) – updates the position of the rock.


--------------------------------------------------------------------
--
-- rocks.lua 
-- 
--------------------------------------------------------------------
local M = {}
--------------------------------------------------------------------

local game_loop = require("game_loop")

local xMax = display.contentWidth
local xMin = 0
local yMax = display.contentHeight + 40
local yMin = 0

local function make()
	local rock = display.newRect( 0, 0, 40, 40 )
	rock:setFillColor( 0, 1, 0 )
	rock.x = math.random( xMin, xMax )
	rock.y = yMin - 40
	rock.speed = math.random() * 3 
	
	function rock:remove()
		game_loop.remove( self )
		display.remove( self ) 
	end
	
	function rock:update( dt ) 				-- receive dt here as a parameter. 
		self.y = self.y + self.speed * dt 	-- Use dt, multiply by pixels per frame (speed)
		if self.y > yMax then 
			
			self:remove()
		end 
		return self
	end
	
	game_loop.add( rock )
	 
	return rock
end 
M.make = make

--------------------------------------------------------------------
return M

ship.lua –
The ship.lua module is very similar to the rocks module. This module acts as factory that produces ship objects. The ship module has a single method.

make() ship – The make method returns a a ship object.

The ship module requires the game_loop module. Each ship created is added to game_loop.

Ship objects have three methods.

ship:move_to( x, y ) – starts the ship moving towards the x, and y.
ship:remove() – Removes the ship from game_loop and from the display.
ship:update( deltaTime ) – Updates the position of the ship. 


-----------------------------------------------
--
-- ship.lua
-- 
-----------------------------------------------
local M = {}
-----------------------------------------------
local game_loop = require( "game_loop" )
-----------------------------------------------

local function make()
	local ship = display.newRect( display.contentCenterX, display.contentHeight - 100, 20, 40 )
	
	ship.target_x = ship.x
	ship.target_y = ship.y
	
	function ship:move_to( x, y )
		self.target_x = x
		self.target_y = y
	end 
	
	function ship:remove()
		game_loop.remove( self )
		display.remove( self )
	end 
	
	function ship:update( dt )
		ship.x = ship.x - (ship.x - ship.target_x) * 0.1
		ship.y = ship.y - (ship.y - ship.target_y) * 0.1
	end 
	
	game_loop.add( ship )
	
	return ship
end 
M.make = make

-----------------------------------------------
return M

Here’s a simple program to test out the modules defined above. 


-----------------------------------------------------
-- Import the game loop module 
local game_loop = require("game_loop")
game_loop:run()
-----------------------------------------------------

-- Make a player
local ship = require( "ship" ) 
local player = ship.make()

local function move_ship( x, y )
	player:move_to( x, y )
end 

local function on_touch( event )
	if event.phase == "began" then 
		move_ship( event.x, event.y )
	elseif event.phase == "moved" then 
		move_ship( event.x, event.y )
	end
end 
Runtime:addEventListener( "touch", on_touch )
-----------------------------------------------------

-- A function to make rocks
local rocks = require( "rocks" )

local function make_rock()
	local rock = rocks.make()
end 

-- Use a timer to make rocks periodically
local rock_timer = timer.performWithDelay( 900, make_rock, 0 )
-----------------------------------------------------

Let’s take a look at each part. The first two lines load up the game_loop module, and start it running. 


local game_loop = require("game_loop")
game_loop:run()

Next we make a player object. The player object will be made from a ship object. To move the ship I’m calling on the ship objects move_to( x, y ) method. Here I made a touch event that calls on move_to() with the event.x and event.y. Which tells the player/ship to move to the position of the touch event.

The event calls move_ship() during both the began and moved phases. The effect is that the ship will start moving to the position of the point your finger makes contact with the screen, and updates position as you drag your finger across the screen. 


local ship = require( "ship" ) 
local player = ship.make()

local function move_ship( x, y )
	player:move_to( x, y )
end 

local function on_touch( event )
	if event.phase == "began" then 
		move_ship( event.x, event.y )
	elseif event.phase == "moved" then 
		move_ship( event.x, event.y )
	end
end 
Runtime:addEventListener( "touch", on_touch )

Lest create a timer to create a new rock every 900 milliseconds. 


local rocks = require( "rocks" )

local function make_rock()
	local rock = rocks.make()
end 

-- Use a timer to make rocks periodically
local rock_timer = timer.performWithDelay( 900, make_rock, 0 )

Here’s a few ideas to try on your own.

Make a new object type that moves differently from the two examples here.
Add some new features to the one or both of the objects here. For example make the rocks rotate as they move down the screen.
Use images, or better yet sprites, in place of the rectangles I used.

Corona – Game Loop part 2

Here’s a follow up to the previous post on game loops. The game loop works with actor objects. Actors are stored in an array. The game loop sends it’s actors an update message each frame, and passes the delta time.

Here I have created a Lua module to handle the game and appropriately named it: game_loop. Using a module gives us three advantages.

First, the code is easily portable. You can copy this Lua file/Module into another project and use the game there.

Second, the code is isolated. This makes it easier to edit the game loop if necessary.

Last, Lua modules are only loaded once. If we load this file with require() in file A, and then load it again with require() in file B, file B sees the same game_loop with the same variable values. It doesn’t load a new unique instance. In effect Lua modules act like singleton objects. For the game loop this is perfect. We will only need one game loop, and many other modules will want to access it.

Here’s the game_loop module. Move all of the code here into a file named game_loop.lua.


---------------------------------------------------
--
-- game_loop.lua
--
---------------------------------------------------
local M = {}
---------------------------------------------------

local MSPF = 1000 / display.fps
local actors_array = {}
local last_ms = 0
local isRunning = false


function add( obj ) 
	if table.indexOf( actors_array, obj ) == nil then 
		actors_array[#actors_array+1] = obj
	end 
end 
M.add = add
	
function remove( obj )
	local index = table.indexOf( actors_array, obj )
	if index ~= nil then 
		table.remove( actors_array, index )
	end  
end 
M.remove = remove
	
function enterFrame( event )
	local ms = event.time
	local dt = MSPF / ( ms - last_ms )
	last_ms = ms
	
	for i = #actors_array, 1, -1 do 
		local actor = actors_array[i]
		actor:update( dt )
	end 
end 

	
function run()
	if isRunning == false then 
		Runtime:addEventListener( "enterFrame", enterFrame )
		isRunning = true
	end 
end 
M.run = run	


function pause()
	if isRunning then 
		Runtime:removeEventListener( "enterFrame", enterFrame )
		isRunning = false
	end 
end 
M.pause = pause
---------------------------------------------------
return M

Load game_loop.lua with require(“game_loop”). When using require() do not include the .lua file extension.

The game_loop object/table has four methods.

game_loop.add( obj ) — Adds obj to the game loop, obj must implement
the update(dt) method.

game_loop.remove( obj ) — Removes obj from the game loop.

game_loop.run() — Starts the game loop by adding an enterFrame listener.

game_loop.pause() — pauses the game loop by removing the enterFrame listener.

A game_loop keeps track of objects, we’ll refer to these as actors, in an array/table. Each frame game_loop sends each of the actors an update() message and passes the delta time (dt).

To use this in a game all actor objects must implement the update() method.
Actors are added to the game_loop using game_loop.add( actor ). When an actor
is to be removed from the display it should also be removed from the game_loop using

game_loop.remove( actor )

The game_loop can be stopped and started with:

game_loop.run() 
game_loop:pause()

To test this you can add the following code to main.lua in a new project. The code below first creates a new game loop. Touching the screen creates a new box. Boxes are added to the game loop that sends them update messages each frame. Touching a box removes it from the game loop, and then from the screen.

At the bottom I created two buttons to to run and pause the game loop.

-- Import the game loop module 
local game_loop = require("game_loop")
-- Start the game loop 
game_loop.run()

-- make green boxes
local function make_green_box()
	local box = display.newRect( 0, 0, 40, 40 )
	box:setFillColor( 0, 1, 0 )
	box.x = 0
	box.y = math.random( 0, 480)
	box.speed = math.random() * 3 
	
	function box:update( dt ) -- receive dt here as a parameter. 
		self.x = self.x + self.speed * dt -- Use dt, multiply by pixels per frame (speed)
		if self.x > 320 then 
			self.x = 0
		end 
	end 
	return box
end 




-- Tap a box to remove a box
local function remove_box( event )
	if event.phase == "began" then 
		-- Remove the box from the loop
		local box = event.target
		game_loop.remove( box )		-- Remove box from loop
		display.remove( box )	-- Remove box from display
	end 
	return true
end 


-- Tap the screen to add a box
local function add_box( event )
	if event.phase == "began" then
		local box = make_green_box() -- Make a new box
	
		box.x = event.x	-- Position box
		box.y = event.y
	
		box:addEventListener( "touch", remove_box ) -- Add a touch event 
	
		game_loop.add( box )	-- Add box to the game loop 
	end 
	return true
end 

Runtime:addEventListener( "touch", add_box )





local run_button = display.newRect( 31, 21, 60, 40 )
run_button:setFillColor( 0, 200/255, 0 )

local pause_button = display.newRect( 92, 21, 60, 40 )
pause_button:setFillColor( 200/255, 0, 0 )

local function touch_run( event )
	if event.phase == "began" then 
		game_loop.run()
	end 
	return true 
end 

local function touch_pause( event )
	if event.phase == "began" then 
		game_loop.pause()
	end 
	return true
end 

run_button:addEventListener( "touch", touch_run )
pause_button:addEventListener( "touch", touch_pause )

To expand on what’s here, you could imagine every actor in the actor_array as a table. These
actor tables could contain properties describing how the game_loop should treat each
actor. A simple idea might be to give each a type, and active property. As is the game
loop can be paused and run. Using this idea you could pause or run actors in groups.