Using memory efficiently

In some of our earlier examples, we used global variables in our code. Cases like those are an exception since the examples did not contain a high volume of functions, loops to call out to or display objects. Once you start building a game that is heavily involved with function calls and numerous display objects, local variables will increase performance within your application and will be placed on the stack so Lua can interface with them faster.

The following code will cause memory leaks:

-- myImage is a global variable
 myImage = display.newImage( "image.png" )
myImage.x = 160;  myImage.y = 240

-- A touch listener to remove object
local removeBody = function( event )
  local t = event.target
  local phase = event.phase

  if "began" == phase then
    -- variable "myImage" still exists even if it's not displayed
    t:removeSelf() -- Destroy object
  end

  -- Stop further propagation of touch event
  return true
end

myImage:addEventListener( "touch", removeBody )

The preceding code removes myImage from the display hierarchy once it is touched. The only problem is that the memory used by myImage leaks because the variable myImage still refers to it. Since myImage is a global variable, the display object it references will not be freed even though myImage does not display on the screen.

Unlike global variables, localizing variables help speed up the look-up process for your display object. Also, it only exists within the block or chunk of code that it's defined in. Using a local variable in the following code will remove the object completely and free up memory:

-- myImage is a local variable
 local myImage = display.newImage( "image.png" )
myImage.x = 160;  myImage.y = 240

-- A touch listener to remove object
local removeBody = function( event )
  local t = event.target
  local phase = event.phase

  if "began" == phase then
    t:removeSelf() -- Destroy object
  end

  -- Stop further propagation of touch event
  return true
end

myImage:addEventListener( "touch", removeBody )

Optimizing your display images

It's important to optimize your image file size as much as you can. Using full-screen images can impact the performance of your application. They require more time to load on a device and consume a lot of texture memory. When a lot of memory is consumed in an application, in most cases it'll be forced to quit.

iOS devices vary in available memory, depending which one you have:

For example, texture memory on the iPhone 3GS should be kept under 25 MB before performance issues start occurring by slowing down your app or even forcing it to quit. An iPad 2 would have no problem going further downthat boundary since it has more memory available.

Refer to the following link to apply memory warnings for iOS devices: http://developer.anscamobile.com/reference/index/memorywarning-ios.

For Android devices, there is around a 24 MB memory limit. So it's important to be aware of how many display objects you have in your scene and how to manage them when they are not needed anymore in your app.

In cases where you no longer need an image to be displayed on screen use the following code:

image.parent:remove( image ) -- remove image from hierarchy
-- or --
image:removeSelf( ) -- same as above

If you want to remove an image from the scene completely throughout the lifetime of your app, include the following line after your image.parent:remove( image ) or image:removeSelf() code:

image = nil

Keeping memory usage low within your application will prevent crashes and improve performance. For more information on optimization, go to the following URL: http://developer.anscamobile.com/content/performance-and-optimization.

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