removing old tiles in a running game made with as3 and flash - actionscript-3

Question

I started to build a run game app for Android. I chose to make it in flash using tiles and Adobe Air. The game is that a player should run automatically to the right and avoid some obstacles by jumping or sliding along the ground.
I have made a function which always takes the first level in the array and uses as the starting level.
private function createLevel()
{
map_level.levell();
level = map_level.level1;
for(var t = 0; t < level.length; t++)
{
for(var u = 0; u < level[t].length; u++)
{
if(level[t][u] != 0)
{
var new_tile:platform_tile = new platform_tile;
addChild(new_tile);
new_tile.gotoAndStop(level[t][u]);
new_tile.x = u * 32;
new_tile.y = t * 32;
tiles.push(new_tile);
}
}
}
total_tile_width += u;
}
Then I create a function that takes a random level in the array of paths.
private function random_level ()
This level is then added at the end of the first track when the player has reached a certain length along the track, then the track seems endless and then made such that the camera follows the player.
private function update_level ()
{
random_level();
for(var t = 0; t < mid_lvl.length; t++)
{
for(u = 0; u < mid_lvl[t].length; u++)
{
if(mid_lvl[t][u] != 0)
{
var new_tile:platform_tile = new platform_tile;
level[t][u + total_tile_width] = mid_lvl[t][u];
addChild(new_tile);
new_tile.gotoAndStop(mid_lvl[t][u]);
new_tile.x = (u + total_tile_width) * 32;
new_tile.y = t * 32;
tiles.push(new_tile);
}
}
}
// Indstiller hvis spilleren skal have en stigende fart
if( movementspeed < 40)
{
movementspeed = movementspeed + 2;
}
else
movementspeed = movementspeed;
total_tile_width += u;
trace ("speed: " + movementspeed);
}
All this works as it should and game function also perfect as a PC, but the phone seems quick to overload it, since I can not figure out how to remove the old levels that have already been played and therefore there's going to be a lot levels in the phone memory.
I need to something like removeChild("old tiles the left the stage again) but got no idear how to only find the tiles that old and not just all tiles.
Anyone able to help me? btw hope you understand my question as im not the best at writing english.
Morten

Answer 1

You must use 2 BitmapData objects.
2 are the size of the screen. We call them screenBuffer1 and screenBuffer2
the other is larger than the screen by at least the width of a tile. We call it terrainBuffer
Then :
Init the terrainBuffer by drawing all visible tiles of the start screen
Copy the currently visible area on screenBuffer1
Draw sprites on screenBuffer1
Attach screenBuffer1 to stage
Now we start scrolling
Copy the currently visible area on screenBuffer2, offset by scroll amount
Draw sprites on screenBuffer2
Attach screenBuffer2 to stage
Continue increasing offset and alternate screenBuffer1 and screenBuffer2 so one is visible while you draw on the other
When offset reach the witdh of a tile :
if(offset>tileWidth){
offset-=tileWidth;
// Move content of terrainBuffer to the left by tileWidth pixels
// Draw a new column of tile on the right
}
Then keep on looping !