Building Games in Flash 5 - Part 2 - Scrolling - Page 4

onClipEvent(enterFrame) code:

Directly below that type the following code:

onClipEvent (enterFrame) {

	this._x-=groundSpeed;
	if (this._x<= (groundStartx-ground._width)){
		this._x=groundStartx-groundSpeed;
	}


}

This is contained within an enterFrame onClipEvent so it will be run over and over again. Remember that the code within an enterFrame clipEvent is run everytime the movieclip enters a new frame. The mainGround movieclip has just one frame - but it is still constantly entering that one frame.

So what does it do?
The second line: this._x-=groundSpeed; reduces the x-coordinate of the ground by the value of groundSpeed. As we set groundSpeed to 10, then the code moves the mainGround to the left by 10 pixels.

The third line checks to see if the mainGround has moved so far to the left that the first ground movie clip is completely off the stage, if this is true then the fourth line sets the x-coordintate of the mainGround such that it is positioned back where it started.

But what are all those variables about?
this._x is the current x-coordinate of the mainGround.
ground._width is the width of the ground movie clip.
groundStartx is the variable we set up earlier. When the game starts the first ground exactly fills the stage. So this variable stores the x-coordinate at which the first ground fills the stage.

So groundStartx - ground._width is the start x-coordinate minus the width of the ground.

When the first ground is completely to the left of the stage its x-coordinate will be equal to its start x-coordinate minus its width. At this point the second ground will be exactly filling the stage.

When we reset the mainGround back to its start position on the stage we have to also subtract the groundSpeed because the mainGround still needs to be moving left by the amount specified in groundSpeed.

You can now test the scrolling ground. In the flash application the ground movement might look a little strange because in test mode the flash application allows you to see graphics that are off the stage - so test in a browser if you want to see only the stage.

OK, so now you have a nice scrolling ground below your spaceship.
If you haven't got it working you can grab the source file by downloading this tutorials zip file.