Someone

var listenerStageResize:Object = new Object();
listenerStageResize.onResize = function()
{
	if	(Stage.width < Stage.height || objectBackground._width <= objectBackground._height)
	{
		objectBackground._height = Stage.width / (objectBackground._width / objectBackground._height);
		objectBackground._width = Stage.width;
	}
	
	else if	(Stage.width > Stage.height || objectBackground._width >= objectBackground._height)
	{
		objectBackground._width = Stage.height / (objectBackground._height / objectBackground._width);
		objectBackground._height = Stage.height;
	}
	
	objectBackground._x = (Stage.width / 2) - (objectBackground._width / 2);
	objectBackground._y = (Stage.height / 2) - (objectBackground._height / 2);
};

Stage.addListener(listenerStageResize);

// Call the function at the start to ensure the first frame is rendered correctly as well.
listenerStageResize.onResize();