Controlling and Scrolling - Learn SpriteBuilder for iOS Game Development - page 64

turns the absolute position in points, and vice versa during assignment. The contentS-
ize and scale properties both have equivalent contentSizeInPoints and
scaleInPoints methods that you should prefer whenever you calculate in points.
The levelSize variable is assigned from _levelNode.contentSizeInPoints .
It is used in the two following lines, which clamp the viewPos to the level's size using
the MIN and MAX macros.
The viewPos coordinates are first clamped to the level extents using the inner MIN
macro. Here viewSize is subtracted from the levelSize . This is an inset calculation
because the screen should never scroll closer than viewCenter to the level's borders,
on both sides for both axes. The insets on all four borders of the level are visualized as
two-faced arrows, which are shown in Figure 3-3 . Both border distances added together
are conveniently equal to viewSize . Or, in other words, the scrollable area is two times
viewCenter or one viewSize smaller than the level area.
Figure 3-3 . Relation of Level Area to Scrollable Area. Arrows depict the Scrollable Area inset. Note the play-
er is not tied to its center position when near level borders. The diagram is not drawn to scale
The smaller of the two candidates, viewPos or levelSize minus viewSize , is re-
turned by the MIN macro to the enclosing MAX macro, which in turn ensures that the
viewPos is always equal to or greater than 0.0. Together, this prevents the
_levelNode from scrolling past its borders.
Finally, the viewPos is negated (multiplied by -1.0) and then assigned to the
_levelNode.positionInPoints . Remember what I said earlier about moving the
level node in the opposite direction? That's what this line does. For instance, if the calcu-
lated viewPos is 850,340, the ccpNeg method would change the coordinates to -850,-
-340 so that movement of the _levelNode goes in the opposite direction of where the
player is going.
Publish the SpriteBuilder project. Then build and run the Xcode project. You are now able
to scroll over the level's area, but no further.