Physics Joints - Learn SpriteBuilder for iOS Game Development - page 312

Note The springboard's base ( springstopper_metal ) needs no specific Category
or Masks settings. Because it's a static body, it won't collide with other static
bodies anyway, and all other bodies should certainly collide with the spring-
board base.
If you run this now, you will notice that the spring is still passing through the springstop-
per node. There's one minor yet important detail I haven't mentioned yet. Select the
CCPhysicsSpringJoint in Spring.ccb , and switch to the Item Properties tab. You'll see the
Collide bodies check box isn't selected. This means that the two bodies connected to this
joint will not collide, regardless of their collision categories and masks. In this case, you
want the springboard to stop at the springstopper, so select the Collide bodies check box.
You can now publish and try this out. You may want to enable physics debug drawing to
verify that the springboard is still slightly inside the shaft when it stops at the springstop-
per. If it isn't, simply increase or decrease the height of the bottom segment of the spring-
stopper's collision shape. Also, verify that the player isn't blocked by the springstopper.
Adding a Spring Lock
A springboard that springs up the instant the level starts is pretty useless. You can easily
turn this springboard into a touch-activated spring that can even reset itself to its locked
position after a short time by using a distance joint.
In Spring.ccb , drag and drop a Physics Distance Joint onto the stage. Connect Body A
with the springstopper. This will generate a static “world anchor” for this joint end, so it
doesn't actually matter what the position of the connected static physics body is. In fact,
you'll move that anchor far away shortly. Connect Body B with the springboard_spring .
On the joint's Item Properties tab, change Anchor A to 0x-200 . This moves the world an-
chor far below the spring. Set Anchor B to 32x32 , which about centers that end of the joint
on the spring.
The trick now is to enable both the Minimum distance and Maximum distance check
boxes. By default, the distances encompass the full length of the joint as it is now. The
joint will also draw two drag handles similar to the joint shown in Figure 9-21 . Don't drag
them. However, if you did drag them, you could click the Enable check box twice so that
it is turned off and on. Doing this will reset the Minimum distance and Maximum distance
fields.