@shiffman nature-of-code/noc-examples-processing#45 fixed in ruby ver…

…sion

chp5_physicslibraries/box2d/distance_joint/distance_joint.rb

@@ -25,10 +25,17 @@ def setup
 end
 
 def draw
-  background(255)
-  pairs.each(&:display)
+  background(255)  
+  # Look at all pairs, in reverse order
+  pairs.reverse_each do |p|
+    p.display
+    # pairs that leave the screen, we delete them
+    # (note they have to be deleted from both the box2d world and our list
+    next unless p.done?
+    pairs.shift
+  end
   # Display all the boundaries
-  boundaries.each(&:display)
+  boundaries.each(&:display)  
   fill(0)
   text('Click mouse to add connected particles.', 10, 20)
 end

chp5_physicslibraries/box2d/distance_joint/pair.rb

@@ -7,7 +7,7 @@
 class Pair
   extend Forwardable
   def_delegators(:@app, :box2d, :stroke, :line, :stroke_weight)
-  attr_reader :p1, :p2, :len
+  attr_reader :p1, :p2, :len, :w, :h
   # Chain constructor
   def initialize(x, y)
     @app = $app
@@ -27,6 +27,26 @@ def initialize(x, y)
     # We might need to someday, but for now it's ok
     box2d.world.create_joint(djd)
   end
+
+  def kill_bodies
+    box2d.destroyBody(p1.body)
+    box2d.destroyBody(p2.body)
+  end
+
+    # Is the pair ready for deletion?
+  def done?
+    # Let's find the screen position of the particle
+    pos1 = box2d.body_coord(p1.body)
+    pos2 = box2d.body_coord(p2.body)
+    # Is it off the screen?
+    if (0..@app.width).include?(pos1.x) || (0..@app.width).include?(pos2.x)
+      if (0..@app.height).include?(pos1.y) || (0..@app.width).include?(pos2.y)
+        return false
+      end
+    end
+    kill_bodies
+    return true
+  end
 
   def display
     pos1 = box2d.body_coord(p1.body)