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--- dev:abstract_elements [2014/09/23 18:38] – created tsmall1
+++ dev:abstract_elements [2014/09/23 19:35] (current) – [Create abstract class] tsmall1
@@ Line 3: / Line 3: @@
 It may be useful to abstract the behavior of your Elements. In this tutorial, we are going to create two Elements which inherit from a common base Element.
+For this tutorial, we are going to make three Elements: ''AbstractElement_Wanderer'', ''Element_Wanderer_Cyan'', and ''Element_Wanderer_Magenta''. These are in the repository.
+===== Create abstract class =====
+First we need to create ''AbstractElement_Wanderer''. This is going to be very similar to creating any other element, besides a few important differences.
+First, we cannot use ElementParameters in the base class. This creates conflicts when saving **.mfs** files and all children will be loaded with the same parameters.
+Second, our constructor must take a ''UUID'' and have no body, like this:
+<code>
+AbstractElement_Wanderer(UUID u) : Element<CC>(u)
+{ }
+</code>
+We should also not define ''GetDefaultPhysicsColor'', since the children will want to be different colors.
+We also cannot define **THE_INSTANCE**, since an abstract class cannot be instantiated.
+We need a behavior method which wanders the same for all children. This looks like:
+<code>
+virtual void Behavior(EventWindow<CC>& window) const
+{
+  SPoint wanderPt;
+  Random& rand = window.GetRandom();
+  MDist<R> md = MDist<R>::get();
+  Dir d = (Dir)rand.Create(Dirs::DIR_COUNT);
+  Dirs::FillDir(wanderPt, d);
+  wanderPt *= Dirs::IsCorner(d) ? (GetWanderDistance() / 2) : GetWanderDistance();
+  if(window.IsLiveSite(wanderPt))
+  {
+    if(window.GetRelativeAtom(wanderPt).GetType() ==
+       Element_Empty<CC>::THE_INSTANCE.GetType())
+       {
+         window.SwapAtoms(wanderPt, SPoint(0, 0));
+       }
+    }
+  }
+}
+</code>
+Finally, we need a new pure abstract method. This will allow a sub-class to tell this base class how far it would like to wander per event. We write it like this, in a ''protected'' section:
+<code>
+virtual u32 GetWanderDistance() const = 0;
+</code>
+===== Create child classes =====
+Next, we need some child classes to inherit from this one. We will create two classes, which are nearly identical, to do this.
+First, each of these classes gets an ''ElementParameterS32'' named ''m_wanderDistance''. **These need to be initialized with unique tags!**
+Next, change the class definition and the constructor to use ''AbstractElement_Wanderer<CC>'' instead of ''Element<CC>''.
+Next, define ''DefaultPhysicsColor'' to return ''0xff00ffff'' for the Cyan element, and ''0xffff00ff'' for the Magenta element.
+Finally, define ''GetWanderDistance'' as such:
+<code>
+protected:
+ virtual u32 GetWanderDistance() const
+ {
+    return (u32)m_wanderDistance.GetValue();
+ }
+</code>