Add a damage step for each modifier, resulting in trees of steps

src/main/java/org/spongepowered/api/event/cause/entity/damage/DamageModifier.java

@@ -24,18 +24,44 @@
  */
 package org.spongepowered.api.event.cause.entity.damage;
 
+import org.spongepowered.api.Sponge;
+
 /**
  * A damage modifier that will be applied before or after a {@link DamageStep}.
  */
-@FunctionalInterface
 public interface DamageModifier {
 
     /**
-     * Modifies the damage.
+     * Gets the {@link DamageStepType} of this modifier.
+     *
+     * @return the modifier type.
+     */
+    DamageStepType type();
+
+    /**
+     * Gets the function that modify the damage.
      *
-     * @param step The damage step this modifier is associated with.
-     * @param damage The current damage value.
-     * @return The next damage value
+     * @return the modifier function.
      */
-    double modify(DamageStep step, double damage);
+    Function function();
+
+    @FunctionalInterface
+    interface Function {
+        /**
+         * Modifies the damage.
+         *
+         * @param step   The damage step this modifier is associated with.
+         * @param damage The current damage value.
+         * @return The next damage value
+         */
+        double modify(DamageStep step, double damage);
+    }
+
+    static DamageModifier of(DamageStepType type, Function function) {
+        return Sponge.game().factoryProvider().provide(Factory.class).of(type, function);
+    }
+
+    interface Factory {
+        DamageModifier of(DamageStepType type, Function function);
+    }
 }

src/main/java/org/spongepowered/api/event/cause/entity/damage/DamageStep.java

@@ -27,30 +27,26 @@
 import org.spongepowered.api.event.Cause;
 
 import java.util.List;
+import java.util.OptionalDouble;
 
 /**
- * Represents a step in the damage calculation.
+ * Represents a tree of steps in the damage calculation.
+ * A damage calculation is made of multiple trees of steps.
  */
 public interface DamageStep {
 
     /**
-     * Gets the {@link DamageStepType} for this {@link DamageStep}.
+     * Gets the {@link DamageStepType} for this step.
      *
      * @return The damage step type
      */
     DamageStepType type();
 
-    /**
-     * Gets the cause of this {@link DamageStep}.
-     *
-     * @return The cause of this step
-     */
-    Cause cause();
-
     /**
      * Gets whether this step is skipped.
-     * When skipped, only the step and its side effects are ignored, modifiers are still applied.
-     * A modifier willing to ignore every previous modifiers should revert the damage to {@link #damageBeforeModifiers()}.
+     * When skipped, only the step itself and its side effects are ignored, children are still applied.
+     * A modifier willing to ignore every previous children should revert the damage to {@link #damageBeforeChildren()},
+     * or call {@link #skip} on each child.
      *
      * @return Whether this step is skipped
      */
@@ -75,47 +71,74 @@ default void skip() {
     }
 
     /**
-     * The damage just before the modifiers of this step.
+     * The damage just before the children of this step.
+     * Returns empty if the value is not known yet.
      *
-     * @return The damage before this step
+     * @return The damage before the children of this step
      */
-    double damageBeforeModifiers();
+    OptionalDouble damageBeforeChildren();
 
     /**
      * The damage just before this step.
+     * Returns empty if the value is not known yet.
      *
      * @return The damage before this step
-     * @throws IllegalStateException if called before the "before" modifiers have finished.
      */
-    double damageBeforeStep();
+    OptionalDouble damageBeforeSelf();
 
     /**
      * The damage just after this step.
+     * Returns empty if the value is not known yet.
      *
      * @return The damage after this step
-     * @throws IllegalStateException if called before this step has finished
      */
-    double damageAfterStep();
+    OptionalDouble damageAfterSelf();
 
     /**
-     * The damage just after the modifiers of this step.
+     * The damage just after the children of this step.
+     * Returns empty if the value is not known yet.
      *
      * @return The damage after this step
-     * @throws IllegalStateException if called before the modifiers have finished.
      */
-    double damageAfterModifiers();
+    OptionalDouble damageAfterChildren();
 
     /**
-     * Gets an immutable list of all modifiers that applies just before this step.
+     * Gets an immutable list of all children steps that applies just before this step.
      *
-     * @return The list of modifiers
+     * @return The list of children steps
      */
-    List<DamageModifier> modifiersBefore();
+    List<DamageStep.Child> childrenBefore();
 
     /**
-     * Gets an immutable list of all modifiers that applies just after this step.
+     * Gets an immutable list of all children steps that applies just after this step.
      *
-     * @return The list of modifiers
+     * @return The list of children steps
+     */
+    List<DamageStep.Child> childrenAfter();
+
+    /**
+     * Represents a step made by the platform (vanilla and mods).
      */
-    List<DamageModifier> modifiersAfter();
+    interface Root extends DamageStep {
+
+        /**
+         * Gets the {@link Cause} of this step.
+         *
+         * @return The cause of this step
+         */
+        Cause cause();
+    }
+
+    /**
+     * Represents a step made by a plugin (a modifier).
+     */
+    interface Child extends DamageStep, DamageModifier {
+
+        /**
+         * Gets the parent of this step.
+         *
+         * @return The parent of this step
+         */
+        DamageStep parent();
+    }
 }

src/main/java/org/spongepowered/api/event/entity/DamageCalculationEvent.java

@@ -117,12 +117,12 @@ interface Post extends DamageCalculationEvent {
         double finalDamage();
 
         /**
-         * Gets the list of the captured steps during the damage calculation in the order they have been applied.
+         * Gets the list of the captured root steps during the damage calculation in the order they have been applied.
          * Note that this list is not an exhaustive representation of all the operations applied,
          * especially in a modded environment.
          *
-         * @return The list of steps
+         * @return The list of root steps
          */
-        List<DamageStep> steps();
+        List<DamageStep.Root> steps();
     }
 }