Added state watch functionality

src/main/kotlin/net/axay/kspigot/inventory/InventoryGUI.kt

@@ -27,13 +27,23 @@ abstract class InventoryGUI<T : ForInventory>(
 
     internal abstract val bukkitInventory: Inventory
 
+    internal abstract fun loadPageUnsafe(
+        page: Int,
+        offsetHorizontally: Int = 0,
+        offsetVertically: Int = 0
+    )
+
     internal abstract fun loadPageUnsafe(
         page: InventoryGUIPage<*>,
         offsetHorizontally: Int = 0,
         offsetVertically: Int = 0
     )
 
-    internal abstract fun loadPageUnsafe(page: Int, offsetHorizontally: Int = 0, offsetVertically: Int = 0)
+    internal abstract fun loadContent(
+        content: Map<Int, InventoryGUISlot<*>>,
+        offsetHorizontally: Int = 0,
+        offsetVertically: Int = 0
+    )
 
     /**
      * @return True, if the [inventory] belongs to this GUI.
@@ -85,29 +95,47 @@ class InventoryGUIShared<T : ForInventory>(
 
     override fun isThisInv(inventory: Inventory) = inventory == bukkitInventory
 
+    override fun loadPageUnsafe(page: Int, offsetHorizontally: Int, offsetVertically: Int) {
+        data.pages[page]?.let { loadPageUnsafe(it, offsetHorizontally, offsetVertically) }
+    }
+
     override fun loadPageUnsafe(page: InventoryGUIPage<*>, offsetHorizontally: Int, offsetVertically: Int) {
 
         val ifOffset = offsetHorizontally != 0 || offsetVertically != 0
 
-        if (!ifOffset)
+        if (!ifOffset) {
+
+            // unregister this inv from all elements on the previous page
+            HashSet(currentPage.slots.values).forEach { if (it is InventoryGUIElement) it.stopUsing(this) }
+
             currentPageInt = page.number
 
-        page.slots.let { slots ->
+        }
+
+        loadContent(page.slots, offsetHorizontally, offsetVertically)
+
+    }
+
+    override fun loadContent(
+        content: Map<Int, InventoryGUISlot<*>>,
+        offsetHorizontally: Int,
+        offsetVertically: Int
+    ) {
+
+        val ifOffset = offsetHorizontally != 0 || offsetVertically != 0
 
         val dimensions = data.inventoryType.dimensions
 
+        // clear the space which will be redefined
         if (ifOffset) {
             dimensions.invSlots.forEach {
-                    dimensions.invSlotsWithRealSlots[it.add(offsetHorizontally, offsetVertically)]?.let { slotToClear ->
+                val slotToClear = dimensions.invSlotsWithRealSlots[it.add(offsetHorizontally, offsetVertically)]
-                        if (dimensions.realSlots.contains(slotToClear))
+                if (slotToClear != null) bukkitInventory.clear(slotToClear)
-                            bukkitInventory.clear(slotToClear)
-                    }
-                }
-            } else {
-                bukkitInventory.clear()
             }
+        } else bukkitInventory.clear()
+
+        content.forEach {
 
-            slots.forEach {
             val slot = it.value
             if (slot is InventoryGUIElement) {
 
@@ -115,11 +143,11 @@ class InventoryGUIShared<T : ForInventory>(
                     val invSlot = InventorySlot.fromRealSlot(it.key, dimensions)
                     if (invSlot != null) {
                         val offsetSlot = invSlot.add(offsetHorizontally, offsetVertically).realSlotIn(dimensions)
-                            if (offsetSlot != null)
+                        if (offsetSlot != null) bukkitInventory.setItem(offsetSlot, slot.getItemStack(offsetSlot))
-                                bukkitInventory.setItem(offsetSlot, slot.getItemStack(offsetSlot))
                     }
                 } else {
                     bukkitInventory.setItem(it.key, slot.getItemStack(it.key))
+                    slot.startUsing(this)
                 }
 
             }
@@ -128,12 +156,6 @@ class InventoryGUIShared<T : ForInventory>(
 
     }
 
-    }
-
-    override fun loadPageUnsafe(page: Int, offsetHorizontally: Int, offsetVertically: Int) {
-        data.pages[page]?.let { loadPageUnsafe(it, offsetHorizontally, offsetVertically) }
-    }
-
     override operator fun set(slot: InventorySlotCompound<T>, value: ItemStack) {
         slot.realSlotsWithInvType(data.inventoryType).forEach {
             bukkitInventory.setItem(it, value)

src/main/kotlin/net/axay/kspigot/inventory/InventoryGUIElements.kt

@@ -19,4 +19,7 @@ abstract class InventoryGUIElement<T : ForInventory> : InventoryGUISlot<T>() {
 
     protected abstract fun onClickElement(clickEvent: InventoryGUIClickEvent<T>)
 
+    internal open fun startUsing(gui: InventoryGUI<*>) { }
+    internal open fun stopUsing(gui: InventoryGUI<*>) { }
+
 }