Inventory GUI compound scroll functionality

src/main/kotlin/net/axay/kspigot/inventory/elements/InventoryGUISpaceCompound.kt

@@ -32,7 +32,8 @@ class InventoryGUISpaceCompound<T : ForInventory, E>(
 
     private val realInternalSlots = ArrayList<Int>()
 
-    private val currentInternalSlots: List<Int> get() {
+    private val currentInternalSlots: List<Int>
+        get() {
 
             val result = ArrayList(realInternalSlots)
 
@@ -46,12 +47,18 @@ class InventoryGUISpaceCompound<T : ForInventory, E>(
 
         }
 
-    internal var scrolledLines: Int = 0
+    internal var scrolledLines: Int = 0; private set
-        set(value) {
+
-            if (((value - 1) * invType.dimensions.width) < content.size) {
+    fun scroll(distance: Int): Boolean {
-                field = value
+        val value = scrolledLines + distance
+        return if (
+            value >= 0 &&
+            ((value - 1) * invType.dimensions.width) < content.size
+        ) {
+            scrolledLines = value
             onChange()
-            }
+            true
+        } else false
     }
 
     private var contentSort: () -> Unit = { }

src/main/kotlin/net/axay/kspigot/inventory/elements/InventoryGUISpaceCompoundScrollButton.kt

@@ -0,0 +1,28 @@
+package net.axay.kspigot.inventory.elements
+
+import net.axay.kspigot.inventory.ForInventory
+import net.axay.kspigot.inventory.InventoryGUIClickEvent
+import net.axay.kspigot.inventory.InventoryGUIElement
+import net.axay.kspigot.runnables.task
+import org.bukkit.inventory.ItemStack
+
+class InventoryGUISpaceCompoundScrollButton<T : ForInventory>(
+    private val icon: ItemStack,
+    private val compound: InventoryGUISpaceCompound<T, *>,
+    private val scrollDistance: Int = compound.invType.dimensions.height,
+    private val reverse: Boolean = false
+) : InventoryGUIElement<T>() {
+
+    override fun getItemStack(slot: Int) = icon
+
+    override fun onClickElement(clickEvent: InventoryGUIClickEvent<T>) {
+        task(
+            period = 1,
+            howOften = scrollDistance.toLong()
+        ) {
+            val ifScrolled = if (reverse) compound.scroll(-1) else compound.scroll(1)
+            if (!ifScrolled) it.cancel()
+        }
+    }
+
+}