inventory ui transition wip

src/net/torvald/terrarum/ui/UIItemHorizontalFadeSlide.kt

@@ -0,0 +1,41 @@
+package net.torvald.terrarum.ui
+
+import kotlin.math.roundToInt
+
+/**
+ * @param width size of the canvas where transition occurs
+ * @param height size of the canvas where transition occurs
+ */
+class UIItemHorizontalFadeSlide(
+        parent: UICanvas,
+        initialX: Int,
+        initialY: Int,
+        width: Int,
+        height: Int,
+        //transitionLength: Float,
+        currentPosition: Float,
+        vararg uis: UICanvas
+) : UIItemTransitionContainer(parent, initialX, initialY, width, height, 0.212f, currentPosition, uis) {
+
+    fun getOffX(index: Int) = ((currentPosition - index) * width).roundToInt()
+    fun getOpacity(index: Int) = (currentPosition - index).coerceIn(0f, 1f)
+
+    init {
+        // re-position the uis according to the initial choice of currentPosition
+        uis.forEachIndexed { index, it ->
+            it.posX = posX + getOffX(index)
+            it.initialX = posX + getOffX(index)
+            it.posY = posY
+            it.initialY = posY
+            it.opacity = getOpacity(index)
+        }
+    }
+
+    override fun onTransition(currentPosition: Float, uis: Array<out UICanvas>) {
+        uis.forEachIndexed { index, it ->
+            it.posX = it.initialX + getOffX(index)
+            it.posY = posY
+            it.opacity = getOpacity(index)
+        }
+    }
+}