Update ParentPanelController+ModernSlider.swift

Clocker/Panel/ParentPanelController+ModernSlider.swift

@@ -16,8 +16,29 @@ extension ParentPanelController: NSCollectionViewDataSource {
 }
 
 extension ParentPanelController {
+    @IBAction func goForward(_: NSButton) {
+        navigateModernSliderToSpecificIndex(1)
+    }
+
+    @IBAction func goBackward(_: NSButton) {
+        navigateModernSliderToSpecificIndex(-1)
+    }
+
+    private func navigateModernSliderToSpecificIndex(_ index: Int) {
+        let contentView = modernSlider.superview as! NSClipView
+        let changedOrigin = contentView.documentVisibleRect.origin
+        let newPoint = NSPoint(x: changedOrigin.x + contentView.frame.width / 2, y: changedOrigin.y)
+        if let indexPath = modernSlider.indexPathForItem(at: newPoint) {
+            let previousIndexPath = IndexPath(item: indexPath.item + index, section: indexPath.section)
+            modernSlider.scrollToItems(at: Set([previousIndexPath]), scrollPosition: .centeredHorizontally)
+        }
+    }
+
     @objc func collectionViewDidScroll(_ notification: NSNotification) {
-        let contentView = notification.object as! NSClipView
+        guard let contentView = notification.object as? NSClipView else {
+            return
+        }
+
         let changedOrigin = contentView.documentVisibleRect.origin
         let newPoint = NSPoint(x: changedOrigin.x + contentView.frame.width / 2, y: changedOrigin.y)
         let indexPath = modernSlider.indexPathForItem(at: newPoint)