From 04a5eb17357ca29013c3ef150e4bfee27eb30e14 Mon Sep 17 00:00:00 2001
From: Dan Barber <github@danbarber.me>
Date: Fri, 8 Mar 2019 15:52:01 -0500
Subject: [PATCH] Maintain relative scroll position on resize

---
 Persephone/Controllers/AlbumViewController.swift |  9 +++++++++
 Persephone/Layouts/AlbumViewLayout.swift         | 12 ++++++++++++
 Persephone/Views/AlbumItemView.swift             |  6 +++++-
 3 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/Persephone/Controllers/AlbumViewController.swift b/Persephone/Controllers/AlbumViewController.swift
index 42bec54..21ee848 100644
--- a/Persephone/Controllers/AlbumViewController.swift
+++ b/Persephone/Controllers/AlbumViewController.swift
@@ -44,6 +44,15 @@ class AlbumViewController: NSViewController,
     albumCollectionView.collectionViewLayout?.invalidateLayout()
   }
 
+  override func viewDidLayout() {
+    super.viewDidLayout()
+
+    guard let layout = albumCollectionView.collectionViewLayout as? AlbumViewLayout
+      else { return }
+
+    layout.setScrollPosition()
+  }
+
   @objc func updateAlbums(_ notification: Notification) {
     guard let albums = notification.userInfo?[Notification.albumsKey] as? [MPDClient.Album]
       else { return }
diff --git a/Persephone/Layouts/AlbumViewLayout.swift b/Persephone/Layouts/AlbumViewLayout.swift
index aaf9c6f..5a93b65 100644
--- a/Persephone/Layouts/AlbumViewLayout.swift
+++ b/Persephone/Layouts/AlbumViewLayout.swift
@@ -11,6 +11,7 @@ import Cocoa
 class AlbumViewLayout: NSCollectionViewFlowLayout {
   let maxItemWidth: CGFloat = 180
   let albumInfoHeight: CGFloat = 39
+  var scrollPosition: CGFloat = 0
 
   required init?(coder aDecoder: NSCoder) {
     super.init()
@@ -35,6 +36,10 @@ class AlbumViewLayout: NSCollectionViewFlowLayout {
     var divider: CGFloat = 1
     var itemWidth: CGFloat = 0
 
+    if let scrollView = collectionView.enclosingScrollView {
+      scrollPosition = scrollView.documentVisibleRect.minY / collectionView.bounds.height
+    }
+
     repeat {
       let totalPaddingWidth = sectionInset.left + sectionInset.right
       let totalGutterWidth = (divider - 1) * (minimumInteritemSpacing)
@@ -46,4 +51,11 @@ class AlbumViewLayout: NSCollectionViewFlowLayout {
 
     itemSize = NSSize(width: itemWidth, height: itemHeight)
   }
+
+  func setScrollPosition() {
+    guard let collectionView = collectionView
+      else { return }
+
+    collectionView.scroll(NSPoint(x: 0, y: scrollPosition * collectionView.bounds.height))
+  }
 }
diff --git a/Persephone/Views/AlbumItemView.swift b/Persephone/Views/AlbumItemView.swift
index dca1eb0..06162f0 100644
--- a/Persephone/Views/AlbumItemView.swift
+++ b/Persephone/Views/AlbumItemView.swift
@@ -43,7 +43,7 @@ class AlbumItemView: NSView {
 
     NotificationCenter.default.addObserver(
       self,
-      selector: #selector(viewWillScroll(_:)),
+      selector: #selector(viewDidScroll(_:)),
       name: NSScrollView.didLiveScrollNotification,
       object: nil
     )
@@ -59,6 +59,10 @@ class AlbumItemView: NSView {
     hidePlayButton()
   }
 
+  @objc func viewDidScroll(_ notification: Notification) {
+    viewWillScroll(notification)
+  }
+
   override func resize(withOldSuperviewSize oldSize: NSSize) {
     hidePlayButton()
   }