From 924b8b18ba8fd48fc42fa956b02dda1677e10a22 Mon Sep 17 00:00:00 2001
From: Michael Kirk <michael@signal.org>
Date: Fri, 11 Jan 2019 18:15:34 -0700
Subject: [PATCH] Scroll to bottom when switching albums

Fixes

1. start on "All Photos" which has many pages of content

You're at the bottom, as expected.

2. switch to "Video", which is much shorter

You're at the bottom, as expected.

3. Switch back to "All Photos"

Now you're unexpectedly in the middle of the content somewhere - at whatever
offset the "Videos" content height was.

Note that changes to "scrollToBottom". For some reason calling scroll to bottom
with infinity works fine initially, but subsequent calls hang. Because we only
used the "infinity" hack to account for the fact that we didn't have accurate
safeAreaInsets before `viewDidAppear`, we can accurately use safeAreaInsets
after that point (like here, when switching albums).
---
 .../PhotoLibrary/ImagePickerController.swift  | 52 +++++++++++--------
 1 file changed, 30 insertions(+), 22 deletions(-)

diff --git a/Signal/src/ViewControllers/PhotoLibrary/ImagePickerController.swift b/Signal/src/ViewControllers/PhotoLibrary/ImagePickerController.swift
index 6f181c926..f2ab1e277 100644
--- a/Signal/src/ViewControllers/PhotoLibrary/ImagePickerController.swift
+++ b/Signal/src/ViewControllers/PhotoLibrary/ImagePickerController.swift
@@ -116,13 +116,14 @@ class ImagePickerGridController: UICollectionViewController, PhotoLibraryDelegat
 
         reloadDataAndRestoreSelection()
         if !hasEverAppeared {
-            hasEverAppeared = true
             scrollToBottom(animated: false)
         }
     }
 
     override func viewDidAppear(_ animated: Bool) {
         super.viewDidAppear(animated)
+
+        hasEverAppeared = true
         // done button may have been disable from the last time we hit "Done"
         // make sure to re-enable it if appropriate upon returning to the view
         hasPressedDoneSinceAppeared = false
@@ -156,31 +157,37 @@ class ImagePickerGridController: UICollectionViewController, PhotoLibraryDelegat
             return
         }
 
-        let verticalOffset: CGFloat
-        if #available(iOS 11, *) {
-            // On iOS10 and earlier, we can be precise, but as of iOS11 `collectionView.contentInset`
-            // is based on `safeAreaInsets`, which isn't accurate until `viewDidAppear` at the earliest.
-            //
-            // from https://developer.apple.com/documentation/uikit/uiview/positioning_content_relative_to_the_safe_area
-            // > Make your modifications in [viewDidAppear] because the safe area insets for a view are
-            // > not accurate until the view is added to a view hierarchy.
-            //
-            // Overshooting like this works without visible animation glitch. on iOS11+
-            // However, before iOS11, "overshooting" the contentOffset like this produces a broken
-            // layout or hanging. Luckily for those versions, before the safeAreaInset feature
-            // existed, we can accurately accesse colletionView.contentInset before `viewDidAppear`
-            // and calculate a precise content offset.
-            verticalOffset = CGFloat.greatestFiniteMagnitude
+        var verticalOffset: CGFloat
+
+        let visibleHeight = collectionView.bounds.height - collectionView.contentInset.top
+        let contentHeight = collectionView.contentSize.height
+        if contentHeight <= visibleHeight {
+            verticalOffset = -collectionView.contentInset.top
         } else {
-            let visibleHeight = collectionView.bounds.height - collectionView.contentInset.top
-            let contentHeight = collectionView.contentSize.height
-            if contentHeight <= visibleHeight {
-                verticalOffset = -collectionView.contentInset.top
+            let topOfLastPage = contentHeight - collectionView.bounds.height
+            verticalOffset = topOfLastPage
+        }
+
+        if #available(iOS 11, *) {
+            if hasEverAppeared {
+                verticalOffset += collectionView.safeAreaInsets.bottom
             } else {
-                let topOfLastPage = contentHeight - collectionView.bounds.height
-                verticalOffset = topOfLastPage
+                // On iOS10 and earlier, we can be precise, but as of iOS11 `collectionView.contentInset`
+                // is based on `safeAreaInsets`, which isn't accurate until `viewDidAppear` at the earliest.
+                //
+                // from https://developer.apple.com/documentation/uikit/uiview/positioning_content_relative_to_the_safe_area
+                // > Make your modifications in [viewDidAppear] because the safe area insets for a view are
+                // > not accurate until the view is added to a view hierarchy.
+                //
+                // Overshooting like this works without visible animation glitch. on iOS11+
+                // However, before iOS11, "overshooting" the contentOffset like this produces a broken
+                // layout or hanging. Luckily for those versions, before the safeAreaInset feature
+                // existed, we can accurately access collectionView.contentInset before `viewDidAppear`
+                // and calculate a precise content offset.
+                verticalOffset += 122
             }
         }
+
         collectionView.setContentOffset(CGPoint(x: 0, y: verticalOffset), animated: animated)
     }
 
@@ -478,6 +485,7 @@ class ImagePickerGridController: UICollectionViewController, PhotoLibraryDelegat
         }
 
         collectionView?.reloadData()
+        scrollToBottom(animated: false)
         hideCollectionPicker()
     }