Reworked the SignalAttachmentItem hash function

SignalUtilitiesKit/Media Viewing & Editing/Attachment Approval/AttachmentItemCollection.swift

@@ -30,6 +30,7 @@ class SignalAttachmentItem: Hashable {
         case noThumbnail
     }
 
+    let uniqueIdentifier: UUID
     let attachment: SignalAttachment
 
     // This might be nil if the attachment is not a valid image.
@@ -66,7 +67,20 @@ class SignalAttachmentItem: Hashable {
     // MARK: Hashable
     
     func hash(into hasher: inout Hasher) {
-        attachment.hash(into: &hasher)
+        /// There was a crash in `AttachmentApprovalViewController` when trying to generate the hash
+        /// value to store in a dictionary, this crash persisted even after refactoring `DataSource` into Swift and
+        /// using custom `hash(into:)` functions on everything in order to exclude values which might have
+        /// been unsafe.
+        ///
+        /// Since the crash is still occurring the most likely culprit is now that one of the values used to generate the
+        /// hash was mutated after the value was stored (as `SignalAttachment` is a class and it was previously
+        /// used for generating the hash) - in order to avoid this we now generate a `uniqueIdentifier` when
+        /// initialising this type and use _only_ that for the hash (this `SignalAttachmentItem` is only used for
+        /// the `AttachmentApprovalViewController` and based on it's usage we shouldn't run into issues
+        /// with this hash not being deterministic
+        ///
+        /// If the crash still occurs it's likely a red herring and there is some other, larger, issue that is causing it
+        uniqueIdentifier.hash(into: &hasher)
     }
 
     // MARK: Equatable