SwiftUI utilities done (for now)

Sources/ListSnapshot.SectionInfo.swift

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+//
+//  ListSnapshot.SectionInfo.swift
+//  CoreStore
+//
+//  Copyright © 2018 John Rommel Estropia
+//
+//  Permission is hereby granted, free of charge, to any person obtaining a copy
+//  of this software and associated documentation files (the "Software"), to deal
+//  in the Software without restriction, including without limitation the rights
+//  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+//  copies of the Software, and to permit persons to whom the Software is
+//  furnished to do so, subject to the following conditions:
+//
+//  The above copyright notice and this permission notice shall be included in all
+//  copies or substantial portions of the Software.
+//
+//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+//  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+//  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+//  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+//  SOFTWARE.
+//
+
+import CoreData
+
+
+// MARK: - ListSnapshot
+
+extension ListSnapshot {
+  
+    // MARK: - SectionInfo
+
+    public struct SectionInfo: Hashable, RandomAccessCollection {
+        
+        // MARK: Private
+        
+        public let sectionID: SectionID
+        
+        public let itemIDs: [ItemID]
+        
+        
+        // MARK: RandomAccessCollection
+        
+        public var startIndex: Index {
+            
+            return self.itemIDs.startIndex
+        }
+        
+        public var endIndex: Index {
+            
+            return self.itemIDs.endIndex
+        }
+
+        public func index(after i: Index) -> Index {
+
+            return self.itemIDs.index(after: i)
+        }
+
+        public func formIndex(after i: inout Index) {
+
+            self.itemIDs.formIndex(after: &i)
+        }
+
+        public func index(before i: Index) -> Index {
+
+            return self.itemIDs.index(before: i)
+        }
+
+        public func formIndex(before i: inout Index) {
+
+            self.itemIDs.formIndex(before: &i)
+        }
+
+
+        // MARK: BidirectionalCollection
+        
+        public subscript(position: Int) -> ObjectPublisher<O> {
+            
+            let itemID = self.itemIDs[position]
+            return self.context.objectPublisher(objectID: itemID)
+        }
+
+        public func index(_ i: Index, offsetBy distance: Int) -> Index {
+
+            return self.itemIDs.index(i, offsetBy: distance)
+        }
+
+        public func index(_ i: Index, offsetBy distance: Int, limitedBy limit: Int) -> Index? {
+
+            return self.itemIDs.index(i, offsetBy: distance, limitedBy: limit)
+        }
+
+        public func distance(from start: Index, to end: Index) -> Int {
+
+            return self.itemIDs.distance(from: start, to: end)
+        }
+
+        public subscript(bounds: Range<Index>) -> ArraySlice<Element> {
+
+            let itemIDs = self.itemIDs[bounds]
+            return ArraySlice(itemIDs.map(self.context.objectPublisher(objectID:)))
+        }
+
+        
+        // MARK: Sequence
+        
+        public typealias Element = ObjectPublisher<O>
+        
+        public typealias Index = Int
+        
+        
+        // MARK: Internal
+        
+        internal let context: NSManagedObjectContext
+        
+        internal init?(
+            sectionID: SectionID,
+            listSnapshot: ListSnapshot<O>
+        ) {
+            
+            guard let context = listSnapshot.context else {
+                
+                return nil
+            }
+            self.sectionID = sectionID
+            self.itemIDs = listSnapshot.itemIDs(inSectionWithID: sectionID)
+            self.context = context
+        }
+    }
+}