Allow purging of datasource

Sources/DiffableDataSource.BaseAdapter.swift

@@ -101,7 +101,28 @@ extension DiffableDataSource {
         }
 
         /**
-         Reloads the `UITableView` using a `ListSnapshot`. This is typically from the `snapshot` property of a `ListPublisher`:
+         Clears the target.
+         - parameter animatingDifferences: if `true`, animations may be applied accordingly. Defaults to `true`.
+         */
+        open func purge(animatingDifferences: Bool = true, completion: @escaping () -> Void = {}) {
+
+            self.dispatcher.purge(
+                target: self.target,
+                animatingDifferences: animatingDifferences,
+                performUpdates: { target, changeset, setSections in
+
+                    target.reload(
+                        using: changeset,
+                        animated: animatingDifferences,
+                        setData: setSections
+                    )
+                },
+                completion: completion
+            )
+        }
+
+        /**
+         Reloads the target using a `ListSnapshot`. This is typically from the `snapshot` property of a `ListPublisher`:
          ```
          listPublisher.addObserver(self) { [weak self] (listPublisher) in
             self?.dataSource?.apply(
@@ -109,11 +130,9 @@ extension DiffableDataSource {
                 animatingDifferences: true
             )
          }
-         ```
-         If the `defaultRowAnimation` is configured to, animations are also applied accordingly.
-
-         - parameter snapshot: the `ListSnapshot` used to reload the `UITableView` with. This is typically from the `snapshot` property of a `ListPublisher`.
-         - parameter animatingDifferences: if `true`, animations will be applied as configured by the `defaultRowAnimation` value. Defaults to `true`.
+         ``
+         - parameter snapshot: the `ListSnapshot` used to reload the target with. This is typically from the `snapshot` property of a `ListPublisher`.
+         - parameter animatingDifferences: if `true`, animations may be applied accordingly. Defaults to `true`.
          */
         open func apply(_ snapshot: ListSnapshot<O>, animatingDifferences: Bool = true, completion: @escaping () -> Void = {}) {
 

Sources/DiffableDataSource.CollectionViewAdapter-AppKit.swift

@@ -152,7 +152,7 @@ extension DiffableDataSource {
 
         public typealias Base = T
 
-        public internal(set) weak var base: Base?
+        public private(set) weak var base: Base?
 
         public init(_ base: Base) {
 

Sources/DiffableDataSource.CollectionViewAdapter-UIKit.swift

@@ -152,7 +152,7 @@ extension DiffableDataSource {
 
         public typealias Base = T
 
-        public internal(set) weak var base: Base?
+        public private(set) weak var base: Base?
 
         public init(_ base: Base) {
 

Sources/DiffableDataSource.TableViewAdapter-UIKit.swift

@@ -174,7 +174,7 @@ extension DiffableDataSource {
 
         public typealias Base = T
 
-        public internal(set) weak var base: Base?
+        public private(set) weak var base: Base?
 
         public init(_ base: Base) {
 

Sources/Internals.DiffableDataUIDispatcher.swift

@@ -51,6 +51,26 @@ extension Internals {
             
             self.dataStack = dataStack
         }
+
+        func purge<Target: DiffableDataSource.Target>(
+            target: Target?,
+            animatingDifferences: Bool,
+            performUpdates: @escaping (
+                Target,
+                StagedChangeset<[Internals.DiffableDataSourceSnapshot.Section]>,
+                @escaping ([Internals.DiffableDataSourceSnapshot.Section]) -> Void
+            ) -> Void,
+            completion: @escaping () -> Void
+        ) {
+
+            self.apply(
+                .init(),
+                target: target,
+                animatingDifferences: animatingDifferences,
+                performUpdates: performUpdates,
+                completion: completion
+            )
+        }
         
         func apply<Target: DiffableDataSource.Target>(
             _ snapshot: DiffableDataSourceSnapshot,

Sources/ListSnapshot.swift

@@ -83,7 +83,10 @@ public struct ListSnapshot<O: DynamicObject>: RandomAccessCollection, Hashable {
      */
     public subscript(safeIndex index: Index) -> ObjectPublisher<O>? {
 
-        let context = self.context!
+        guard let context = self.context else {
+
+            return nil
+        }
         let itemIDs = self.diffableSnapshot.itemIdentifiers
         guard itemIDs.indices.contains(index) else {
 
@@ -118,7 +121,10 @@ public struct ListSnapshot<O: DynamicObject>: RandomAccessCollection, Hashable {
      */
     public subscript(safeSectionIndex sectionIndex: Int, safeItemIndex itemIndex: Int) -> ObjectPublisher<O>? {
 
-        let context = self.context!
+        guard let context = self.context else {
+
+            return nil
+        }
         let snapshot = self.diffableSnapshot
         let sectionIDs = snapshot.sectionIdentifiers
         guard sectionIDs.indices.contains(sectionIndex) else {