added ObjectSnapshot as foundation for datasources API

Sources/ObjectSnapshot.swift

@@ -0,0 +1,143 @@
+//
+//  ObjectSnapshot.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
+import Foundation
+
+
+// MARK: - ObjectSnapshot
+
+/**
+ An `ObjectSnapshot` contains "snapshot" values from a `DynamicObject` instance copied at a specific point in time.
+ */
+@dynamicMemberLookup
+public struct ObjectSnapshot<O: DynamicObject> {
+    
+    // MARK: FilePrivate
+    
+    fileprivate var attributes: [KeyPathString: Any]
+    
+    // MARK: Private
+    
+    private init() {
+        
+        self.attributes = [:]
+    }
+}
+
+// MARK: - ObjectSnapshot where O: NSManagedObject
+
+extension ObjectSnapshot where O: NSManagedObject {
+    
+    /**
+     Initializes an `ObjectSnapshot` instance by copying all attribute values from the given `NSManagedObject`.
+     */
+    public init(from object: O) {
+        
+        self.attributes = object.dictionaryWithValues(
+            forKeys: Array(object.entity.attributesByName.keys)
+        )
+    }
+    
+    /**
+     Returns the value for the property identified by a given key.
+     */
+    public subscript<V: AllowedObjectiveCKeyPathValue>(dynamicMember member: KeyPath<O, V>) -> V {
+        
+        get {
+            
+            let key = String(keyPath: member)
+            return self.attributes[key]! as! V
+        }
+        set {
+            
+            let key = String(keyPath: member)
+            self.attributes[key] = newValue
+        }
+    }
+}
+
+
+// MARK: - ObjectSnapshot where O: CoreStoreObject
+
+extension ObjectSnapshot where O: CoreStoreObject {
+    
+    /**
+     Initializes an `ObjectSnapshot` instance by copying all attribute values from the given `CoreStoreObject`.
+     */
+    public init(from object: O) {
+    
+        var attributes: [KeyPathString: Any] = [:]
+        Self.initializeAttributes(
+            mirror: Mirror(reflecting: object),
+            object: object,
+            into: &attributes
+        )
+        self.attributes = attributes
+    }
+    
+    /**
+     Returns the value for the property identified by a given key.
+     */
+    public subscript<K: AttributeKeyPathStringConvertible>(dynamicMember member: KeyPath<O, K>) -> K.ReturnValueType {
+        
+        get {
+            
+            let key = String(keyPath: member)
+            return self.attributes[key]! as! K.ReturnValueType
+        }
+        set {
+            
+            let key = String(keyPath: member)
+            self.attributes[key] = newValue
+        }
+    }
+    
+    
+    // MARK: Private
+    
+    private static func initializeAttributes(mirror: Mirror, object: CoreStoreObject, into attributes: inout [KeyPathString: Any]) {
+        
+        if let superClassMirror = mirror.superclassMirror {
+            
+            self.initializeAttributes(
+                mirror: superClassMirror,
+                object: object,
+                into: &attributes
+            )
+        }
+        for child in mirror.children {
+            
+            switch child.value {
+                
+            case let property as AttributeProtocol:
+                attributes[property.keyPath] = property.valueForSnapshot
+                
+            default:
+                continue
+            }
+        }
+    }
+}