added a way to lazily-initialize user info data

Sources/Setup/DataStack.swift

@@ -454,7 +454,6 @@ public final class DataStack: Equatable {
      ```
      - Important: Do not use this method to store thread-sensitive data.
      - parameter userInfoKey: the key for custom data. Make sure this is a static pointer that will never be changed.
-     - returns: A custom data identified by  `userInfoKey`
      */
     public subscript(userInfoKey key: UnsafeRawPointer) -> Any? {
         
@@ -478,6 +477,35 @@ public final class DataStack: Equatable {
         }
     }
     
+    /**
+     Allow external libraries to store custom data in the `DataStack`. App code should rarely have a need for this.
+     ```
+     enum Static {
+     static var myDataKey: Void?
+     }
+     CoreStore.defaultStack[userInfoKey: &Static.myDataKey, lazyInit: { MyObject() }] = myObject
+     ```
+     - Important: Do not use this method to store thread-sensitive data.
+     - parameter userInfoKey: the key for custom data. Make sure this is a static pointer that will never be changed.
+     - parameter lazyInit: a closure to use to lazily-initialize the data
+     - returns: A custom data identified by  `userInfoKey`
+     */
+    public subscript(userInfoKey key: UnsafeRawPointer, lazyInit closure: () -> Any) -> Any? {
+        
+        self.userInfoLock.lock()
+        defer {
+            
+            self.userInfoLock.unlock()
+        }
+        if let value = self.userInfo[key] {
+            
+            return value
+        }
+        let value = closure()
+        self.userInfo[key] = value
+        return value
+    }
+    
     
     // MARK: Equatable
     

Sources/Transactions/BaseDataTransaction.swift

@@ -474,6 +474,35 @@ public /*abstract*/ class BaseDataTransaction {
         }
     }
     
+    /**
+     Allow external libraries to store custom data in the transaction. App code should rarely have a need for this.
+     ```
+     enum Static {
+     static var myDataKey: Void?
+     }
+     CoreStore.defaultStack[userInfoKey: &Static.myDataKey, lazyInit: { MyObject() }] = myObject
+     ```
+     - Important: Do not use this method to store thread-sensitive data.
+     - parameter userInfoKey: the key for custom data. Make sure this is a static pointer that will never be changed.
+     - parameter lazyInit: a closure to use to lazily-initialize the data
+     - returns: A custom data identified by  `userInfoKey`
+     */
+    public subscript(userInfoKey key: UnsafeRawPointer, lazyInit closure: () -> Any) -> Any? {
+        
+        self.userInfoLock.lock()
+        defer {
+            
+            self.userInfoLock.unlock()
+        }
+        if let value = self.userInfo[key] {
+            
+            return value
+        }
+        let value = closure()
+        self.userInfo[key] = value
+        return value
+    }
+    
     
     // MARK: Internal