[84] Add helper method for copying file

Sources/Disk+Helpers.swift

@@ -223,6 +223,40 @@ public extension Disk {
             throw error
         }
     }
+
+    /// Copy file to a new directory
+    ///
+    /// - Parameters:
+    ///   - path: path of file relative to directory
+    ///   - directory: directory the file is currently in
+    ///   - newFilePath: path to the new copy
+    ///   - newDirectory: new directory to copy file in
+    /// - Throws: Error if file could not be copied
+    static func copy(_ path: String, from directory: Directory, to newFilePath: String, in newDirectory: Directory) throws {
+        do {
+            let currentUrl = try getExistingFileURL(for: path, in: directory)
+            let newUrl = try createURL(for: newFilePath, in: newDirectory)
+            try createSubfoldersBeforeCreatingFile(at: newUrl)
+            try FileManager.default.copyItem(at: currentUrl, to: newUrl)
+        } catch {
+            throw error
+        }
+    }
+
+    /// Copy file to a new directory
+    ///
+    /// - Parameters:
+    ///   - originalURL: url of file
+    ///   - newURL: new url for the copy
+    /// - Throws: Error if file could not be moved
+    static func copy(_ originalURL: URL, to newURL: URL) throws {
+        do {
+            try createSubfoldersBeforeCreatingFile(at: newURL)
+            try FileManager.default.copyItem(at: originalURL, to: newURL)
+        } catch {
+            throw error
+        }
+    }
 
     /// Rename a file
     ///

Tests/DiskTests.swift

@@ -880,4 +880,34 @@ class DiskTests: XCTestCase {
             fatalError(convertErrorToString(error))
         }
     }
+
+    func testCopy() {
+        do {
+            try Disk.save(messages[0], to: .caches, as: "message.json")
+            XCTAssert(Disk.exists("message.json", in: .caches))
+            try Disk.copy("message.json", from: .caches, to: "message-copy.json", in: .documents)
+            XCTAssert(Disk.exists("message-copy.json", in: .documents))
+
+            let existingFileUrl = try Disk.url(for: "message.json", in: .caches)
+            let newFileUrl = try Disk.url(for: "message-copy.json", in: .temporary)
+            try Disk.copy(existingFileUrl, to: newFileUrl)
+            XCTAssert(Disk.exists("message.json", in: .caches))
+            XCTAssert(Disk.exists("message-copy.json", in: .temporary))
+
+            // ... in folder hierarchy
+            try Disk.save(messages[0], to: .caches, as: "Messages/Bob/message.json")
+            XCTAssert(Disk.exists("Messages/Bob/message.json", in: .caches))
+            try Disk.copy("Messages/Bob/message.json", from: .caches, to: "Messages/Joe/message.json", in: .caches)
+            XCTAssert(Disk.exists("Messages/Joe/message.json", in: .caches))
+
+            // Array of structs
+            try Disk.save(messages, to: .caches, as: "messages.json")
+            XCTAssert(Disk.exists("messages.json", in: .caches))
+            try Disk.copy("messages.json", from: .caches, to: "messages.json", in: .documents)
+            XCTAssert(Disk.exists("messages.json", in: .documents))
+
+        } catch {
+            fatalError(convertErrorToString(error))
+        }
+    }
 }