Add nested decoding

Sources/Codextended/Codextended.swift

@@ -103,6 +103,31 @@ public extension Decoder {
         return try container.decode(type, forKey: key)
     }
 
+    /// Decode a nested value for array of keys, specified as a `CodingKey`.
+    /// Throws an error if keys array is empty
+    func decode<T: Decodable>(_ keys: [CodingKey], as type: T.Type = T.self) throws -> T {
+
+        // Throw an error here?
+        guard !keys.isEmpty else {
+            throw CodextendedDecodingError.emptyCodingKey
+        }
+
+        let keys = keys.map({AnyCodingKey($0.stringValue)})
+
+        var container = try self.container(keyedBy: AnyCodingKey.self)
+        for key in keys.dropLast() {
+            container = try container.nestedContainer(keyedBy: AnyCodingKey.self, forKey: key)
+        }
+        return try container.decode(type, forKey: keys.last!)
+    }
+
+    /// Decode a nested value for array of keys, specified as a string.
+    /// Throws an error if keys array is empty
+    func decode<T: Decodable>(_ keys: [String], as type: T.Type = T.self) throws -> T {
+        return try decode(keys.map({AnyCodingKey($0)}))
+    }
+
+
     /// Decode an optional value for a given key, specified as a string. Throws an error if the
     /// specified key exists but is not able to be decoded as the inferred type.
     func decodeIfPresent<T: Decodable>(_ key: String, as type: T.Type = T.self) throws -> T? {
@@ -142,6 +167,19 @@ public extension Decoder {
     }
 }
 
+// MARK: - Errors
+
+public enum CodextendedDecodingError: Error, LocalizedError {
+    case emptyCodingKey
+
+    public var errorDescription: String? {
+        switch self {
+        case .emptyCodingKey:
+            return "Coding keys array was empty"
+        }
+    }
+}
+
 // MARK: - Date formatters
 
 /// Protocol acting as a common API for all types of date formatters,

Tests/CodextendedTests/CodextendedTests.swift

@@ -212,6 +212,58 @@ final class CodextendedTests: XCTestCase {
         }
     }
 
+    func testDecodingNested() throws {
+        struct Value: Decodable, Equatable {
+            let string: String
+
+            init(from decoder: Decoder) throws {
+                string = try decoder.decode(["a","b","c"])
+            }
+        }
+
+        let jsonString = #"""
+        {
+            "a": {
+                "b": {
+                    "c": "hello world"
+                }
+            }
+        }
+        """#
+
+        let value = try Data(jsonString.utf8).decoded() as Value
+        XCTAssertEqual(value.string, "hello world")
+    }
+
+    func testDecodingSingleValueNested() throws {
+        struct Value: Decodable, Equatable {
+            let string: String
+
+            init(from decoder: Decoder) throws {
+                string = try decoder.decode(["a"])
+            }
+        }
+
+        let value = try Data(#"{"a": "hello world"}"#.utf8).decoded() as Value
+        XCTAssertEqual(value.string, "hello world")
+    }
+
+    func testDecodingNestedWithEmptyKeysThrows() {
+        struct Value: Decodable, Equatable {
+            let string: String
+
+            init(from decoder: Decoder) throws {
+                string = try decoder.decode([String]())
+            }
+        }
+
+        let data = Data(#"{"a": "hello world"}"#.utf8)
+        XCTAssertThrowsError(try data.decoded() as Value) { error in
+                        XCTAssertEqual(error as? CodextendedDecodingError, CodextendedDecodingError.emptyCodingKey,
+                          "Expected CodextendedDecodingError.emptyCodingKey but got \(error)")
+        }
+    }
+
     func testAllTestsRunOnLinux() {
         verifyAllTestsRunOnLinux(excluding: ["testDateWithISO8601Formatter"])
     }
@@ -226,6 +278,9 @@ extension CodextendedTests: LinuxTestable {
         ("testUsingStringAsKey", testUsingStringAsKey),
         ("testUsingCodingKey", testUsingCodingKey),
         ("testDateWithCustomFormatter", testDateWithCustomFormatter),
-        ("testDecodingErrorThrownForInvalidDateString", testDecodingErrorThrownForInvalidDateString)
+        ("testDecodingErrorThrownForInvalidDateString", testDecodingErrorThrownForInvalidDateString),
+        ("testDecodingNested", testDecodingNested),
+        ("testDecodingSingleValueNested", testDecodingSingleValueNested),
+        ("testDecodingNestedWithEmptyKeysThrows", testDecodingNestedWithEmptyKeysThrows)
     ]
 }