Add Decoding support for choice elements

Sources/XMLCoder/Auxiliaries/Box/SingleElementBox.swift

@@ -0,0 +1,25 @@
+//
+//  SingleElementBox.swift
+//  XMLCoder
+//
+//  Created by James Bean on 7/15/19.
+//
+
+/// A `Box` which contains a single `key` and `element` pair. This is useful for disambiguating elements which could either represent
+/// an element nested in a keyed or unkeyed container, or an choice between multiple known-typed values (implemented in Swift using
+/// enums with associated values).
+struct SingleElementBox: SimpleBox {
+    let key: String
+    let element: Box
+}
+
+extension SingleElementBox: Box {
+
+    var isNull: Bool {
+        return false
+    }
+
+    func xmlString() -> String? {
+        return nil
+    }
+}

Sources/XMLCoder/Auxiliaries/XMLCoderElement.swift

@@ -51,7 +51,10 @@ struct XMLCoderElement: Equatable {
         elements.append(element)
     }
 
-    func transformToBoxTree() -> KeyedBox {
+    func transformToBoxTree() -> Box {
+        if let value = value, self.attributes.isEmpty, self.elements.isEmpty {
+            return SingleElementBox(key: key, element: StringBox(value))
+        }
         let attributes = KeyedStorage(self.attributes.map { attribute in
             (key: attribute.key, value: StringBox(attribute.value) as SimpleBox)
         })
@@ -63,9 +66,7 @@ struct XMLCoderElement: Equatable {
         if elements.isEmpty, let value = value {
             elements.append(StringBox(value), at: "value")
         }
-        let keyedBox = KeyedBox(elements: elements, attributes: attributes)
-
-        return keyedBox
+        return KeyedBox(elements: elements, attributes: attributes)
     }
 
     func toXMLString(with header: XMLHeader? = nil,

Sources/XMLCoder/Auxiliaries/XMLStackParser.swift

@@ -23,7 +23,7 @@ class XMLStackParser: NSObject {
         errorContextLength length: UInt,
         shouldProcessNamespaces: Bool,
         trimValueWhitespaces: Bool
-    ) throws -> KeyedBox {
+    ) throws -> Box {
         let parser = XMLStackParser(trimValueWhitespaces: trimValueWhitespaces)
 
         let node = try parser.parse(

Sources/XMLCoder/Decoder/XMLDecoderImplementation.swift

@@ -138,9 +138,10 @@ class XMLDecoderImplementation: Decoder {
         case let unkeyed as SharedBox<UnkeyedBox>:
             return XMLUnkeyedDecodingContainer(referencing: self, wrapping: unkeyed)
         case let keyed as SharedBox<KeyedBox>:
-            guard let firstKey = keyed.withShared({ $0.elements.keys.first }) else { fallthrough }
-
-            return XMLUnkeyedDecodingContainer(referencing: self, wrapping: SharedBox(keyed.withShared { $0.elements[firstKey] }))
+            return XMLUnkeyedDecodingContainer(
+                referencing: self,
+                wrapping: SharedBox(keyed.withShared { $0.elements.map(SingleElementBox.init) })
+            )
         default:
             throw DecodingError.typeMismatch(
                 at: codingPath,
@@ -372,6 +373,20 @@ extension XMLDecoderImplementation {
         return urlBox.unboxed
     }
 
+    func unbox<T: Decodable>(_ box: SingleElementBox) throws -> T {
+        do {
+            return try unbox(box.element)
+        } catch {
+            // FIXME: Find a more economical way to unbox a `SingleElementBox` !
+            return try unbox(
+                KeyedBox(
+                    elements: KeyedStorage([(box.key, box.element)]),
+                    attributes: []
+                )
+            )
+        }
+    }
+
     func unbox<T: Decodable>(_ box: Box) throws -> T {
         let decoded: T?
         let type = T.self
@@ -392,6 +407,8 @@ extension XMLDecoderImplementation {
             type == String.self || type == NSString.self,
             let value = (try unbox(box) as String) as? T {
             decoded = value
+        } else if let singleElementBox = box as? SingleElementBox {
+            decoded = try unbox(singleElementBox)
         } else {
             storage.push(container: box)
             defer {

Tests/XMLCoderTests/CompositeChoiceTests.swift

@@ -21,7 +21,7 @@ private enum IntOrStringWrapper: Equatable {
     case string(StringWrapper)
 }
 
-extension IntOrStringWrapper: Codable {
+extension IntOrStringWrapper: XMLChoiceCodable {
 
     enum CodingKeys: String, CodingKey {
         case int
@@ -85,6 +85,4 @@ class CompositeChoiceTests: XCTestCase {
         ]
         XCTAssertEqual(result, expected)
     }
-
-    #warning("TODO: Add encoding and round-trip tests")
 }

Tests/XMLCoderTests/Minimal/BoxTreeTests.swift

@@ -9,7 +9,7 @@ import XCTest
 @testable import XMLCoder
 
 class BoxTreeTests: XCTestCase {
-    func testNestedValues() {
+    func testNestedValues() throws {
         let e1 = XMLCoderElement(
             key: "foo",
             value: "456",
@@ -29,17 +29,8 @@ class BoxTreeTests: XCTestCase {
             attributes: []
         )
 
-        let boxTree = root.transformToBoxTree()
-
-        guard let foo = boxTree.elements["foo"] as? UnkeyedBox else {
-            XCTAssert(
-                false,
-                """
-                flattened.elements["foo"] is not an UnkeyedBox
-                """
-            )
-            return
-        }
+        let boxTree = try XCTUnwrap(root.transformToBoxTree() as? KeyedBox)
+        let foo = try XCTUnwrap(boxTree.elements["foo"])
 
         XCTAssertEqual(foo.count, 2)
     }

Tests/XMLCoderTests/SimpleChoiceTests.swift

@@ -13,7 +13,8 @@ private enum IntOrString: Equatable {
     case string(String)
 }
 
-extension IntOrString: Codable {
+extension IntOrString: XMLChoiceCodable {
+
     enum CodingKeys: String, CodingKey {
         case int
         case string

XMLCoder.xcodeproj/project.pbxproj

@@ -26,6 +26,7 @@
 		1482D59E22DD2A6B00AE2D6E /* XMLChoiceEncodable.swift in Sources */ = {isa = PBXBuildFile; fileRef = 1482D59D22DD2A6B00AE2D6E /* XMLChoiceEncodable.swift */; };
 		1482D5A222DD2D9400AE2D6E /* SimpleChoiceTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 1482D5A122DD2D9400AE2D6E /* SimpleChoiceTests.swift */; };
 		1482D5A422DD2F4D00AE2D6E /* CompositeChoiceTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 1482D5A322DD2F4D00AE2D6E /* CompositeChoiceTests.swift */; };
+		1482D5A822DD6AEE00AE2D6E /* SingleElementBox.swift in Sources */ = {isa = PBXBuildFile; fileRef = 1482D5A722DD6AED00AE2D6E /* SingleElementBox.swift */; };
 		A61DCCD821DF9CA200C0A19D /* ClassTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = A61DCCD621DF8DB300C0A19D /* ClassTests.swift */; };
 		A61FE03921E4D60B0015D993 /* UnkeyedIntTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = A61FE03721E4D4F10015D993 /* UnkeyedIntTests.swift */; };
 		A61FE03C21E4EAB10015D993 /* KeyedIntTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = A61FE03A21E4EA8B0015D993 /* KeyedIntTests.swift */; };
@@ -153,6 +154,7 @@
 		1482D59D22DD2A6B00AE2D6E /* XMLChoiceEncodable.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = XMLChoiceEncodable.swift; sourceTree = "<group>"; };
 		1482D5A122DD2D9400AE2D6E /* SimpleChoiceTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = SimpleChoiceTests.swift; sourceTree = "<group>"; };
 		1482D5A322DD2F4D00AE2D6E /* CompositeChoiceTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = CompositeChoiceTests.swift; sourceTree = "<group>"; };
+		1482D5A722DD6AED00AE2D6E /* SingleElementBox.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = SingleElementBox.swift; sourceTree = "<group>"; };
 		A61DCCD621DF8DB300C0A19D /* ClassTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ClassTests.swift; sourceTree = "<group>"; };
 		A61FE03721E4D4F10015D993 /* UnkeyedIntTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = UnkeyedIntTests.swift; sourceTree = "<group>"; };
 		A61FE03A21E4EA8B0015D993 /* KeyedIntTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = KeyedIntTests.swift; sourceTree = "<group>"; };
@@ -310,6 +312,7 @@
 				BF9457A021CBB497005ACFDE /* UnkeyedBox.swift */,
 				BF94579F21CBB497005ACFDE /* KeyedBox.swift */,
 				BF63EF1721CEB6BD001D38C5 /* SharedBox.swift */,
+				1482D5A722DD6AED00AE2D6E /* SingleElementBox.swift */,
 			);
 			path = Box;
 			sourceTree = "<group>";
@@ -645,6 +648,7 @@
 				BF9457AE21CBB498005ACFDE /* Box.swift in Sources */,
 				BF9457DA21CBB5D2005ACFDE /* DataBox.swift in Sources */,
 				BF9457AB21CBB498005ACFDE /* DecimalBox.swift in Sources */,
+				1482D5A822DD6AEE00AE2D6E /* SingleElementBox.swift in Sources */,
 				OBJ_56 /* XMLKeyedEncodingContainer.swift in Sources */,
 				D158F12F2229892C0032B449 /* DynamicNodeDecoding.swift in Sources */,
 				D1E0C85521D91EBF0042A261 /* Metatypes.swift in Sources */,