Adds .noEmptyElements option to OutputFormatting (#264)

* Adds `.noEmptyElements` option to OutputFormatting

Opts out of empty element shorthand.

* Remove unnecessary `if #available` check

Author: nighthawk

Sources/XMLCoder/Auxiliaries/XMLCoderElement.swift

@@ -291,12 +291,14 @@ struct XMLCoderElement: Equatable {
             formatXMLAttributes(formatting, &string, escapedCharacters.attributes)
         }
 
-        if !elements.isEmpty {
+        if !elements.isEmpty || formatting.contains(.noEmptyElements) {
             let prettyPrintElements = prettyPrinted && !containsTextNodes
             if !key.isEmpty {
                 string += prettyPrintElements ? ">\n" : ">"
             }
-            formatXMLElements(escapedCharacters, formatting, indentation, &string, level, prettyPrintElements)
+            if !elements.isEmpty {
+                formatXMLElements(escapedCharacters, formatting, indentation, &string, level, prettyPrintElements)
+            }
 
             if prettyPrintElements { string += prefix }
             if !key.isEmpty {

Sources/XMLCoder/Encoder/XMLEncoder.swift

@@ -27,6 +27,9 @@ open class XMLEncoder {
 
         /// Produce XML with keys sorted in lexicographic order.
         public static let sortedKeys = OutputFormatting(rawValue: 1 << 1)
+
+        /// Produce XML with no short-hand annotation for empty elements, e.g., use `<p></p>` over `</p>`
+        public static let noEmptyElements = OutputFormatting(rawValue: 1 << 2)
     }
 
     /// The indentation to use when XML is pretty-printed.

Tests/XMLCoderTests/NodeEncodingStrategyTests.swift

@@ -309,4 +309,23 @@ final class NodeEncodingStrategyTests: XCTestCase {
             XCTAssert(false, "failed to decode the example: \(error)")
         }
     }
+    
+
+    func testNoEmptyElements() {
+        let encoder = XMLEncoder()
+        encoder.outputFormatting = [.noEmptyElements]
+
+        do {
+            let data = try encoder.encode(UnkeyedContainer(elements: []), withRootKey: "container")
+            let xml = String(data: data, encoding: .utf8)!
+
+            let expected =
+                """
+                <container></container>
+                """
+            XCTAssertEqual(xml, expected)
+        } catch {
+            XCTAssert(false, "failed to decode the example: \(error)")
+        }
+    }
 }