Add performance test case for JSON serialization

Author: djones6

TestFoundation/TestNSJSONSerialization.swift

@@ -15,7 +15,7 @@
     import SwiftFoundation
     import SwiftXCTest
 #endif
-
+import Dispatch
 
 class TestNSJSONSerialization : XCTestCase {
 
@@ -598,6 +598,7 @@ extension TestNSJSONSerialization {
             ("test_invalidJsonObjectToStreamBuffer", test_invalidJsonObjectToStreamBuffer),
             ("test_jsonObjectToOutputStreamInsufficeintBuffer", test_jsonObjectToOutputStreamInsufficeintBuffer),
             ("test_booleanJSONObject", test_booleanJSONObject),
+            ("test_serialization_perf", test_serialize_perf),
         ]
     }
 
@@ -743,6 +744,35 @@ extension TestNSJSONSerialization {
         XCTAssertEqual(try trySerialize(jsonArr4), "[{\"a\":null},{\"b\":null}]")
     }
 
+    func test_serialize_perf() {
+        let numItems = 20                  // Number of elements in the payload
+        let loops = 500                    // Number of times to invoke serialization per measurement
+        let concurrency = 4                // Number of concurrent threads
+        var PAYLOAD = [String:Any]()       // The string to convert
+        
+        for i in 1...numItems {
+            PAYLOAD["Item \(i)"] = Double(i)
+        }
+        
+        let queue = DispatchQueue(label: "testcaseQueue", attributes: .concurrent)
+        let group = DispatchGroup()
+        
+        self.measure {
+            for _ in 1...concurrency {
+                queue.async(group: group) {
+                    for _ in 1...loops {
+                        do {
+                            _ = try JSONSerialization.data(withJSONObject: PAYLOAD)
+                        } catch {
+                            XCTFail("Could not serialize to JSON: \(error)")
+                        }
+                    }
+                }
+            }
+            _ = group.wait(timeout: .distantFuture) // forever
+        }
+    }
+    
     func test_nested_array() {
         var arr: [Any] = ["a"]
         XCTAssertEqual(try trySerialize(arr), "[\"a\"]")