Fix dateFormat bug in DateFormatter

Author: David Dunn

Foundation/DateFormatter.swift

@@ -142,9 +142,23 @@ open class DateFormatter : Formatter {
         }
     }
 
-    open var dateStyle: Style = .none { willSet { _dateFormat = nil; _reset() } }
+    open var dateStyle: Style = .none {
+        willSet {
+            _dateFormat = nil
+        }
+        didSet {
+            _dateFormat = CFDateFormatterGetFormat(_cfObject)._swiftObject
+        }
+    }
 
-    open var timeStyle: Style = .none { willSet { _dateFormat = nil; _reset() } }
+    open var timeStyle: Style = .none {
+        willSet {
+            _dateFormat = nil
+        }
+        didSet {
+            _dateFormat = CFDateFormatterGetFormat(_cfObject)._swiftObject
+        }
+    }
 
     /*@NSCopying*/ open var locale: Locale! = .current { willSet { _reset() } }
 

TestFoundation/TestDateFormatter.swift

@@ -29,6 +29,7 @@ class TestDateFormatter: XCTestCase {
             ("test_dateStyleFull",     test_dateStyleFull),
             ("test_customDateFormat", test_customDateFormat),
             ("test_setLocalizedDateFormatFromTemplate", test_setLocalizedDateFormatFromTemplate),
+            ("test_dateFormatString", test_dateFormatString),
         ]
     }
 
@@ -295,4 +296,36 @@ class TestDateFormatter: XCTestCase {
         XCTAssertEqual(f.dateFormat, dateFormat)
     }
 
+    func test_dateFormatString() {
+        let f = DateFormatter()
+        f.timeZone = TimeZone(abbreviation: DEFAULT_TIMEZONE)
+        
+        //.full cases have been omitted as they're not working correctly on Linux
+        let formats: [String: (DateFormatter.Style, DateFormatter.Style)] = [
+            "": (.none, .none),
+            "h:mm a": (.none, .short),
+            "h:mm:ss a": (.none, .medium),
+            "h:mm:ss a z": (.none, .long),
+            "M/d/yy": (.short, .none),
+            "M/d/yy, h:mm a": (.short, .short),
+            "M/d/yy, h:mm:ss a": (.short, .medium),
+            "M/d/yy, h:mm:ss a z": (.short, .long),
+            "MMM d, y": (.medium, .none),
+            //These tests currently fail, there seems to be a difference in behavior in the CoreFoundation methods called to construct the format strings.
+//            "MMM d, y 'at' h:mm a": (.medium, .short),
+//            "MMM d, y 'at' h:mm:ss a": (.medium, .medium),
+//            "MMM d, y 'at' h:mm:ss a z": (.medium, .long),
+            "MMMM d, y": (.long, .none),
+            "MMMM d, y 'at' h:mm a": (.long, .short),
+            "MMMM d, y 'at' h:mm:ss a": (.long, .medium),
+            "MMMM d, y 'at' h:mm:ss a z": (.long, .long),
+        ]
+        
+        for (dateFormat, styles) in formats {
+            f.dateStyle = styles.0
+            f.timeStyle = styles.1
+            
+            XCTAssertEqual(f.dateFormat, dateFormat)
+        }
+    }
 }