Use a default date formatter factory for additional thread safety.

Sources/TemplateKit/Tag/DateFormat.swift

@@ -4,37 +4,39 @@
 ///
 /// If no date format is supplied, a default will be used.
 public final class DateFormat: TagRenderer {
-    private let defaultDateFormatter: DateFormatter
+    public typealias DateFormatterFactory = () -> DateFormatter
 
-    private static let dateAndTimeFormatter: DateFormatter = {
+    private let defaultDateFormatterFactory: DateFormatterFactory
+
+    private static let dateAndTimeFormatterFactory: DateFormatterFactory = {
         let dateFormatter = DateFormatter()
         dateFormatter.dateFormat = "yyyy-MM-dd HH:mm:ss"
         return dateFormatter
-    }()
+    }
 
-    private static let iso8601Formatter: DateFormatter = {
+    private static let iso8601FormatterFactory: DateFormatterFactory = {
         let dateFormatter = DateFormatter()
         dateFormatter.calendar = Calendar(identifier: .iso8601)
         dateFormatter.locale = Locale(identifier: "en_US_POSIX")
         dateFormatter.timeZone = TimeZone(secondsFromGMT: 0)
         dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSXXXXX"
         return dateFormatter
-    }()
+    }
 
     /// Creates a new `DateFormat` tag renderer.
     public convenience init() {
-        self.init(defaultDateFormatter: DateFormat.dateAndTimeFormatter)
+        self.init(defaultDateFormatterFactory: DateFormat.dateAndTimeFormatterFactory)
     }
 
     /// Creates a new `DateFormat` tag renderer.
     /// - parameter defaultDateFormatter: The date formatter to use when the tag invocation
     ///   does not specify a date format.
-    public init(defaultDateFormatter: DateFormatter) {
-        self.defaultDateFormatter = defaultDateFormatter
+    public init(defaultDateFormatterFactory: @escaping DateFormatterFactory) {
+        self.defaultDateFormatterFactory = defaultDateFormatterFactory
     }
 
     /// A `DateFormat` tag renderer that uses ISO 8601 date formatting by default.
-    public static let iso8601 = DateFormat(defaultDateFormatter: DateFormat.iso8601Formatter)
+    public static let iso8601 = DateFormat(defaultDateFormatterFactory: DateFormat.iso8601FormatterFactory)
 
     /// See `TagRenderer`.
     public func render(tag: TagContext) throws -> Future<TemplateData> {
@@ -58,17 +60,23 @@ public final class DateFormat: TagRenderer {
         }
 
         let dateFormatter: DateFormatter
-        /// Set format as the second param or default to ISO-8601 format.
-        if tag.parameters.count == 2, let dateFormat = tag.parameters[1].string {
-            if let formatter = dateFormatterCache.dateFormatters[dateFormat] {
-                dateFormatter = formatter
-            } else {
+        var dateFormat: String?
+        if tag.parameters.count == 2 {
+            /// Set format as the second param. If that's not available, we'll use `self.defaultDateFormatterFactory`.
+            dateFormat = tag.parameters[1].string
+        }
+
+        let cacheKey = dateFormat ?? DateFormatterCache.defaultFormatterPlaceholderKey
+        if let formatter = dateFormatterCache.dateFormatters[cacheKey] {
+            dateFormatter = formatter
+        } else {
+            if let dateFormat = dateFormat {
                 dateFormatter = DateFormatter()
                 dateFormatter.dateFormat = dateFormat
-                dateFormatterCache.dateFormatters[dateFormat] = dateFormatter
+            } else {
+                dateFormatter = self.defaultDateFormatterFactory()
             }
-        } else {
-            dateFormatter = self.defaultDateFormatter
+            dateFormatterCache.dateFormatters[cacheKey] = dateFormatter
         }
 
         /// Return formatted date
@@ -78,6 +86,7 @@ public final class DateFormat: TagRenderer {
 
 private class DateFormatterCache {
     static let userInfoKey = "TemplateKit.DateFormatterCache"
+    static let defaultFormatterPlaceholderKey = "DEFAULT"
 
     var dateFormatters: [String: DateFormatter] = [:]
 }