class XCTestCase : XCTest {
  /// The names of the performance metrics to measure when invoking `XCTestCase.measure()`.
  /// Returns `XCTPerformanceMetric_WallClockTime` by default. Subclasses can override this to
  /// change the behavior of `XCTestCase.measure()`.
  class func defaultPerformanceMetrics() -> [String]

  /// Call from a test method to measure resources (`XCTestCase.defaultPerformanceMetrics()`)
  /// used by the block in the current process.
  ///     func testPerformanceOfMyFunction() {
  ///         measure {
  ///             // Do that thing you want to measure.
  ///             myFunction()
  ///         }
  ///     }
  /// - Parameter block: A closure whose performance to measure.
  func measure(_ block: () -> Void)

  /// Call from a test method to measure resources (XCTPerformanceMetrics) used by the
  /// block in the current process. Each metric will be measured across calls to the block.
  /// The number of times the block will be called is undefined and may change in the
  /// future. For one example of why, as long as the requested performance metrics do
  /// not interfere with each other the API will measure all metrics across the same
  /// calls to the block. If the performance metrics may interfere the API will measure
  /// them separately.
  /// 
  ///     func testMyFunction2_WallClockTime() {
  ///         measureMetrics(self.dynamicType.defaultPerformanceMetrics(), automaticallyStartMeasuring: false) {
  ///             // Do setup work that needs to be done for every iteration
  ///             // but you don't want to measure before the call to `XCTestCase.startMeasuring()`.
  ///             setupSomething()
  ///             startMeasuring()
  ///             // Do that thing you want to measure.
  ///             myFunction()
  ///             stopMeasuring()
  ///             // Do teardown work that needs to be done for every iteration
  ///             // but you don't want to measure after the call to `XCTestCase.stopMeasuring()`.
  ///             teardownSomething()
  ///         }
  ///     }
  ///
  /// Caveats:
  /// • If true was passed for `automaticallyStartMeasuring` and `XCTestCase.startMeasuring()`
  ///   is called anyway, the test will fail.
  /// • If false was passed for `automaticallyStartMeasuring` then `XCTestCase.startMeasuring()`
  ///   must be called once and only once before the end of the block or the test will fail.
  /// • If `XCTestCase.stopMeasuring()` is called multiple times during the block the test will fail.
  ///
  /// - Parameter metrics: An array of NSStrings (XCTPerformanceMetrics) to measure. Providing an unrecognized string is a test failure.
  /// - Parameter automaticallyStartMeasuring: If false, XCTestCase will not take any measurements until `XCTesrCase.startMeasuring()` is called.
  /// - Parameter block: A closure whose performance to measure.
  func measureMetrics(_ metrics: [String], automaticallyStartMeasuring automaticallyStartMeasuring: Bool, for block: () -> Void)

  /// Call this from within a measure block to set the beginning of the critical section.
  /// Measurement of metrics will start at this point.
  func startMeasuring()

  /// Call this from within a measure block to set the ending of the critical section.
  /// Measurement of metrics will stop at this point.
  func stopMeasuring()
}
/// Records wall clock time in seconds between startMeasuring/stopMeasuring.
let XCTPerformanceMetric_WallClockTime: String