Function XCTAssert​Difference(_:​_:​operation:​changes:​file:​line:​)

@available(iOS 13, macOS 10.15, tvOS 13, watchOS 6, *)
public func XCTAssertDifference<T>(
  _ expression: @autoclosure () throws -> T,
  _ message: @autoclosure () -> String = "",
  operation: () throws -> Void = {},
  changes updateExpectingResult: (inout T) throws -> Void,
  file: StaticString = #filePath,
  line: UInt = #line
) where T: Equatable  

This function evaluates a given expression before and after a given operation and then compares the results. The comparison is done by invoking the changes closure with a mutable version of the initial value, and then asserting that the modifications made match the final value using XCTAssertNoDifference.

For example, given a very simple counter structure, we can write a test against its incrementing functionality: `

struct Counter {
  var count = 0
  var isOdd = false
  mutating func increment() {
    self.count += 1
    self.isOdd.toggle()
  }
}

var counter = Counter()
XCTAssertDifference(counter) {
  counter.increment()
} changes: {
  $0.count = 1
  $0.isOdd = true
}

If the changes does not exhaustively describe all changed fields, the assertion will fail.

By omitting the operation you can write a "non-exhaustive" assertion against a value by describing just the fields you want to assert against in the changes closure:

counter.increment()
XCTAssertDifference(counter) {
  $0.count = 1
  // Don't need to further describe how `isOdd` has changed
}