Add XCTAssertDifference for exhaustively testing changes to values (#51)

* Add `XCTAssertDifference` for testing changes to values

* wip

* wip

* Docs

* Update Tests/CustomDumpTests/XCTAssertDifferenceTests.swift

Co-authored-by: Brandon Williams <[email protected]>

* wip

* wip

---------

Co-authored-by: Brandon Williams <[email protected]>

Author: stephencelis

Sources/CustomDump/XCTAssertDifference.swift

@@ -0,0 +1,159 @@
+import XCTestDynamicOverlay
+
+/// Asserts that a value has a set of changes.
+///
+/// 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:
+/// `
+/// ```swift
+/// 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:
+///
+/// ```swift
+/// counter.increment()
+/// XCTAssertDifference(counter) {
+///   $0.count = 1
+///   // Don't need to further describe how `isOdd` has changed
+/// }
+/// ```
+///
+/// - Parameters:
+///   - expression: An expression that is evaluated before and after `operation`, and then compared.
+///   - message: An optional description of a failure.
+///   - operation: An optional operation that is performed in between an initial and final
+///     evaluation of `operation`. By omitting this operation, you can write a "non-exhaustive"
+///     assertion against an already-changed value by describing just the fields you want to assert
+///     against in the `changes` closure.
+///   - updateExpectingResult: A closure that asserts how the expression changed by supplying a
+///     mutable version of the initial value. This value must be modified to match the final value.
+@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 {
+  do {
+    var expression1 = try expression()
+    try updateExpectingResult(&expression1)
+    try operation()
+    let expression2 = try expression()
+    let message = message()
+    guard expression1 != expression2 else { return }
+    let format = DiffFormat.proportional
+    guard let difference = diff(expression1, expression2, format: format)
+    else {
+      XCTFail(
+        """
+        XCTAssertDifference failed: ("\(expression1)" is not equal to ("\(expression2)"), but no \
+        difference was detected.
+        """,
+        file: file,
+        line: line
+      )
+      return
+    }
+    let failure = """
+      XCTAssertDifference failed: …
+
+      \(difference.indenting(by: 2))
+      
+      (Expected: \(format.first), Actual: \(format.second))
+      """
+    XCTFail(
+      "\(failure)\(message.isEmpty ? "" : " - \(message)")",
+      file: file,
+      line: line
+    )
+  } catch {
+    XCTFail(
+      """
+      XCTAssertDifference failed: threw error "\(error)"
+      """,
+      file: file,
+      line: line
+    )
+  }
+}
+
+/// Asserts that a value has a set of changes.
+///
+/// An async version of ``XCTAssertDifference(_:_:operation:changes:)``.
+@available(iOS 13, macOS 10.15, tvOS 13, watchOS 6, *)
+public func XCTAssertDifference<T: Sendable>(
+  _ expression: @autoclosure @Sendable () throws -> T,
+  _ message: @autoclosure @Sendable () -> String = "",
+  operation: @Sendable () async throws -> Void = {},
+  changes updateExpectingResult: @Sendable (inout T) throws -> Void,
+  file: StaticString = #filePath,
+  line: UInt = #line
+) async where T: Equatable {
+  do {
+    var expression1 = try expression()
+    try updateExpectingResult(&expression1)
+    try await operation()
+    let expression2 = try expression()
+    let message = message()
+    guard expression1 != expression2 else { return }
+    let format = DiffFormat.proportional
+    guard let difference = diff(expression1, expression2, format: format)
+    else {
+      XCTFail(
+        """
+        XCTAssertDifference failed: ("\(expression1)" is not equal to ("\(expression2)"), but no \
+        difference was detected.
+        """,
+        file: file,
+        line: line
+      )
+      return
+    }
+    let failure = """
+      XCTAssertDifference failed: …
+
+      \(difference.indenting(by: 2))
+
+      (Expected: \(format.first), Actual: \(format.second))
+      """
+    XCTFail(
+      "\(failure)\(message.isEmpty ? "" : " - \(message)")",
+      file: file,
+      line: line
+    )
+  } catch {
+    XCTFail(
+      """
+      XCTAssertDifference failed: threw error "\(error)"
+      """,
+      file: file,
+      line: line
+    )
+  }
+}

Tests/CustomDumpTests/XCTAssertDifferenceTests.swift

@@ -0,0 +1,44 @@
+import CustomDump
+import XCTest
+
+@available(iOS 13, macOS 10.15, tvOS 13, watchOS 6, *)
+class XCTAssertDifferencesTests: XCTestCase {
+  func testXCTAssertDifference() {
+    var user = User(id: 42, name: "Blob")
+    func increment<Value>(_ root: inout Value, at keyPath: WritableKeyPath<Value, Int>) {
+      root[keyPath: keyPath] += 1
+    }
+
+    XCTAssertDifference(user) {
+      increment(&user, at: \.id)
+    } changes: {
+      $0.id = 43
+    }
+  }
+
+  func testXCTAssertDifference_NonExhaustive() {
+    let user = User(id: 42, name: "Blob")
+
+    XCTAssertDifference(user) {
+      $0.id = 42
+      $0.name = "Blob"
+    }
+  }
+
+  #if DEBUG && compiler(>=5.4) && (os(iOS) || os(macOS) || os(tvOS) || os(watchOS))
+    func testXCTAssertDifference_Failure() {
+      var user = User(id: 42, name: "Blob")
+      func increment<Value>(_ root: inout Value, at keyPath: WritableKeyPath<Value, Int>) {
+        root[keyPath: keyPath] += 1
+      }
+
+      XCTExpectFailure()
+
+      XCTAssertDifference(user) {
+        increment(&user, at: \.id)
+      } changes: {
+        $0.id = 44
+      }
+    }
+  #endif
+}