Exhaustive RenderTester state assertion (#238)

- Before: To perform exhaustive state assertions with a RenderTester you have to manually keep track of the initial state.
- After: Added RenderTesterResult/assertState which makes it easy to perform exhaustive state assertions by mutating the initial state

Author: gilgtm

WorkflowTesting/Sources/RenderTesterResult.swift

@@ -20,11 +20,13 @@ import XCTest
 /// The result of a `RenderTester` rendering. Used to verify state, output, and actions that were produced as a result of
 /// actions performed during the render (such as child workflow output being produced).
 public struct RenderTesterResult<WorkflowType: Workflow> {
+    let initialState: WorkflowType.State
     let state: WorkflowType.State
     let appliedAction: AppliedAction<WorkflowType>?
     let output: WorkflowType.Output?
 
-    internal init(state: WorkflowType.State, appliedAction: AppliedAction<WorkflowType>?, output: WorkflowType.Output?) {
+    internal init(initialState: WorkflowType.State, state: WorkflowType.State, appliedAction: AppliedAction<WorkflowType>?, output: WorkflowType.Output?) {
+        self.initialState = initialState
         self.state = state
         self.appliedAction = appliedAction
         self.output = output
@@ -120,6 +122,22 @@ extension RenderTesterResult where WorkflowType.State: Equatable {
         XCTAssertEqual(state, expectedState, file: file, line: line)
         return self
     }
+
+    /// Exhaustive state testing against the initial state.
+    /// - Parameters:
+    ///   - modifications: A function that receives the initial state
+    ///   and is expected to mutate it to match the new state.
+    @discardableResult
+    public func assertStateModifications(
+        file: StaticString = #file,
+        line: UInt = #line,
+        _ modifications: (inout WorkflowType.State) throws -> Void
+    ) rethrows -> RenderTesterResult<WorkflowType> {
+        var initialState = self.initialState
+        try modifications(&initialState)
+        XCTAssertEqual(state, initialState, "Expected state does not match", file: file, line: line)
+        return self
+    }
 }
 
 extension RenderTesterResult where WorkflowType.Output: Equatable {

WorkflowTesting/Sources/WorkflowRenderTester.swift

@@ -275,6 +275,7 @@ public struct RenderTester<WorkflowType: Workflow> {
         try assertions(rendering)
 
         return RenderTesterResult<WorkflowType>(
+            initialState: state,
             state: contextImplementation.state,
             appliedAction: contextImplementation.appliedAction,
             output: contextImplementation.producedOutput

WorkflowTesting/Tests/WorkflowRenderTesterFailureTests.swift

@@ -340,6 +340,18 @@ final class WorkflowRenderTesterFailureTests: XCTestCase {
             }
         }
     }
+
+    func test_assertState() {
+        let result = TestWorkflow()
+            .renderTester(initialState: .idle)
+            .render { _ in }
+
+        expectingFailure("Expected state does not match") {
+            result.assertStateModifications { state in
+                state = .sideEffect(key: "nah")
+            }
+        }
+    }
 }
 
 private struct TestWorkflow: Workflow {

WorkflowTesting/Tests/WorkflowRenderTesterTests.swift

@@ -185,6 +185,9 @@ final class WorkflowRenderTesterTests: XCTestCase {
             .verifyState { state in
                 XCTAssertEqual("Failed", state.text)
             }
+            .assertStateModifications { state in
+                state.text = "Failed"
+            }
     }
 }