Save failed snapshot if it differs from reference image

.ruby-version

@@ -1,2 +1,2 @@
-2.3.5
+2.4.3
 

Sources/SnapshotCoordinator.swift

@@ -54,6 +54,7 @@ extension SnapshotCoordinator : SnapshotCoordinating {
         let referenceImage = try fileManager.referenceImage(filename: filename, className: className)
 
         guard snapshot.compare(withImage: referenceImage) else {
+            try fileManager.save(referenceImage: snapshot, filename: filename.failed, className: className)
             throw SnapshotError.imageMismatch(filename: filename)
         }
     }
@@ -65,3 +66,10 @@ extension SnapshotCoordinator : SnapshotCoordinating {
         return try fileManager.save(referenceImage: referenceImage, filename: filename, className: className)
     }
 }
+
+fileprivate extension String {
+
+    var failed: String {
+        return "\(self)_failed"
+    }
+}

Sources/SnapshotFileManager.swift

@@ -50,6 +50,7 @@ class DataHandler : DataHandling {
 }
 
 protocol SnapshotFileManaging {
+    @discardableResult
     func save(referenceImage: UIImage, filename: String, className: String) throws -> URL
     func referenceImage(filename: String, className: String) throws -> UIImage
 }

Tests/SnapshotTestTests/SnapshotCoordinatorTests.swift

@@ -72,6 +72,22 @@ class SnapshotCoordinatorTests: XCTestCase {
             XCTAssertEqual(error as? SnapshotError, SnapshotError.imageMismatch(filename: "redSquare"))
         }
     }
+
+    func testCompareSnapshot_withViewNotEqualToReferenceImage_shouldInvokeSaveWithFailedSnapshotAndFailedFilenameOnFileManager() {
+
+        // Given
+        let view = UIView(frame: CGRect(x: 0, y: 0, width: 200, height: 200))
+        view.backgroundColor = .blue
+        fileManagerMock.referenceImageReturnValue = UIImage(testFilename: "redSquare", ofType: "png")
+
+        // When
+        XCTAssertThrowsError(try sut.compareSnapshot(of: view, options: [], functionName: "redSquare", line: 0)) { error in
+            XCTAssertEqual(fileManagerMock.saveInvokeCount, 1)
+            XCTAssertNotNil(fileManagerMock.saveReferenceImageArgument)
+            XCTAssertEqual(fileManagerMock.saveFilenameArgument, "redSquare_failed")
+            XCTAssertEqual(fileManagerMock.saveClassNameArgument, "CustomButtonTests")
+        }
+    }
 
     func testCompareSnapshot_shouldInvokeReferenceImageWithCorrectFilenameAndClassNameOnFileManager() {