Fix false positive when asserting old image with new empty image (#453)

Author: nuno-vieira

Sources/SnapshotTesting/AssertSnapshot.swift

@@ -265,8 +265,10 @@ public func verifySnapshot<Value, Format>(
       let reference = snapshotting.diffing.fromData(data)
 
       #if os(iOS) || os(tvOS)
-      // If the image generation fails for the diffable part use the reference
-      if let localDiff = diffable as? UIImage, localDiff.size == .zero {
+      // If the image generation fails for the diffable part and the reference was empty, use the reference
+      if let localDiff = diffable as? UIImage,
+         let refImage = reference as? UIImage,
+         localDiff.size == .zero && refImage.size == .zero {
         diffable = reference
       }
       #endif

Sources/SnapshotTesting/Snapshotting/UIImage.swift

@@ -29,7 +29,8 @@ extension Diffing where Value == UIImage {
       let difference = SnapshotTesting.diff(old, new)
       let oldAttachment = XCTAttachment(image: old)
       oldAttachment.name = "reference"
-      let newAttachment = XCTAttachment(image: new)
+      let isEmptyImage = new.size == .zero
+      let newAttachment = XCTAttachment(image: isEmptyImage ? emptyImage() : new)
       newAttachment.name = "failure"
       let differenceAttachment = XCTAttachment(image: difference)
       differenceAttachment.name = "difference"

Tests/SnapshotTestingTests/SnapshotTestingTests.swift

@@ -1064,6 +1064,17 @@ final class SnapshotTestingTests: XCTestCase {
     #endif
   }
 
+  func testViewAgainstEmptyImage() {
+    #if os(iOS) || os(tvOS)
+    let rect = CGRect(x: 0, y: 0, width: 0, height: 0)
+    let view = UIView(frame: rect)
+    view.backgroundColor = .blue
+
+    let failure = verifySnapshot(matching: view, as: .image, named: "notEmptyImage")
+    XCTAssertNotNil(failure)
+    #endif
+  }
+
   func testEmbeddedWebView() throws {
     #if os(iOS)
     let label = UILabel()