From 69489a9a248143c663aa3d07f4d8df54f19be553 Mon Sep 17 00:00:00 2001
From: Nuno Vieira <nuno.fcvieira93@gmail.com>
Date: Thu, 8 Apr 2021 19:43:39 +0100
Subject: [PATCH] Fix false positive when asserting old image with new empty
 image

---
 Sources/SnapshotTesting/AssertSnapshot.swift     |   6 ++++--
 .../SnapshotTesting/Snapshotting/UIImage.swift   |   3 ++-
 .../SnapshotTestingTests.swift                   |  11 +++++++++++
 .../testViewAgainstEmptyImage.notEmptyImage.png  | Bin 0 -> 1831 bytes
 4 files changed, 17 insertions(+), 3 deletions(-)
 create mode 100644 Tests/SnapshotTestingTests/__Snapshots__/SnapshotTestingTests/testViewAgainstEmptyImage.notEmptyImage.png

diff --git a/Sources/SnapshotTesting/AssertSnapshot.swift b/Sources/SnapshotTesting/AssertSnapshot.swift
index dd6ca4fb6..85acb0aba 100644
--- a/Sources/SnapshotTesting/AssertSnapshot.swift
+++ b/Sources/SnapshotTesting/AssertSnapshot.swift
@@ -256,8 +256,10 @@ 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
diff --git a/Sources/SnapshotTesting/Snapshotting/UIImage.swift b/Sources/SnapshotTesting/Snapshotting/UIImage.swift
index bf87a1c07..903e58cbf 100644
--- a/Sources/SnapshotTesting/Snapshotting/UIImage.swift
+++ b/Sources/SnapshotTesting/Snapshotting/UIImage.swift
@@ -30,7 +30,8 @@ extension Diffing where Value == UIImage {
         : "Newly-taken snapshot@\(new.size) does not match reference@\(old.size)."
       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"
diff --git a/Tests/SnapshotTestingTests/SnapshotTestingTests.swift b/Tests/SnapshotTestingTests/SnapshotTestingTests.swift
index 4f7a8ba64..bd887842d 100644
--- a/Tests/SnapshotTestingTests/SnapshotTestingTests.swift
+++ b/Tests/SnapshotTestingTests/SnapshotTestingTests.swift
@@ -998,6 +998,17 @@ 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()
diff --git a/Tests/SnapshotTestingTests/__Snapshots__/SnapshotTestingTests/testViewAgainstEmptyImage.notEmptyImage.png b/Tests/SnapshotTestingTests/__Snapshots__/SnapshotTestingTests/testViewAgainstEmptyImage.notEmptyImage.png
new file mode 100644
index 0000000000000000000000000000000000000000..341e06ebdb1064f31cba7e1b6067216df967eb3a
GIT binary patch
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