FBSnapshotTestCase

What it does

A “snapshot test case” takes a configured UIView or CALayer and uses the
renderInContext: method to get an image snapshot of its contents. It
compares this snapshot to a “reference image” stored in your source code
repository and fails the test if the two images don’t match.

Why?

At Facebook we write a lot of UI code. As you might imagine, each type of
feed story is rendered using a subclass of UIView. There are a lot of edge
cases that we want to handle correctly:

• What if there is more text than can fit in the space available?
• What if an image doesn’t match the size of an image view?
• What should the highlighted state look like?

It’s straightforward to test logic code, but less obvious how you should test
views. You can do a lot of rectangle asserts, but these are hard to understand
or visualize. Looking at an image diff shows you exactly what changed and how
it will look to users.

We developed FBSnapshotTestCase to make snapshot tests easy.

Installation with CocoaPods

1. Add the following lines to your Podfile:

   target "Tests" do
     use_frameworks!
     pod 'FBSnapshotTestCase'
   end
   

   If you support iOS 7 use FBSnapshotTestCase/Core instead, which doesn’t contain Swift support.

   Replace “Tests” with the name of your test project.

2. There are three ways of setting reference image directories, the recommended one is to define FB_REFERENCE_IMAGE_DIR in your scheme. This should point to the directory where you want reference images to be stored. At Facebook, we normally use this:

Name	Value
FB_REFERENCE_IMAGE_DIR	$(SOURCE_ROOT)/$(PROJECT_NAME)Tests/ReferenceImages
IMAGE_DIFF_DIR	$(SOURCE_ROOT)/$(PROJECT_NAME)Tests/FailureDiffs

Define the IMAGE_DIFF_DIR to the directory where you want to store diffs of failed snapshots.

Creating a snapshot test

1. Subclass FBSnapshotTestCase instead of XCTestCase.
2. From within your test, use FBSnapshotVerifyView.
3. Run the test once with self.recordMode = YES; in the test’s -setUp
   method. (This creates the reference images on disk.)
4. Remove the line enabling record mode and run the test.

Features

• Automatically names reference images on disk according to test class and
  selector.
• Prints a descriptive error message to the console on failure. (Bonus:
  failure message includes a one-line command to see an image diff if
  you have Kaleidoscope installed.)
• Supply an optional “identifier” if you want to perform multiple snapshots
  in a single test method.
• Support for CALayer via FBSnapshotVerifyLayer.
• usesDrawViewHierarchyInRect to handle cases like UIVisualEffect, UIAppearance and Size Classes.
• isDeviceAgnostic to allow appending the device model (iPhone, iPad, iPod Touch, etc), OS version and screen size to the images (allowing to have multiple tests for the same «snapshot» for different OSs and devices).

Notes

Your unit test must be an “application test”, not a “logic test.” (That is, it
must be run within the Simulator so that it has access to UIKit.) In Xcode 5
and later new projects only offer application tests, but older projects will
have separate targets for the two types.

Authors

FBSnapshotTestCase was written at Facebook by
Jonathan Dann with significant contributions by
Todd Krabach.

License

FBSnapshotTestCase is BSD-licensed. See LICENSE.