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.
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:
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.
Add the following lines to your Podfile:
target "Tests" do 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.
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 |
FBSnapshotTestCase
instead of XCTestCase
.FBSnapshotVerifyView
.self.recordMode = YES;
in the test's -setUp
method. (This creates the reference images on disk.)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 OS
s and devices).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.
FBSnapshotTestCase
was written at Facebook by Jonathan Dann with significant contributions by Todd Krabach.
FBSnapshotTestCase
is BSD-licensed. See LICENSE
.