What it does
A "snapshot test case" takes a configured
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:
- 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.
FBSnapshotTestCase to make snapshot tests easy.
Creating a snapshot test
- From within your test, use
- Run the test once with
self.recordMode = YES;in the test's
-setUpmethod. (This creates the reference images on disk.)
- Remove the line enabling record mode and run the test.
- 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
usesDrawViewHierarchyInRectto handle cases like
UIAppearanceand Size Classes.
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 is BSD-licensed. See