Dealing With Display Scales

[xavierLowmiller]

Question:

How do you deal with size classes on different devices?

Elaborate explanation:

We'd like to generate snapshots of views in different size classes.
To do this, we pass a UITraitCollection with different display scales to the .image function.

Consider the following test case:

func testPrimaryButtonStates() {
    // Given
    let frame = CGRect(x: 0, y: 0, width: 200, height: 48)
    let button = PrimaryButton(frame: frame)
    button.awakeFromNib()
    button.setTitle("PrimaryButton", for: .normal)

    // When
    button.testState = .enabled

    // Then
    assertSnapshot(matching: button, as: .image(traits: .init(displayScale: 2)), named: "enabled@2x")
    assertSnapshot(matching: button, as: .image(traits: .init(displayScale: 3)), named: "enabled@3x")
}

This test passes on Simulators with the same display scale (say iPhone 7 -> iPhone 8), but fails when the display scale changes (iPhone 7 -> iPhone X).
Since we run the tests on our CI on multiple devices, we can't guarantee the display scale to always be the same.

Here is a diff of the screenshots of one of the failing test cases:

[stephencelis]

This unfortunately appears to be a limitation of how differently-scaled simulators render things 😞. Ideally, UIGraphicsImageRenderer would guarantee a consistent rendering given a view and a constant trait collection, but there are artifacts that can’t be accounted for. Filing a radar may help get this fixed in the future, but in the meantime you have to make one of the following choices:

• Run tests in simulators of the same scale
• Run tests in simulators of both scales, guarding assertions using UIScreen.main.scale
• Introduce precision: to the assertion

Really wish there were a better solution here! Maybe in the future!

[xavierLowmiller]

Thanks for the quick response!

I just checked how we dealt with this in the FBSnapshotTestCase stone age.
They append a @2x and @3x to the generated image name by default.

Would this be an option for this library, too?

[stephencelis]

@xavierLowmiller This is definitely a problem that could have a better solution, but for now you can encode this in the named argument:

assertSnapshot(matching: vc, as: .image, named: “@\(Int(UIScreen.main.scale))x”)

Not ideal, but a helper in your test suite could save you the extra work each time. We’ll try to address this better in the future, though!

[xavierLowmiller]

Here's the radar: rdar://46554126

Thanks for your support!

[xavierLowmiller]

Sadly, the radar was closed with "Behaving as intended": http://openradar.appspot.com/radar?id=4941361895702528....