Upload failing iOS snapshot tests cases to S3
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Latest commit 9737efd Nov 23, 2015


Second Curtain

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If you're using the cool FBSnapshotTestCase to test your iOS view logic, awesome! Even better if you have continuous integration, like on Travis, to automate running those tests!


Isn't it frustrating that you can't see the results of your tests? At best, you'll get this kind of error output:

  a_view_controller_with_a_loaded_view_should_have_a_valid_snapshot, expected a matching snapshot in a_view_controller_with_a_loaded_view_should_have_a_valid_snapshot

        it(@"should have a valid snapshot", ^{

    Executed 1 test, with 1 failure (1 unexpected) in 0.952 (0.954) seconds

Wouldn't it be awesome if we could upload the failing test snapshots somewhere, so we can see exactly what's wrong? That's what we aim to do here.


Usage is pretty simple. Have an S3 bucket that is world-readable (that is, include the following bucket policy).

  "Version": "2008-10-17",
  "Statement": [
      "Sid": "AllowPublicRead",
      "Effect": "Allow",
      "Principal": {
        "AWS": "*"
      "Action": "s3:GetObject",
      "Resource": "arn:aws:s3:::bucket-name/*"

(Replace "bucket-name" with your bucket name.)

It's also a good idea not to use a general-purpose S3 user for your CI, so create a new one with the following policy that will let them list buckets, but only read from or write to the bucket you're using.

  "Version": "2012-10-17",
  "Statement": [
      "Effect": "Allow",
      "Action": ["s3:ListAllMyBuckets"],
      "Resource": "arn:aws:s3:::*"
      "Effect": "Allow",
      "Action": [
      "Resource": "arn:aws:s3:::bucket-name"
      "Effect": "Allow",
      "Action": [
      "Resource": "arn:aws:s3:::bucket-name/*"

OK, so the hard part is mostly done. Now that we have a place to upload our images, let's configure our build.

I use a Makefile to build and run tests on my iOS project, so my .travis.yml file looks something like this:

language: objective-c
cache: bundler


  - bundle install

  - export LANG=en_US.UTF-8
  - make ci

  - make test

(You can take a look at how to encrypt information in your config file, but this has limitations due to how ecrypted variables are accessed via PRs on forks.)

My Makefile looks like this:

WORKSPACE = Demo/Demo.xcworkspace

all: ci

	set -o pipefail && xcodebuild -workspace $(WORKSPACE) -scheme $(SCHEME) -sdk iphonesimulator build | xcpretty -c

	xcodebuild -workspace $(WORKSPACE) -scheme $(SCHEME) clean

	set -o pipefail && xcodebuild -workspace $(WORKSPACE) -scheme $(SCHEME) -configuration Debug test -sdk iphonesimulator | second_curtain | xcpretty -c --test

ci:	build

Notice that we're piping the output from xcodebuild into second_curtain.

Note also that we're using xcpretty, as you should, too. The xcpretty invocation must come after the second_curtain invocation, since Second Curtain relies on parsing the output from xcodebuild directly.

And finally, our Gemfile:

source 'https://rubygems.org'

gem 'cocoapods'
gem 'xcpretty'
gem 'second_curtain', '~> 0.2'

And when any snapshot tests fail, they'll be uploaded to S3 and an HTML page will be generated with links to the images so you can download them. Huzzah!

Sample diff

Note that when the S3 directory is created, it needs to be unique. You can provide a custom folder name with the UPLOAD_IOS_SNAPSHOT_FOLDER_NAME environment variable. If one is not provided, Second Curtain will fall back to TRAVIS_JOB_ID or CIRCLE_BUILD_NUM, and then onto one generated by the current date and time.