constraint-error-ci-example

Catch broken Auto Layout constraints during test execution using LLDB breakpoints, and surface them as PR warnings via Danger.

Why?

UIViewAlertForUnsatisfiableConstraints fires at runtime but is invisible in CI. Broken constraints silently ship to production, causing layout bugs that are hard to trace back to a specific change.

This project demonstrates a technique to intercept these warnings during tests and block them at the PR level.

What it does

• Sets an LLDB breakpoint on UIViewAlertForUnsatisfiableConstraints that runs in the background during test execution
• A Python script attached to the breakpoint walks the call stack, identifies the failing XCTestCase, and logs it to brokenConstraints.txt
• Danger reads the output file and posts constraint violations as warnings on the pull request
• See PR #3 for an example of the Danger output

Requirements

• Xcode 14.1+
• macOS (CI runs on macos-latest)
• danger-swift (installed via Homebrew in CI)

Quick start

# Run tests locally
make test

# Run LLDB constraint detection (background) + tests
lldb --source lldb-command &
make test

How it works

1. LLDB attaches to the test process and sets a breakpoint on UIViewAlertForUnsatisfiableConstraints with auto-continue enabled
2. On each hit, a Python command iterates the thread's frames, finds the XCTestCase subclass, and appends the module/file/function to brokenConstraints.txt
3. After tests complete, Danger checks for the output file and warns on the PR with the logged violations

CI pipeline

Defined in .github/workflows/snapshot.yml, triggered on pull requests:

checkout -> lldb --source lldb-command & -> make test -> danger-swift ci

Project structure

lldb-command          # LLDB breakpoint script (Python)
Dangerfile.swift      # Danger config - reads constraint errors, warns on PR
Makefile              # xcodebuild wrapper
constraint-error-ci-example/
  BrokenView.swift    # UIView with intentionally broken constraints (demo)
constraint-error-ci-snapshots/
  BrokenViewTests.swift  # Snapshot tests that trigger constraint violations

Limitations

• LLDB must attach to the process before tests start - requires background launch (&)
• The lldb-command script relies on GITHUB_WORKSPACE env var, so local runs won't write to brokenConstraints.txt without modification
• Only detects constraints broken during test execution - no static analysis

License

MIT