Hashing {#hashing}

Features like <LocalizedLink href="/guides/features/cache">caching</LocalizedLink> or selective test execution require a way to determine whether a target has changed. Tuist calculates a hash for each target in the dependency graph to determine if a target has changed. The hash is calculated based on the following attributes:

• The target's attributes (e.g., name, platform, product, etc.)
• The target's files
• The hash of the target's dependencies

Cache attributes {#cache-attributes}

Additionally, when calculating the hash for <LocalizedLink href="/guides/features/cache">caching</LocalizedLink>, we also hash the following attributes.

Swift version {#swift-version}

We hash the Swift version obtained from running the command /usr/bin/xcrun swift --version to prevent compilation errors due to Swift version mismatches between the targets and the binaries.

[!NOTE] Module Stability

Previous versions of binary caching relied on the BUILD_LIBRARY_FOR_DISTRIBUTION build setting to enable module stability and enable using binaries with any compiler version. However, it caused compilation issues in projects with targets that don't support module stability. Generated binaries are bound to the Swift version used to compile them, and the Swift version must match the one used to compile the project.

Configuration {#configuration}

The idea behind the flag -configuration was to ensure debug binaries were not used in release builds and viceversa. However, we are still missing a mechanism to remove the other configurations from the projects to prevent them from being used.

Debugging {#debugging}

If you notice non-deterministic behaviors when using the caching across environments or invocations, it might be related to differences across the environments or a bug in the hashing logic. We recommend following these steps to debug the issue:

1. Run tuist hash cache or tuist hash selective-testing (hashes for <LocalizedLink href="/guides/features/cache">binary caching</LocalizedLink> or <LocalizedLink href="/guides/features/selective-testing">selective testing</LocalizedLink>), copy the hashes, rename the project directory, and run the command again. The hashes should match.
2. If the hashes don't match, it's likely that the generated project depends on the environment. Run tuist graph --format json in both cases and compare the graphs. Alternatively, generate the projects and compare their project.pbxproj files with a diff tool such as Diffchecker.
3. If the hashes are the same but differ across environments (for example, CI and local), make sure the same configuration and Swift version are used everywhere. The Swift version is tied to the Xcode version, so confirm the Xcode versions match.

If the hashes are still non-deterministic, let us know and we can help with the debugging.

[!NOTE] Better Debugging Experience Planned

Improving our debugging experience is in our roadmap. The print-hashes command, which lacks the context to understand the differences, will be replaced by a more user-friendly command that uses a tree-like structure to show the differences between the hashes.