Explore CMAKE options to include ImagingSuite automatically

[anderskaestner]

Including dependencies built from other CMake projects into your own CMake-based project can be streamlined using several approaches. The best method often depends on the specifics of your project, the external dependencies, and how you manage your build environment. Below are some effective strategies:

1. add_subdirectory (For Source Inclusion)

If you have access to the source code of the dependencies and can include them in your project's source tree (or as Git submodules), add_subdirectory() is a straightforward method. This command tells CMake to add another project source directory to the build.

# Assuming the dependency's source is in an external/ directory
add_subdirectory(external/dependency)

This approach makes it easy to build your project and its dependencies with the correct settings all at once. However, it requires that the dependency's CMakeLists is compatible with being included in this way.

2. find_package (For Pre-installed Libraries)

For dependencies that are installed separately and findable via CMake's find_package() command, this approach allows for linking against pre-built libraries. find_package() searches for library binaries, header files, and necessary CMake files (like Config.cmake or Find<Package>.cmake) that are installed on your system.

find_package(Dependency REQUIRED)

After finding the package, you can link against it using target_link_libraries(). This method requires that the dependency is correctly installed and that CMake can find it, which might involve setting CMAKE_PREFIX_PATH or similar variables.

3. FetchContent (For Downloading During Configure Time)

Introduced in CMake 3.11, FetchContent is a powerful command that downloads dependencies directly into your build tree during the configuration phase. It's especially useful for dependencies not available as pre-built binaries or when you want to pin a specific version of a dependency.

include(FetchContent)

FetchContent_Declare(
  dependency
  GIT_REPOSITORY <repository_url>
  GIT_TAG <tag/branch/commit>
)
FetchContent_MakeAvailable(dependency)

FetchContent_MakeAvailable makes the dependency's targets available for linking as if they were part of your own project.

4. ExternalProject (For More Complex Builds)

ExternalProject_Add() is a flexible command that can download, configure, build, and install external projects at build time. It's more complex than FetchContent and is executed during the build phase, not the configure phase. This method is useful when dependencies require a complex build process or when you want to keep the dependency builds entirely separate from your project.

include(ExternalProject)
ExternalProject_Add(
  dependency
  GIT_REPOSITORY <repository_url>
  GIT_TAG <tag/branch/commit>
  CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=<install_path>
)

Choosing the Best Method

• For tight integration and simplicity: add_subdirectory or FetchContent.
• For pre-installed libraries: find_package.
• For complex external dependencies: ExternalProject.

Each of these methods has its place depending on your project's needs and the nature of the dependencies. For projects that aim to have a straightforward build process and manage dependencies efficiently, FetchContent offers a good balance of ease of use and control, making it a popular choice for modern CMake projects. However, leveraging find_package for widely-used, pre-installed libraries can significantly simplify dependency management and improve compatibility with various systems and package managers.

ExternalProject works on a folder too

The ExternalProject_Add command in CMake is highly versatile and does not strictly require a Git repository as the source for external projects. It can work with local directories, tarballs (compressed files), or even a URL to download the source code. This flexibility makes ExternalProject_Add a powerful tool for integrating complex dependencies into your build process, regardless of where the source code resides.

Using a Local Directory

To include a dependency from a different folder on the same computer, you can specify the SOURCE_DIR option in ExternalProject_Add. This tells CMake to use a local directory as the source:

include(ExternalProject)
ExternalProject_Add(
  name_of_your_dependency
  SOURCE_DIR "/path/to/your/local/dependency"
  CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=<install_path>
  # Specify other relevant options as needed
)

In this setup, CMake does not attempt to download the source code since SOURCE_DIR points to an existing local directory. It's a convenient way to integrate external projects that you might already have on your system without needing to fetch them from a remote repository.

Additional Considerations

• Building: CMake will configure, build, and optionally install the external project according to the options you provide in ExternalProject_Add.
• Dependencies: If the external project has its dependencies, you'll need to ensure they are resolved. This might involve specifying additional CMake arguments (CMAKE_ARGS) or setting up the environment so that the external project's build system can find its dependencies.
• Path Management: When specifying paths (e.g., SOURCE_DIR, BINARY_DIR, INSTALL_DIR), absolute paths are recommended to avoid confusion and errors related to relative path resolution.

Using ExternalProject_Add with a local source directory is particularly useful for integrating large external projects or dependencies that are under development or customization, allowing you to maintain a clear separation between your project and its dependencies while leveraging CMake's build automation capabilities.

Handling Multiple Dependencies with ExternalProject

When ExternalProject is used to build several dependencies, there are a couple of strategies you can use to manage this complexity:

1. Sequential Builds: You can add multiple ExternalProject_Add commands to your CMakeLists.txt file, one for each dependency. If these dependencies need to be built in a specific order (for instance, if one depends on another), you can use the DEPENDS option in ExternalProject_Add to manage this dependency chain.

ExternalProject_Add(dependency1
  # Configuration for dependency1
)

ExternalProject_Add(dependency2
  DEPENDS dependency1
  # Configuration for dependency2
)

2. Superbuild Approach: For managing complex dependencies, a superbuild project can be created. This is essentially a CMake project whose sole purpose is to orchestrate the building of multiple dependencies and your project itself. Each dependency (and possibly your project) is included as an external project. This method keeps the main project's CMakeLists.txt cleaner and centralizes dependency management.

3. Custom Aggregate Targets: If you need more control over the build process, you can create custom targets using add_custom_target and then use add_dependencies to manage the build order of your external projects.

add_custom_target(build_all_dependencies)
add_dependencies(build_all_dependencies dependency1 dependency2)

This approach allows you to explicitly control the build process and order of your external dependencies.

Fetch from git if needed

ExternalProject_Add itself is designed to execute a predefined sequence of steps (such as download, update, configure, build, install) and does not include built-in conditional logic to check if a local path is present before deciding whether to fetch from a Git repository. However, you can achieve this behavior by incorporating CMake's conditional logic around the ExternalProject_Add call.

Here’s how you can conditionally use a local path if present or fall back to fetching from a Git repository if the local path is not available:

1. Define the Local Path and Git Repository: Specify variables for both the local path and the Git repository URL.

2. Check for the Local Path: Use CMake's if(EXISTS ...) command to check if the directory at the local path exists.

3. Conditionally Configure the External Project: Depending on the existence of the local path, configure ExternalProject_Add to either use the local directory or fetch from the Git repository.

Here is an example illustrating this approach:

cmake_minimum_required(VERSION 3.14)
project(ConditionalDependency)

include(ExternalProject)

# Define the local path and Git repository
set(local_path "${CMAKE_CURRENT_SOURCE_DIR}/path/to/local/dependency")
set(git_repo "https://github.com/example/dependency.git")

# Check if the local path exists
if(EXISTS "${local_path}")
  # Configure ExternalProject to use the local directory
  ExternalProject_Add(
    my_dependency
    SOURCE_DIR "${local_path}"
    CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=${CMAKE_BINARY_DIR}/dependency_install
  )
else()
  # Configure ExternalProject to fetch from the Git repository
  ExternalProject_Add(
    my_dependency
    GIT_REPOSITORY "${git_repo}"
    GIT_TAG "master" # Specify the branch, tag, or commit
    CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=${CMAKE_BINARY_DIR}/dependency_install
  )
endif()

This script first checks if the local path to the dependency exists. If it does, ExternalProject_Add is configured to use the local source directory. If not, it falls back to fetching the source code from the specified Git repository.

This approach provides flexibility in managing dependencies, allowing developers to work with local copies of dependencies for development or testing purposes while also supporting automated fetching of dependencies from remote repositories when necessary.