# Configuring Component Dependencies

A component declares the other components it uses, and v2 lets those declarations depend on the project configuration. Expressing dependencies in terms of Kconfig options, so that the set of components built into a project follows its configuration, is the main new capability of v2. This page covers how to declare dependencies and, in particular, how to make them conditional on configuration.

重要

Configuration-conditional dependencies are a v2-only feature. A component that makes its dependencies conditional on `CONFIG_*` options will not work as expected under v1. To keep such a component working under both v1 and v2, see Managing Component Backward Compatibility.

## How Dependencies Are Declared

A component brings another component into the build and links it. With idf_component_register, this is the `REQUIRES` and `PRIV_REQUIRES` arguments (see Backward-Compatible Component). A native CMake component does it explicitly, by calling idf_component_include and linking the dependency's `idf::<name>` interface target (see Native CMake Component):

    idf_component_include(log)
    target_link_libraries(${COMPONENT_TARGET} PRIVATE idf::log)

With `idf_component_register`, the common components (`freertos`, `log`, and so on) are added automatically, as in v1. A native CMake component receives nothing automatically and must include every component it uses with idf_component_include, including the common ones.

## Configuration-Driven Dependencies

A dependency is declared in one of two ways (see How Dependencies Are Declared): with the `REQUIRES` or `PRIV_REQUIRES` arguments of idf_component_register, or with idf_component_include in a native CMake component. Because the configuration of all discovered components is available while a component is evaluated, either form can be made conditional on `CONFIG_*` options.

The option that drives the dependency is declared in a Kconfig file (see Project Configuration):

main/Kconfig.projbuild

    config EXAMPLE_ENABLE_LOGGING
        bool "Enable Logging Utility Component"
        default y

### Using `idf_component_register`

A component that uses idf_component_register builds its `PRIV_REQUIRES` list (or `REQUIRES` for a public dependency) conditionally and passes it to the call. This is usually the most natural form for a component coming from v1:

    set(reqs)
    if(CONFIG_EXAMPLE_ENABLE_LOGGING)
        list(APPEND reqs logging_util)
    endif()

    idf_component_register(SRCS main.c PRIV_REQUIRES ${reqs})

Initialize the list with `set(reqs)` before appending to it, so it does not inherit a value from an enclosing component's scope; see Recursive Evaluation of Components.

### Using `idf_component_include`

A native CMake component expresses the same condition around the idf_component_include call and the `target_link_libraries`. The conditional_component example is written this way: its `main` component depends on the `logging_util` and `math_util` components only when the corresponding options are enabled.

main/CMakeLists.txt

    add_library(${COMPONENT_TARGET} STATIC main.c)

    if(CONFIG_EXAMPLE_ENABLE_LOGGING)
        idf_component_include(logging_util)
        target_link_libraries(${COMPONENT_TARGET} PRIVATE idf::logging_util)
    endif()

In both forms, the component's source code uses the same option to compile the dependent code conditionally:

    #ifdef CONFIG_EXAMPLE_ENABLE_LOGGING
    #include "logging_util.h"
    #endif

重要

A dependency must be included with idf_component_include before it is used. In v2 the configuration of every discovered component is visible, so a `CONFIG_*` option being set does not mean the component that defines it is part of the build. Do not assume a component is available just because its configuration option is set; include it explicitly. This visibility change from v1 is detailed in Component Configuration Visibility.

## Optional Dependencies

A component can declare an optional dependency with idf_component_optional_requires, which links another component only if it is already part of the build. This function exists mainly for backward compatibility with v1 and should generally be avoided in v2; prefer making dependencies conditional on configuration options, as described above. When it is used, v2 resolves the dependency in one of two modes, IMMEDIATE or DEFERRED, depending on how the project is built; the modes and their constraints are described in The Behavior of idf_component_optional_requires has Changed.

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