如何将文件下载从配置步骤移动到构建步骤

Question

假设我的CMakeLists.txt中有下面的块。

file (DOWNLOAD http://.../file.zip "${CMAKE_BINARY_DIR}/file.zip")
execute_process (
COMMAND "${CMAKE_COMMAND}" -E tar -xf file.zip
WORKING_DIRECTORY "${CMAKE_BINARY_DIR}"
)
add_custom_command (
OUTPUT output.txt
COMMAND "${MY_COMMAND}" file-found-in-zip.txt output.txt
WORKING_DIRECTORY "${CMAKE_BINARY_DIR}"
)

基本步骤是：

1. 下载
2. 摘录
3. 添加一个自定义命令，该命令使用在步骤2中提取的文件

在构建步骤中，可以执行也可以不执行自定义命令，但下载和提取始终会执行。如何使下载和提取成为有条件的，以便只有在执行需要它的自定义命令时才会发生？

Answer 1

我猜有什么：

file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/download.cmake 
"file(DOWNLOAD http://.../file.zip ${CMAKE_CURRENT_BINARY_DIR}/file.zip"
)
add_custom_command(
OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/file.zip
COMMAND ${CMAKE_COMMAND} -P ${CMAKE_CURRENT_BINARY_DIR}/download.cmake
VERBATIM
)
execute_process (
OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/file-found-in-zip.txt
DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/file.zip
COMMAND ${CMAKE_COMMAND} -E tar -xf file.zip
WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
VERBATIM
)
add_custom_command (
OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/output.txt
DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/file-found-in-zip.txt
COMMAND "${MY_COMMAND}" file-found-in-zip.txt output.txt
WORKING_DIRECTORY "${CMAKE_BINARY_DIR}"
VERBATIM
)

或者只是：

file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/download.cmake 
"file(DOWNLOAD http://.../file.zip ${CMAKE_CURRENT_BINARY_DIR}/file.zip"
)
add_custom_command(
OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/output.txt
COMMAND ${CMAKE_COMMAND} -P ${CMAKE_CURRENT_BINARY_DIR}/download.cmake
COMMAND ${CMAKE_COMMAND} -E tar -xf file.zip
COMMAND "${MY_COMMAND}" file-found-in-zip.txt output.txt
WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
VERBATIM
)