CMake构建Flutter Android插件报错：URL条目含无效路径

问题详情 #

为自研Flutter插件配置CMake时，编写了从GitHub仓库下载Zip包的代码，Windows平台构建正常，但Android平台构建失败，报错：At least one entry of URL is a path (invalid in a list)。

相关CMake代码片段 #

# Download the zipped folder of dart-sdk-api from the dependencies repo
set(dart_sdk_api_zip_url "https://github.com/Haidar0096/image_magick_ffi_deps/raw/master/dart_sdk_api.zip")
message(STATUS "Downloading dart-sdk-api from ${dart_sdk_api_zip_url}")
FetchContent_Declare(
        dart_sdk_api
        URL ${dart_sdk_api_zip_url}
        DOWNLOAD_EXTRACT_TIMESTAMP TRUE
)
FetchContent_MakeAvailable(dart_sdk_api)

# Include the include folder of dart_sdk_api from the downloaded zip
include_directories(${dart_sdk_api_SOURCE_DIR}/include)

# Create a target from the sources of the downloaded zip and link it to the executable
file(GLOB_RECURSE DART_SDK_API_SOURCES "${dart_sdk_api_SOURCE_DIR}/src/*.c")
add_library(dart_sdk_api STATIC ${DART_SDK_API_SOURCES})
target_link_libraries(image_magick_ffi dart_sdk_api)

Android构建错误日志 #

[        ] Successfully started process 'command 'C:\Users\haidar\AppData\Local\Android\Sdk\cmake\3.18.1\bin\cmake.exe''
[        ] C/C++: debug|arm64-v8a :CMake Error at C:/Users/haidar/AppData/Local/Android/Sdk/cmake/3.18.1/share/cmake-3.18/Modules/ExternalProject.cmake:2480 (message):
[        ] C/C++: debug|arm64-v8a :  At least one entry of URL is a path (invalid in a list)
[        ] C/C++: debug|arm64-v8a :Call Stack (most recent call first):
[        ] C/C++: debug|arm64-v8a :  C:/Users/haidar/AppData/Local/Android/Sdk/cmake/3.18.1/share/cmake-3.18/Modules/ExternalProject.cmake:3228 (_ep_add_download_command)
[        ] C/C++: debug|arm64-v8a :  CMakeLists.txt:13 (ExternalProject_Add)
[        ] C/C++: debug|arm64-v8a :-- Configuring incomplete, errors occurred!
[        ] C/C++: debug|arm64-v8a :See also "D:/dev_haidar/flutter/image_magick_ffi/android/.cxx/Debug/42552b6e/arm64-v8a/_deps/dart_sdk_api-subbuild/CMakeFiles/CMakeOutput.log".

错误原因 #

报错源于Android SDK自带的CMake版本（3.18.1）过旧，在Android交叉编译场景下，旧版本CMake对FetchContent_Declare的URL参数解析存在缺陷，会误将远程URL识别为本地路径，从而触发该错误。

解决方法 #

方法1：升级Android SDK中的CMake版本 #

打开Android Studio，进入SDK Manager -> SDK Tools，找到CMake选项，选择更新到3.21及以上版本（如3.22.1），重新构建项目即可。新版本CMake优化了FetchContent的URL处理逻辑，能正确识别远程资源。

方法2：添加URL_HASH参数增强兼容性 #

在FetchContent_Declare中加入URL_HASH参数，指定下载文件的SHA256哈希值，帮助旧版本CMake明确这是远程资源。示例代码：

FetchContent_Declare(
        dart_sdk_api
        URL ${dart_sdk_api_zip_url}
        URL_SHA256 替换为dart_sdk_api.zip的SHA256哈希值
        DOWNLOAD_EXTRACT_TIMESTAMP TRUE
)

可通过以下命令获取哈希值：

• Windows：certutil -hashfile dart_sdk_api.zip SHA256
• Linux/macOS：shasum -a 256 dart_sdk_api.zip

方法3：改用ExternalProject_Add手动配置（兼容旧CMake） #

若无法升级CMake，可直接使用ExternalProject_Add替代FetchContent，手动配置下载逻辑：

include(ExternalProject)
ExternalProject_Add(
        dart_sdk_api
        URL ${dart_sdk_api_zip_url}
        DOWNLOAD_DIR ${CMAKE_CURRENT_BINARY_DIR}/_deps
        SOURCE_DIR ${CMAKE_CURRENT_BINARY_DIR}/_deps/dart_sdk_api-src
        CONFIGURE_COMMAND ""
        BUILD_COMMAND ""
        INSTALL_COMMAND ""
        DOWNLOAD_EXTRACT_TIMESTAMP TRUE
)

# 确保依赖顺序
add_dependencies(image_magick_ffi dart_sdk_api)

# 配置头文件和源码
include_directories(${CMAKE_CURRENT_BINARY_DIR}/_deps/dart_sdk_api-src/include)
file(GLOB_RECURSE DART_SDK_API_SOURCES "${CMAKE_CURRENT_BINARY_DIR}/_deps/dart_sdk_api-src/src/*.c")

# 构建静态库并链接
add_library(dart_sdk_api STATIC ${DART_SDK_API_SOURCES})
add_dependencies(dart_sdk_api dart_sdk_api)
target_link_libraries(image_magick_ffi dart_sdk_api)

内容的提问来源于stack exchange，提问作者Haidar