Android Studio 2.2支持CMakeList编译脚本，这是个很不错的特性。本人并不熟悉CMakeList，只是不想再每次手动执行ndk-build、拷贝动态库到指定目录的shell脚本。
在向CMakeList指定链接自定义动态库时遇到了需要同时指定libs及src/main/libs才能编译成本的问题。
同时指定libs及src/main/libs
起初以为是gradle命令写错了。经测试，发现srcDir指定为libs时，app层级下的libs和src/main层级下的libs都能被拷贝到apk中。
疑似gradle引发的问题
如果删除src/main层级下的libs、只有app/libs时，则编译报错找不到src/main/libs中的动态库。
只有app/libs时链接失败
只用src/main层级下的libs，删除app/libs时，打包的apk找不到自定义的动态库。
只用src/main层级下的libs
为避免歧义，建议删除build.gradle中的sourceSets与拷贝预编译库相关的部分。
经检查，CMakeList指定的set_target_properties语句表明了动态库的路径，也是前面失败的根本原因。
set_target_properties语句导致的失败
修复后则顺利通过编译。
链接静态库只需修改add_library语句为STATIC IMPORTED。另外，指定链接库时容易出现yourLib-NOFOUND问题，原因是必须按语法要求使用前面add_library语句引入的变量名，而非直接填写。示例如下
add_library(libyuv SHARED IMPORTED)
set_target_properties( libyuv
PROPERTIESIMPORT_LOCATION
${CMAKE_SOURCE_DIR}/libs/${ANDROID_ABI}/libyuv.so )
target_link_libraries( ${libyuv})
# 下面是错误的写法
# target_link_libraries(libyuv)
完整示例如下所示。
# Sets the minimum version of CMake required to build the native
# library. You should either keep the default value or only pass a
# value of 3.4.0 or lower.
cmake_minimum_required(VERSION 3.4.1)
####################################################################################################
set(ffmpeg-headers ${CMAKE_SOURCE_DIR}/src/main/cpp/ffmpeg-header)
include_directories(${ffmpeg-headers})
set(PNG-headers ${CMAKE_SOURCE_DIR}/src/main/cpp/PNG)
include_directories(${PNG-headers})
set(include-headers ${CMAKE_SOURCE_DIR}/src/main/cpp/include)
include_directories(${include-headers})
set(SDL-headers ${CMAKE_SOURCE_DIR}/src/main/SDL/include)
include_directories(${SDL-headers})
#---------------------------------------------------------------------------------------------------
add_library(libffmpeg SHARED IMPORTED)
set_target_properties( libffmpeg
PROPERTIES
IMPORT_LOCATION ${CMAKE_SOURCE_DIR}/libs/${ANDROID_ABI}/libffmpeg.so )
add_library(libyuv SHARED IMPORTED)
set_target_properties( libyuv
PROPERTIES
IMPORT_LOCATION ${CMAKE_SOURCE_DIR}/libs/${ANDROID_ABI}/libyuv.so )
add_library(libpng STATIC IMPORTED)
set_target_properties( libpng
PROPERTIES
IMPORT_LOCATION ${CMAKE_SOURCE_DIR}/libs/${ANDROID_ABI}/libpng.a )
####################################################################################################
# Creates and names a library, sets it as either STATIC
# or SHARED, and provides the relative paths to its source code.
# You can define multiple libraries, and CMake builds it for you.
# Gradle automatically packages shared libraries with your APK.
add_library( # Sets the name of the library.
native-lib
# Sets the library as a shared library.
SHARED
# Provides a relative path to your source file(s).
# Associated headers in the same location as their source
# file are automatically included.
src/main/cpp/native-lib.cpp )
# Searches for a specified prebuilt library and stores the path as a
# variable. Because system libraries are included in the search path by
# default, you only need to specify the name of the public NDK library
# you want to add. CMake verifies that the library exists before
# completing its build.
find_library( # Sets the name of the path variable.
log-lib
# Specifies the name of the NDK library that
# you want CMake to locate.
log )
# Specifies libraries CMake should link to your target library. You
# can link multiple libraries, such as libraries you define in the
# build script, prebuilt third-party libraries, or system libraries.
target_link_libraries( # Specifies the target library.
native-lib
# Links the target library to the log library
# included in the NDK.
${log-lib}
${libffmpeg}
${libyuv}
${libpng}
)
