Android Studio 2.2以上支持了Cmake的配置JNI的相关参数

Android Studio 2.2以上支持了Cmake的配置JNI的相关参数，简化了通过Android.mk配置。并很好的继承了C++的编辑方式。以下是对应的引入第三方so和第三方.cpp文件的路径脚本编写。对应于：CMakeLists.txt

设置CMake插件的版本

# Sets the minimum version of CMake required to build your native library.
# This ensures that a certain set of CMake features is available to
# your build.

cmake_minimum_required(VERSION 3.4.1)

定义要编译的源代码和最终要生成的库文件名称及类型

# Specifies a library name, specifies whether the library is STATIC or
# SHARED, and provides relative paths to the source code. You can
# define multiple libraries by adding multiple add.library() commands,
# and CMake builds them for you. When you build your app, Gradle
# automatically packages shared libraries with your APK.

#语法，参考：https://cmake.org/cmake/help/latest/command/add_library.html
add_library(<name> [STATIC | SHARED | MODULE]
            [EXCLUDE_FROM_ALL]
            source1 [source2 ...])
# 语法#########################################

# 示例
add_library(
             #编译生成的库的名称，注意最终生成时会在前面加`lib`.
             # Specifies the name of the library.
             native-lib

             # Sets the library as a shared library.
             # 生成的库的类型，有SHARED，STATIC，MODULE
             SHARED

             # Provides a relative path to your source file(s).
             # 要编译的源代码文件
             src/main/cpp/native-lib.cpp )

包含要编译的源码头文件

如果有源码文件有头文件，自然需要包含:

# 语法， 参考：https://cmake.org/cmake/help/latest/command/include_directories.html
include_directories([AFTER|BEFORE] [SYSTEM] dir1 [dir2 ...])
# 语法#########################################

# Specifies a path to native header files.
# 注意这里指定的是头文件的目录
include_directories(src/main/cpp/include/)

使用NDK中的Api

有时候我们发现需要使用到NDK中定义的Api，因为NDK中的API都已经是编译好的，而且CMake会自动去NDK目录中查找这些我们需要使用的库，所以这里只需要提供库的名称就可以了，一下举例加载NDK中的log库，因为我们在NDK开发中，总是少不了要打印日志的。 
- 首先查找要使用的库，使用变量保存它的路径。

# 语法，参考：https://cmake.org/cmake/help/latest/command/find_library.html
# A short-hand signature is:
# name1 保存库路径的变量
find_library (<VAR> name1 [path1 path2 ...])
# The general signature is:

find_library (
          <VAR>
          name | NAMES name1 [name2 ...] [NAMES_PER_DIR]
          [HINTS path1 [path2 ... ENV var]]
          [PATHS path1 [path2 ... ENV var]]
          [PATH_SUFFIXES suffix1 [suffix2 ...]]
          [DOC "cache documentation string"]
          [NO_DEFAULT_PATH]
          [NO_CMAKE_ENVIRONMENT_PATH]
          [NO_CMAKE_PATH]
          [NO_SYSTEM_ENVIRONMENT_PATH]
          [NO_CMAKE_SYSTEM_PATH]
          [CMAKE_FIND_ROOT_PATH_BOTH |
           ONLY_CMAKE_FIND_ROOT_PATH |
           NO_CMAKE_FIND_ROOT_PATH]
         )
# 语法#########################################

# 示例
# 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 )

为了编译时可以使用，需要将上面一步找到的库链接到我们要编译的库中。
# 语法，参考: https://cmake.org/cmake/help/latest/command/target_link_libraries.html
target_link_libraries(<target> ... <item>... ...)
# 语法 end #######################

# 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.
# 注意这里可以同时链接多个库
target_link_libraries( # Specifies the target library.
                       native-lib

                       # Links the log library to the target library.
                       # 此处使用上面保存的变量
                       ${log-lib} )

NDK中也有一些库是以源代码的方式存在的，这个时候如果我们要使用它，就需要把它编译到一个本地静态库中，再链接到我们要生成的库中。

# 这样就会在\app\.externalNativeBuild\cmake\debug\${ANDROID_ABI}\ 目录下生成libapp-glue.a/静态库
add_library( app-glue
             # 以静态的方式添加
             STATIC
             # 该源码用于管理`NativeActivity`生命周期事件和触摸事件
             ${ANDROID_NDK}/sources/android/native_app_glue/android_native_app_glue.c )

# You need to link static libraries against your shared native library.
# 链接到库中
target_link_libraries( native-lib app-glue ${log-lib} )

添加第三方的库

使用add_library添加第三库

# 语法，因为已经编译好了，所以使用`IMPORTED`关键字
add_library(<name> <SHARED|STATIC|MODULE|UNKNOWN> IMPORTED [GLOBAL])

# 示例
add_library( imported-lib
             SHARED
             IMPORTED )

当第三方库有针对不同架构编译了不同的库版本时，有时候我们只需要引入我们想要的版本的库，当我们想要引入多个版本的库时，可以使用ANDROID_ABI变量，它表示默认的ABI架构和NDK支持的架构，如果我们在build.gradle中设置了过滤值，则表示过滤后的架构集合。

set_target_properties( # Specifies the target library.
                       imported-lib

                       # Specifies the parameter you want to define.
                       PROPERTIES IMPORTED_LOCATION

                       # Provides the path to the library you want to import.
                       # 注意下面的imported-lib是在项目的app目录下的
                       imported-lib/src/${ANDROID_ABI}/libimported-lib.so )

同时为了在编译时的使用，我们需要导入所依赖的库的头文件

include_directories( imported-lib/include/ )

添加链接，以便在编译的时候使用，注意如果要添加的第三方库在编译我们要产出的库时不需要直接用到（比如一个库是imported-lib所依赖的），则不需要执行以下步骤。

target_link_libraries( native-lib imported-lib app-glue ${log-lib} )

在build.gradle中配置编译库的脚本

android {
    ......

    defaultConfig {
        ......
        //在默认配置中配置 cmake 的一些参数
        externalNativeBuild {
            cmake {
                arguments "-DANDROID_TOOLCHAIN=clang"
                cFlags  ""
                cppFlags ""
            }
        }

        //ndk 配置
        ndk {
          // Specifies the ABI configurations of your native
          // libraries Gradle should build and package with your APK.
          abiFilters 'x86', 'x86_64', 'armeabi', 'armeabi-v7a',
                       'arm64-v8a'
        }
    }

    //配置库编译的脚本
    externalNativeBuild {
        cmake {
            path "CMakeLists.txt"
        }
    }

    //定义不同的产品类型（多渠道打包），通过复写externalNativeBuild，使用target字段实现打包不同的so库。
    productFlavors {
    ...
    demo {
      ...
      externalNativeBuild {
        cmake {
          ...
          // Specifies which native libraries to build and package for this
          // product flavor. If you don't configure this property, Gradle
          // builds and packages all shared object libraries that you define
          // in your CMake or ndk-build project.
          targets "native-lib-demo"
        }
      }
    }

    paid {
      ...
      externalNativeBuild {
        cmake {
          ...
          targets "native-lib-paid"
        }
      }
    }
  }

}