NDK(23) 使用CMake 构建 c/c++代码库

1.官网

　　https://developer.android.com/studio/projects/add-native-code.html

2.android studio 安装相关工具

在打开的项目中，从菜单栏选择 Tools > Android > SDK Manager。
点击 SDK Tools 标签。
选中 LLDB、CMake 和 NDK 旁的复选框，如图所示.

3.新建支持c/c++的项目

在android studio 3.4.1 新建向导里选 Native C++ 模板.

2.选择想要使用的c++标准,如C++14.

3.最后生成的项目如下:

其中:

- CMakeLists.txt 是cmake构建工程要用的项目配置文件,类似android.mk.
- native-lib.cpp是源文件

4.编辑代码,使用本地程序....

4.在现有项目中添加 c/c++库

4.1 创建 jni目录

　　项目名(或者File) ---> 右键 ---> New ---> Folder ---> JNI Folder

4.2 创建 CMakeLists.txt文件

　　在上一步上创建的jni目录(当前版本目录名是cpp) 右键 ---> New ---> File 输入CMakeLists.txt

4.3 配置CMakeLists.txt 文件

 # 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)

 # 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.

 add_library( # Specifies the name of the library.

              # 本地库名

              student-lib

              # Sets the library as a shared library.

              # 本地库的类型

              SHARED

              # Provides a relative path to your source file(s).

              # 要编译的源文件 ,多个之间用空格.

              student.cpp school.cpp)

 # Specifies a path to native header files.

 include_directories(inc)

 #添加本地代码依赖的其它系统库.

 find_library( # Defines the name of the path variable that stores the

               # location of the NDK library.

               log-lib

               # Specifies the name of the NDK library that

               # CMake needs to locate.

               log )

 # Links your native library against one or more other native libraries.

 target_link_libraries( # Specifies the target library.

                        student-lib

                        # Links the log library to the target library.

                        ${log-lib} )

注意CMakeLists.txt中源文件和include的路径引用问题,这里是和源文件同级.
CMakeLists.txt 常用函数含义
下面是常用的cmake命令.

　　　　cmake命令官网 : https://cmake.org/cmake/help/latest/manual/cmake-commands.7.html

add_library()	向您的 CMake 构建脚本添加源文件或库 add_library( # Specifies the name of the library. native-lib # Sets the library as a shared library. SHARED # Provides a relative path to your source file(s). src/main/cpp/native-lib.cpp )
include_directories	指定头文件的路径 add_library(...) # Specifies a path to native header files. include_directories(src/main/cpp/include/)
find_library()	添加引用的NDK 库 find_library( # Defines the name of the path variable that stores the # location of the NDK library. log-lib # Specifies the name of the NDK library that # CMake needs to locate. log )
target_link_libraries()	链接多个库 target_link_libraries( native-lib imported-lib app-glue ${log-lib} )
add_library()	将其它源代码编译到本地库中. add_library( app-glue STATIC ${ANDROID_NDK}/sources/android/native_app_glue/android_native_app_glue.c ) # You need to link static libraries against your shared native library. target_link_libraries( native-lib app-glue ${log-lib} )
set_target_properties 和 IMPORTED	IMPORTED不是个函数,只是add_library的参数,添加其他预构建的本地库然后，您需要使用 set_target_properties() 命令指定库的路径.

4.4 关联Gradle与 CMakeLists.txt　　

　　右键点击您想要关联的模块（例如 app 模块），并从菜单中选择 Link C++ Project with Gradle。关联到之前创建的CMakeLists.txt,点OK.

4.5 完成本地的c/c++代码工作

MainActivity.java　

 public class MainActivity extends AppCompatActivity {

     // Used to load the 'native-lib' library on application startup.

     static {

         System.loadLibrary("native-lib");

     }

     @Override

     protected void onCreate(Bundle savedInstanceState) {

         super.onCreate(savedInstanceState);

         setContentView(R.layout.activity_main);

         // Example of a call to a native method

         TextView tv = findViewById(R.id.sample_text);

         tv.setText(stringFromJNI());

     }

     /**

      * A native method that is implemented by the 'native-lib' native library,

      * which is packaged with this application.

      */

     public native String stringFromJNI();

 }

native-lib.cpp

 #include <jni.h>

 #include <string>

 extern "C" JNIEXPORT jstring JNICALL

 Java_com_example_cpp14_MainActivity_stringFromJNI(

         JNIEnv* env,

         jobject /* this */) {

     std::string hello = "Hello from C++";

     return env->NewStringUTF(hello.c_str());

 }

5.在 Gradle 中使用 CMake 变量

　　https://developer.android.com/ndk/guides/cmake.html#variables

 android {

   ...

   defaultConfig {

     ...

     // This block is different from the one you use to link Gradle

     // to your CMake or ndk-build script.

     externalNativeBuild {

       // For ndk-build, instead use ndkBuild {}

       cmake {

         // Passes optional arguments to CMake.

         arguments "-DANDROID_ARM_NEON=TRUE", "-DANDROID_TOOLCHAIN=clang"

         // Sets optional flags for the C compiler.

         cFlags "-D_EXAMPLE_C_FLAG1", "-D_EXAMPLE_C_FLAG2"

         // Sets a flag to enable format macro constants for the C++ compiler.

         cppFlags "-D__STDC_FORMAT_MACROS"

       }

     }

   }

   buildTypes {...}

     ...

 }

6.指定本地库的cpu架构

　　默认情况下，Gradle 会针对 NDK 支持的 ABI 将您的原生库构建到单独的 .so 文件中，并将其全部打包到您的 APK 中。如果您希望 Gradle 仅构建和打包原生库的特定 ABI 配置，您可以在模块级 build.gradle 文件中使用 ndk.abiFilters 标志指定这些配置.

 apply plugin: 'com.android.application'

 android {

     compileSdkVersion 28

     defaultConfig {

         applicationId "com.example.tocpp5"

         minSdkVersion 15

         targetSdkVersion 28

         versionCode 1

         versionName "1.0"

         testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"

         // This block is different from the one you use to link Gradle

         // to your CMake or ndk-build script.

         externalNativeBuild {

             // For ndk-build, instead use ndkBuild {}

             cmake {

                 // Passes optional arguments to CMake.

                 arguments "-DANDROID_ARM_NEON=TRUE", "-DANDROID_TOOLCHAIN=clang"

                 // Sets optional flags for the C compiler.

                 cFlags "-D_EXAMPLE_C_FLAG1", "-D_EXAMPLE_C_FLAG2"

                 // Sets a flag to enable format macro constants for the C++ compiler.

                 cppFlags "-D__STDC_FORMAT_MACROS"

                 abiFilters 'armeabi-v7a','arm64-v8a'

             }

         }

30         ndk {

31             // Specifies the ABI configurations of your native

32             // libraries Gradle should build and package with your APK.

33             abiFilters 'x86', 'x86_64', 'armeabi-v7a','arm64-v8a'

34         }

     }

     buildTypes {

         release {

             minifyEnabled false

             proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'

         }

     }

     externalNativeBuild {

         cmake {

             path file('src/main/jni/CMakeLists.txt')

         }

     }

 }

 dependencies {

     implementation fileTree(dir: 'libs', include: ['*.jar'])

     implementation 'androidx.appcompat:appcompat:1.0.2'

     implementation 'androidx.constraintlayout:constraintlayout:1.1.3'

     testImplementation 'junit:junit:4.12'

     androidTestImplementation 'androidx.test:runner:1.2.0'

     androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'

 }

　　在 defaultConfig.externalNativeBuild.cmake {} 块

　　或

　　defaultConfig.externalNativeBuild.ndkBuild {} 块

　　中配置另一个 abiFilters 标志。Gradle 会构建defaultConfig.ndk中的 ABI 配置，不过仅会打包在 defaultConfig.cmake或者ndkBuild{} 块中指定的配置。