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

1. 在打开的项目中，从菜单栏选择 Tools > Android > SDK Manager。
2. 点击 SDK Tools 标签。
3. 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

1. 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.
  • 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。

 　　注： 非特殊情况下，只保留  'arm64-v8a' 就够用了。 参考微信的apk。