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1.2 CMake编译基础语法与实践

上一小节已经通过Android Studio创建了一个支持NDK的项目，这小节来看看AS帮助我们写了哪些代码，与普通的通常有什么不一样。

1、从项目结构上来看，AS帮我们在main目录先创建了一个cpp的文件夹，用来存放CMake脚本，以及native源文件。

项目结构

2、主Module（即app Module）的build.gradle文件也新增了一些内容。

externalNativeBuild {
    cmake {
        cppFlags "-std=c++11"
    }
}

-std=c++11是在创建项目时选择的C++标准。

externalNativeBuild {
    cmake {
        path "src/main/cpp/CMakeLists.txt"
        version "3.10.2"
    }
}

这里声明了CMake的脚本路径，版本号，下面看看CMakeLists.txt写了什么内容：

# For more information about using CMake with Android Studio, read the
# documentation: https://d.android.com/studio/projects/add-native-code.html

# Sets the minimum version of CMake required to build the native library.

cmake_minimum_required(VERSION 3.10.2)

# Declares and names the project.

project("ndktutorials")

# 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 them 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).
             native-lib.cpp )

# Searches for a specified prebuilt library and stores the path as a
# variable. Because CMake includes system libraries 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 this
# 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} )

早期其实可以先只关注这几行：

add_library( # Sets the name of the library.
             native-lib //生成so库的名字

             # Sets the library as a shared library.
             SHARED

             # Provides a relative path to your source file(s).
             native-lib.cpp ) //源码

接下来看看MainActivity

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();
}

1、声明了一个native方法 stringFromJNI()； 2、System.loadLibrary("native-lib")加载so库； 3、调用native方法

下面看看这个native方法的实现：

extern "C" JNIEXPORT jstring JNICALL
Java_com_jdqm_ndktutorials_MainActivity_stringFromJNI(
        JNIEnv* env,
        jobject /* this */) {
    std::string hello = "Hello from C++";
    return env->NewStringUTF(hello.c_str());
}

jstring：返回值 com_jdqm_ndktutorials：包名点换成了下划线 MainActivity：声明native方法的类名 stringFromJNI：方法名