今天，簡單講講android中關于Cmake進行NDK編程的相關知識。

Android studio 2.2版本以后對C/C++的支持可以說很方便了，當然官方推薦使用Cmake完成對C/C++的支持

2.2版本以上的同學新建一個項目就知道了，步驟如下：

File -> New -> New Project,如下圖：

然后勾選Include C++ support,一直next ，最后Finish以后，項目就出現了，和一般的項目略有不同，其實只要多了幾個文件，而已：

1：目錄下多了個CmakeLists.txt文件

2：src目錄下多了一個cpp目錄，里面有個.cpp文件，C++文件都是以.cpp結尾的。

3：就是build.gradle內容中添加了幾行配置

一一解讀一下，先來看看CmakeLists.txt 文件里面的內容是什么：

# 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 3.4.1)# 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).src/main/cpp/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} )這里有設置Cmake版本啊，引用的cpp文件的路徑啊等等，如果自己引用的話 其實值需要修改引用的cpp路徑即可，

其他的暫時不需要動

看下native_lib.cpp文件內容：

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

很簡單 其實就是輸出Hello from C++這段文字：

再看下build.gradle的配置改變：

apply plugin: 'com.android.application'android {compileSdkVersion 25buildToolsVersion "25.0.2"defaultConfig {applicationId "com.process.main.myapplication"minSdkVersion 22targetSdkVersion 25versionCode 1versionName "1.0"testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"externalNativeBuild {cmake {cppFlags ""}}}buildTypes {release {minifyEnabled falseproguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'}}externalNativeBuild {cmake {path "CMakeLists.txt"}} }dependencies {compile fileTree(dir: 'libs', include: ['*.jar'])androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {exclude group: 'com.android.support', module: 'support-annotations'})compile 'com.android.support:appcompat-v7:25.1.1'compile 'com.android.support.constraint:constraint-layout:1.0.0-beta4'testCompile 'junit:junit:4.12' }

主要多了兩個地方的改變：

1：defaultConfig中添加：

externalNativeBuild {cmake {cppFlags ""}}

2：在android{}中添加：

externalNativeBuild {cmake {path "CMakeLists.txt"} }

其實也就是引用CmakeLists.txt文件。

現在重點講講CmakeLists.txt里面的代碼的含義。

以上的CmakeLists.txt對于與圖片的目錄結構，可以看出主要是add_liberary里寫入需要編譯的C文件的路徑，其余的基本不用修改。
還有就是要是在add_liberary中添加.cpp文件的話記得路徑，如果你的CMakeLists和你的.cpp在一個問價夾下就不用管路徑。

Android studio 使用Cmake完成C/C++ 的使用以及生成so文件就講完了。
就這么簡單。
 

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