基于CMake Android NDK學(xué)習(xí)

網(wǎng)上很多NDK開發(fā)入門都是基于Ecplise，ndk-build方式的開發(fā)教程，本文主要介紹在現(xiàn)有項(xiàng)目中用CMake腳本方式添加C/C++代碼。

Android NDK 安裝教程參考官網(wǎng)

官網(wǎng)開發(fā)教程

NDK開發(fā)教程

一.創(chuàng)建源文件

• IDE左側(cè)，切換項(xiàng)目為Project試圖
• 導(dǎo)航到src，右鍵點(diǎn)擊main目錄，選擇New > Directory
• 在目錄中給文件夾取名（官網(wǎng)取名cpp，隨便?。┎Ⅻc(diǎn)擊OK
• 右鍵點(diǎn)擊上一步創(chuàng)建目錄，然后選擇New > C/C++ Source File
• 為源文件輸入一個(gè)名稱，如native-lib（愛咋取咋?。?，然后點(diǎn)擊OK

二.創(chuàng)建CMake腳本

右鍵點(diǎn)擊app，選擇New > File,創(chuàng)建名為CMakeLists.txt作為文件名并點(diǎn)擊OK（名字不能看心情??發(fā)揮了）

接下來我們添加CMake以下命令，開始構(gòu)建腳本（其他相關(guān)操作配置，，根據(jù)相關(guān)需求參考官方文檔）

# 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.
#定義Cmake 版本號(hào)，每個(gè)版本新增功能，參考CMake官方文檔
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.
#定義library名字（生成的so庫名：lib庫名稱.so）和定義library相對(duì)路徑
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 )
#引用庫
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.
# 關(guān)聯(lián)相關(guān)lib
target_link_libraries( # Specifies the target library.
                       native-lib

                       # Links the log library to the target library.
                       ${log-lib} )

三.使用Android GUI關(guān)聯(lián)CMake

• 左側(cè)IDE打開Project選擇app模塊
• 右鍵app文件夾，從菜單中選擇Link C++ Project with Gradle
• 在彈框中，Build System選擇CMake方式，Path選擇你的CMakeLists.txt文件，點(diǎn)擊OK等待gradle構(gòu)建完成

四.開始在java中定義native方法

• 定義native方法

public class NativeDemo {
    //靜態(tài)native方法
    public static native String helloFromJNI();
}
//非靜態(tài)native方法
public class Hello {
    public int property;
    public int function(int foo, Date date, int[] arr) {
        System.out.println("function");
        Log.d("Yxjie","jni輸入數(shù)字："+foo);
        return foo;
    }
    public boolean func(int a ,double b,char c){
        return true;
    }
    public native void test();
}

• 開始寫native-lib.cpp文件
  開始編寫之前我們需要了解一下簽名格式：
  

<font color=red>注：char類型參數(shù)要在前面加個(gè)L，如L'a',java
中字符在Unicode是雙字節(jié)的，而C++中的字符是單字節(jié)，需要加寬</font>

#include "jni.h"

extern "C"
JNIEXPORT jstring  JNICALL
Java_com_example_ndktest_NativeDemo_helloFromJNI(JNIEnv *env, jclass jclz) {
    //靜態(tài)native方法 需要兩個(gè)參數(shù)JNIEnv，以及jclass
    return env->NewStringUTF("Hello From JNI");
}

extern "C"
JNIEXPORT void Java_com_example_ndktest_Hello_test(JNIEnv *env, jobject obj) {
    //非靜態(tài)方法需要JNIEnv以及jobject（java中Hello，class對(duì)象引用）
    jclass hello_clazz = env->GetObjectClass(obj);//獲取java端 hello.class
    jfieldID fieldId_prop = env->GetFieldID(hello_clazz, "property", "I");//獲取屬性
    jint prop_int = env->GetIntField(hello_clazz, fieldId_prop);//獲取java prop默認(rèn)值

    jmethodID methodId_function = env->GetMethodID(hello_clazz, "function",
                                                   "(ILjava/util/Date;[I)I");//獲取對(duì)應(yīng)方法

    //批量傳入?yún)?shù)
    jmethodID methodId_func = env->GetMethodID(hello_clazz, "func", "(IDC)Z");
    jvalue *args = new jvalue[3];
    args[0].i = 10L;
    args[1].d = 3.14;
    args[3].c = L'j';//Java 中字符雙字節(jié)，C++字符為單子接需要編程器寬字節(jié)
    env->CallBooleanMethod(hello_clazz, methodId_func, args);
    delete[] args;
    //直接傳值
    env->SetIntField(hello_clazz, fieldId_prop, 100L);//設(shè)置java類中props的值
    env->CallIntMethod(obj, methodId_function, 11L, NULL, NULL);//調(diào)用方法

}

• 加載load lib
  我是直接放在我的入口Activity里面，直接靜態(tài)代碼塊加載lib

public class MainActivity extends AppCompatActivity {

    static {
        System.loadLibrary("native-lib");
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        TextView txt=findViewById(R.id.txt);
        txt.setText(NativeDemo.helloFromJNI());

        Hello hello=new Hello();
        hello.test();
    }
}

<font color=red>注：如有錯(cuò)誤歡迎留言指正</font>