Android Studio中通過CMake使用NDK並編譯自定義庫和添加預編譯庫

cmake_minimum_required(VERSION 3.4.1)
add_library(add SHARED
            src/main/cpp/add.cpp)
set(distribution_DIR ${CMAKE_CURRENT_SOURCE_DIR}/../distribution)
set_target_properties(add PROPERTIES
                      LIBRARY_OUTPUT_DIRECTORY
                      ${distribution_DIR}/libs/${ANDROID_ABI})
target_include_directories(add
                           PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}/src/main/cpp)
                           
add_custom_command(TARGET add POST_BUILD
                   COMMAND "${CMAKE_COMMAND}" -E
                   copy "${CMAKE_CURRENT_SOURCE_DIR}/src/main/cpp/add.h"
                   "${distribution_DIR}/include/mathlib/add.h"
#                   **** the following 2 lines are for potential future debug purpose ****
#                   COMMAND "${CMAKE_COMMAND}" -E
#                   remove_directory "${CMAKE_CURRENT_BINARY_DIR}"
                   COMMENT "Copying gmath to output directory")

set可以自定義變數。這裡定義生成so文件的目錄

set_target_properties 命令的意思是設置目標的一些屬性來改變它們構建的方式。這個命令中設置了 add的ARCHIVE_OUTPUT_DIRECTORY 屬性。也就是改變了輸出路徑。

add_custom_command 命令是自定義命令。命令中把頭文件也複製到了 distribution_DIR 中。

target_include_directories，它對創建的庫設置include路徑，針對目標來設置，可以避免與其他庫的衝突，並且此時對自定義的庫設置好了此路徑後，後續導入這個庫就不需要再次設置了。但對於預構建的庫，就需要設置，稍後會有詳細講解。

接下來我們在模塊mathlib的build.gradle中的defaultConfig{}中添加如下語句：

externalNativeBuild {
            cmake {
                arguments '-DANDROID_PLATFORM=android-19',
                        '-DANDROID_TOOLCHAIN=clang', '-DANDROID_STL=gnustl_static'
                targets 'add'
            }
        }

這裡arguments是編譯參數，而targets則是相比於add_subdirectory更高許可權的方法。一般來說可以把它刪去，即預設構建所有目標。

然後在android{}最後添加如下語句，將CMakeLists.txt關聯起來。

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

C++庫已經創建好了，接下來就要在主模塊中使用它了。

為了使用自定義C++庫，我們需要一個中間人，它從Android本身的Java程式中獲取請求，然後使用我們的C++庫中的函數計算得到結果，並將數據傳回Android本身的Java程式中。

創建一個中間文件native-math.cpp

#include <jni.h>
#include <string>
#include "mathlib/add.h"

extern "C"
JNIEXPORT jstring

JNICALL
Java_com_example_bill_opencvtest_MainActivity_stringFromJNI(
        JNIEnv *env,
        jobject /* this */) {
    std::string hello = "Hello from C++ From Android openCVTest";
    return env->NewStringUTF(hello.c_str());
}
extern "C"
JNIEXPORT jint JNICALL
Java_com_example_bill_opencvtest_MainActivity_addFromCpp(JNIEnv *env, jobject instance, jint a,
                                                         jint b) {

    // TODO
    return add(a,b);
}

在app/CMakeLists.txt 加上這個自定義庫的引用

set(distribution_DIR ${CMAKE_SOURCE_DIR}/../distribution)
include_directories(${distribution_DIR}/include)
add_library(lib_add SHARED IMPORTED)
set_target_properties(lib_add PROPERTIES IMPORTED_LOCATION
                      ${distribution_DIR}/libs/${ANDROID_ABI}/libadd.so)
add_library( # Sets the name of the library.
             native-math

             # Sets the library as a shared library.
             SHARED

             # Provides a relative path to your source file(s).
             src/main/cpp/native-math.cpp )

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 )
set_target_properties(native-math PROPERTIES
                      LIBRARY_OUTPUT_DIRECTORY
                      ${distribution_DIR}/libs/${ANDROID_ABI})
target_link_libraries( # Specifies the target library.
                       native-math
                       android
                       log
                       lib_add
                       # Links the target library to the log library
                       # included in the NDK.
                       ${log-lib} )

在模塊app的局部build.gradle中，像之前一樣添加好對應的語句：

defaultConfig{}中：

externalNativeBuild {
            cmake {
                arguments '-DANDROID_PLATFORM=android-19',
                        '-DANDROID_TOOLCHAIN=clang', '-DANDROID_STL=gnustl_static'
            }
        }
        ndk {
            //abiFilters 'armeabi-v7a','x86_64'
        }

其中ndk指定abi平臺

ABI（Application binary interface）應用程式二進位介面。不同的CPU 與指令集的每種組合都有定義的 ABI (應用程式二進位介面)，一段程式只有遵循這個介面規範才能在該 CPU 上運行，所以同樣的程式代碼為了相容多個不同的CPU，需要為不同的 ABI 構建不同的庫文件。當然對於CPU來說，不同的架構並不意味著一定互不相容。

• armeabi設備只相容armeabi；
• armeabi-v7a設備相容armeabi-v7a、armeabi；
• arm64-v8a設備相容arm64-v8a、armeabi-v7a、armeabi；
• X86設備相容X86、armeabi；
• X86_64設備相容X86_64、X86、armeabi；
• mips64設備相容mips64、mips；
• mips只相容mips；

接著在src/main/java/*/MainActivity.java中的MainActivity類下麵，載入庫，以及設置好對應的方法聲明：

static {
        System.loadLibrary("native-math");
    }
    /**
     * A native method that is implemented by the 'native-lib' native library,
     * which is packaged with this application.
     */
    public native String stringFromJNI();

    public native int addFromCpp(int a, int b);

然後就可以在onCreate方法中使用這個C++庫定義的函數，在Java中對應的函數了

super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        // Example of a call to a native method
        TextView tv = (TextView) findViewById(R.id.sample_text);
        tv.setText(stringFromJNI() + "____" + addFromCpp(2,88));

最後別忘了在項目中添加模塊的依賴關係才可以正常運行這個Android App。右鍵項目OpenCVTest，選擇Open Module Settings。選擇app->Dependencies，添加Module dependency，選擇mathlib，確定即可

添加OpenCV庫的支持
導入OpenCV進項目

從OpenCV的官網將OpenCV4Android 3.4下載下來，解壓到某個目錄。
點擊Android Studio的File->New->Import Module，然後選擇路徑為OpenCV-android-sdk/sdk/java，確定。併在導入之後，修改build.gradle中的SDK版本。
在Open Module Settings中添加模塊的依賴關係，使app依賴openCVLibrary340。

現在已經可以在.java文件中看得到OpenCV的自動補全了。
配置OpenCV的C++預構建庫

把包含文件夾OpenCV-android-sdk/sdk/native/jni/include和預構建庫文件夾OpenCV-android-sdk/sdk/native/libs也複製到項目的distribution中。
由於之前已經在添加C++庫時修改了app的build.gradle，所以這個步驟現在不需要再執行了。

由於OpenCV是預構建庫，所以沒有編譯的過程，因此模塊openCVLibrary320中不需要添加CMakeLists.txt等。我們直接在app模塊中根目錄下的CMakeLists.txt導入OpenCV的庫即可。
set(libs "${CMAKE_SOURCE_DIR}/src/main/jniLibs")
include_directories(${distribution_DIR}/include)
# set add lib
add_library(libopencv_java3 SHARED IMPORTED )
set_target_properties(libopencv_java3 PROPERTIES
    IMPORTED_LOCATION "${libs}/${ANDROID_ABI}/libopencv_java3.so")
add_library( # Sets the name of the library.
             native-opencv

             # Sets the library as a shared library.
             SHARED

             # Provides a relative path to your source file(s).
             src/main/cpp/native-opencv.cpp )
set_target_properties(native-opencv PROPERTIES
                      LIBRARY_OUTPUT_DIRECTORY
                      ${distribution_DIR}/libs/${ANDROID_ABI})
target_link_libraries( # Specifies the target library.
                       native-opencv
                       android
                       log
                       libopencv_java3
                       # Links the target library to the log library
                       # included in the NDK.
                       ${log-lib} )
需要註意的是.so使用SHARED，.a使用STATIC。
註意：預構建庫：so文件和.a文件必須copy在src/main/jniLibs這個目錄，才可以自動被打包。其他路徑都不可以，連source這個命令也不起作用
現在可以使用openCV庫了，新建一個文件native-opencv.cpp
//
// Created by bill on 2018/1/13.
//
#include <jni.h>
#include <opencv2/opencv.hpp>
#include <vector>

using namespace cv;
using namespace std;

extern "C"
JNIEXPORT void JNICALL
Java_com_example_bill_opencvtest_MainActivity_nativeProcessFrame(JNIEnv *env, jobject instance,
        jlong addrGray, jlong addrRGBA) {
// TODO
    Mat& gray = *(Mat *) addrGray;
    Mat& rgba = *(Mat *) addrRGBA;
    vector<KeyPoint> v;

    Ptr<ORB> orb = ORB::create();
    orb->detect(gray, v, cv::Mat());

    for (int i = 0; i < v.size(); ++i) {
        const KeyPoint& kp = v[i];
        circle(rgba, Point(kp.pt.x, kp.pt.y), 10, Scalar(255,0,0,255));
    }
}
現在就可以在src/main/java/*/MainActivity.java中按照同樣的方法，載入庫，寫上方法聲明。最後，如下所示。
static {
        System.loadLibrary("native-opencv");
        System.loadLibrary("native-math");
    }
    /**
     * A native method that is implemented by the 'native-lib' native library,
     * which is packaged with this application.
     */
    public native String stringFromJNI();

    public native int addFromCpp(int a, int b);
    private native void nativeProcessFrame(long addrGray, long addrRGBA);
完整的MainActivity
package com.example.bill.opencvtest;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.view.WindowManager;
import android.widget.TextView;

import org.opencv.android.CameraBridgeViewBase;
import org.opencv.android.OpenCVLoader;
import org.opencv.core.CvType;
import org.opencv.core.Mat;

public class MainActivity extends Activity implements CameraBridgeViewBase.CvCameraViewListener2{
    static {
        System.loadLibrary("native-opencv");
        System.loadLibrary("native-math");
    }
    /**
     * A native method that is implemented by the 'native-lib' native library,
     * which is packaged with this application.
     */
    public native String stringFromJNI();

    public native int addFromCpp(int a, int b);
    private native void nativeProcessFrame(long addrGray, long addrRGBA);

    private static final String TAG = "MainActivity";

    private Mat rgba;
    private Mat gray;
    private CameraBridgeViewBase mOpenCvCameraView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        // Example of a call to a native method
        TextView tv = (TextView) findViewById(R.id.sample_text);
        tv.setText(stringFromJNI() + "____" + addFromCpp(2,88));
        getWindow().addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);

        mOpenCvCameraView = (CameraBridgeViewBase) findViewById(R.id.activity_camera_view);
        mOpenCvCameraView.setVisibility(CameraBridgeViewBase.VISIBLE);
        mOpenCvCameraView.setCvCameraViewListener(this);
    }

    @Override
    public void onPause() {
        super.onPause();
        if (mOpenCvCameraView != null){
            mOpenCvCameraView.disableView();
        }
    }

    @Override
    public void onResume()
    {
        super.onResume();
        if (!OpenCVLoader.initDebug()) {
            Log.d(TAG, "Internal OpenCV library not found. Using OpenCV Manager for initialization");
        } else {
            Log.d(TAG, "OpenCV library found inside package. Using it!");
            mOpenCvCameraView.enableView();
        }
    }

    public void onDestroy() {
        super.onDestroy();
        if (mOpenCvCameraView != null){
            mOpenCvCameraView.disableView();
        }
    }

    public void onCameraViewStarted(int width, int height){
        rgba = new Mat(height, width, CvType.CV_8UC4);
        gray = new Mat(height, width, CvType.CV_8UC1);
    }

    public void onCameraViewStopped() {
        rgba.release();
        gray.release();
    }

    public Mat onCameraFrame(CameraBridgeViewBase.CvCameraViewFrame inputFrame){
        rgba = inputFrame.rgba();
        gray = inputFrame.gray();
        nativeProcessFrame(gray.getNativeObjAddr(), rgba.getNativeObjAddr());
        return rgba;
    }
}
activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:opencv="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.bill.opencvtest.MainActivity">

    <TextView
        android:id="@+id/sample_text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
    <org.opencv.android.JavaCameraView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/activity_camera_view"
        opencv:show_fps="true"
        opencv:camera_id="any"/>
</android.support.constraint.ConstraintLayout>
為了愉快的使用OpenCV Library，可以直接在AndroidManifest.xml裡面加入如下許可權
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.bill.opencvtest">

    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.CAMERA"/>
    <uses-feature android:name="android.hardware.camera"/>
    <uses-feature android:name="android.hardware.camera.autofocus"/>
    <uses-feature android:name="android.hardware.camera.front"/>
    <uses-feature android:name="android.hardware.camera.front.autofocus"/>

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity"
            android:screenOrientation="landscape"
            android:configChanges="keyboardHidden|orientation">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>