如果你对本系列感兴趣的话，可以看看上一篇，主要说的是如何在NDK使用多线程，还有就是基础的java与c++的相互调用。

最近看了第二个官方示例，主要是说的不使用java代码，用c++写一个activity。

整个项目down下来，结构如下。

运行一下，主要的效果是点击一下，屏幕整个开始进行颜色变化，注意点击屏幕的不同位置，颜色的变化不同，效果如下：

这个项目中只有在cpp文件夹下面有一个main.cpp代码。但是我们首先从AndroidManifest.xml入手。

<?xml version="1.0" encoding="utf-8"?>
<!-- BEGIN_INCLUDE(manifest) -->
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
          package="com.example.native_activity"
          android:versionCode="1"
          android:versionName="1.0">

  <!-- This .apk has no Java code itself, so set hasCode to false. -->
  <application
      android:allowBackup="false"
      android:fullBackupContent="false"
      android:icon="@mipmap/ic_launcher"
      android:label="@string/app_name"
      android:hasCode="false">

    <!-- Our activity is the built-in NativeActivity framework class.
         This will take care of integrating with our NDK code. -->
    <activity android:name="android.app.NativeActivity"
              android:label="@string/app_name"
              android:configChanges="orientation|keyboardHidden">
      <!-- Tell NativeActivity the name of our .so -->
      <meta-data android:name="android.app.lib_name"
                 android:value="native-activity" />
      <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
      </intent-filter>
    </activity>
  </application>

</manifest>
<!-- END_INCLUDE(manifest) -->

我们看到，这里注册了一个activity，代码在android.app.NativeActivity，它还有一个参数<meta-data android:name="android.app.lib_name"android:value="native-activity" />，根据这里的注释，我们的Activity是内建在NativeActivity中的，运行我们的NDK代码。所以android:value="native-activity"这个的值就是NDK代码的库文件的名字。我们去NativeActivity看看。

在NativeActivity.java的onCreate方法中我注意到以下代码：

        String libname = "main";
        String funcname = "ANativeActivity_onCreate";
···

try {
            ai = getPackageManager().getActivityInfo(
                    getIntent().getComponent(), PackageManager.GET_META_DATA);
            if (ai.metaData != null) {
                String ln = ai.metaData.getString(META_DATA_LIB_NAME);
                if (ln != null) libname = ln;
                ln = ai.metaData.getString(META_DATA_FUNC_NAME);
                if (ln != null) funcname = ln;
            }
        } catch (PackageManager.NameNotFoundException e) {
            throw new RuntimeException("Error getting activity info", e);
        }

        BaseDexClassLoader classLoader = (BaseDexClassLoader) getClassLoader();
        String path = classLoader.findLibrary(libname);

        if (path == null) {
            throw new IllegalArgumentException("Unable to find native library " + libname +
                                               " using classloader: " + classLoader.toString());
        }
        
        byte[] nativeSavedState = savedInstanceState != null
                ? savedInstanceState.getByteArray(KEY_NATIVE_SAVED_STATE) : null;

        mNativeHandle = loadNativeCode(path, funcname, Looper.myQueue(),
                getAbsolutePath(getFilesDir()), getAbsolutePath(getObbDir()),
                getAbsolutePath(getExternalFilesDir(null)),
                Build.VERSION.SDK_INT, getAssets(), nativeSavedState,
                classLoader, classLoader.getLdLibraryPath());

这个的代码并没有使用META_DATA_FUNC_NAME = "android.app.func_name"参数，只使用了String META_DATA_LIB_NAME = "android.app.lib_name"参数。所以classLoader.findLibrary(libname);中的libname就是native-activity，而funcname的值并没有改变仍是 "ANativeActivity_onCreate"。

我在CMakeLists.txt中发现了add_library(native-activity SHARED main.cpp)，这刚好符合上面的逻辑。同时CMakeLists.txt中还有两行代码值得注意target_include_directories(native-activity PRIVATE ${ANDROID_NDK}/sources/android/native_app_glue),add_library(native_app_glue STATIC ${ANDROID_NDK}/sources/android/native_app_glue/android_native_app_glue.c)说明我们还用到了android_native_app_glue.c，但是暂时还不知道哪里用到了，我们继续往下看。

最后调用的loadNativeCode方法，把之前取到的参数传了进去。我们看看loadNativeCode方法的实现，这个是个NDK方法，代码在系统源码中\frameworks\base\core\jni\android_app_NativeActivity.cpp

static jlong
loadNativeCode_native(JNIEnv* env, jobject clazz, jstring path, jstring funcName,
        jobject messageQueue, jstring internalDataDir, jstring obbDir,
        jstring externalDataDir, jint sdkVersion,
        jobject jAssetMgr, jbyteArray savedState)
{
    if (kLogTrace) {
        ALOGD("loadNativeCode_native");
    }

    const char* pathStr = env->GetStringUTFChars(path, NULL);
    NativeCode* code = NULL;
    bool needNativeBridge = false;

    void* handle = dlopen(pathStr, RTLD_LAZY);
    if (handle == NULL) {
        if (NativeBridgeIsSupported(pathStr)) {
            handle = NativeBridgeLoadLibrary(pathStr, RTLD_LAZY);
            needNativeBridge = true;
        }
    }
    env->ReleaseStringUTFChars(path, pathStr);

    if (handle != NULL) {
        void* funcPtr = NULL;
        const char* funcStr = env->GetStringUTFChars(funcName, NULL);
        if (needNativeBridge) {
            funcPtr = NativeBridgeGetTrampoline(handle, funcStr, NULL, 0);
        } else {
            funcPtr = dlsym(handle, funcStr);
        }

···
    
    return (jlong)code;
}

首先，代码中根据传入的参数path，加载了lib，然后在lib中查找方法，方法名自然就是我们传入的ANativeActivity_onCreate。这个方法就在android_native_app_glue.c中。

JNIEXPORT
void ANativeActivity_onCreate(ANativeActivity* activity, void* savedState,
                              size_t savedStateSize) {
    LOGV("Creating: %p\n", activity);
    activity->callbacks->onDestroy = onDestroy;
    activity->callbacks->onStart = onStart;
    activity->callbacks->onResume = onResume;
    activity->callbacks->onSaveInstanceState = onSaveInstanceState;
    activity->callbacks->onPause = onPause;
    activity->callbacks->onStop = onStop;
    activity->callbacks->onConfigurationChanged = onConfigurationChanged;
    activity->callbacks->onLowMemory = onLowMemory;
    activity->callbacks->onWindowFocusChanged = onWindowFocusChanged;
    activity->callbacks->onNativeWindowCreated = onNativeWindowCreated;
    activity->callbacks->onNativeWindowDestroyed = onNativeWindowDestroyed;
    activity->callbacks->onInputQueueCreated = onInputQueueCreated;
    activity->callbacks->onInputQueueDestroyed = onInputQueueDestroyed;

    activity->instance = android_app_create(activity, savedState, savedStateSize);
}

在这个方法中绑定一些回调，然后调用了android_app_create方法。再看看android_app_create方法。

static struct android_app* android_app_create(ANativeActivity* activity,
        void* savedState, size_t savedStateSize) {
    struct android_app* android_app = (struct android_app*)malloc(sizeof(struct android_app));
    memset(android_app, 0, sizeof(struct android_app));
    android_app->activity = activity;

    pthread_mutex_init(&android_app->mutex, NULL);
    pthread_cond_init(&android_app->cond, NULL);

    if (savedState != NULL) {
        android_app->savedState = malloc(savedStateSize);
        android_app->savedStateSize = savedStateSize;
        memcpy(android_app->savedState, savedState, savedStateSize);
    }

    int msgpipe[2];
    if (pipe(msgpipe)) {
        LOGE("could not create pipe: %s", strerror(errno));
        return NULL;
    }
    android_app->msgread = msgpipe[0];
    android_app->msgwrite = msgpipe[1];

    pthread_attr_t attr; 
    pthread_attr_init(&attr);
    pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
    pthread_create(&android_app->thread, &attr, android_app_entry, android_app);

    // Wait for thread to start.
    pthread_mutex_lock(&android_app->mutex);
    while (!android_app->running) {
        pthread_cond_wait(&android_app->cond, &android_app->mutex);
    }
    pthread_mutex_unlock(&android_app->mutex);

    return android_app;
}

这里起了一个线程，在线程上执行android_app_entry方法。

pthread_attr_t attr; 
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
pthread_create(&android_app->thread, &attr, android_app_entry, android_app);

看看android_app_entry方法：

static void* android_app_entry(void* param) {
    struct android_app* android_app = (struct android_app*)param;

    android_app->config = AConfiguration_new();
    AConfiguration_fromAssetManager(android_app->config, android_app->activity->assetManager);

    print_cur_config(android_app);

    android_app->cmdPollSource.id = LOOPER_ID_MAIN;
    android_app->cmdPollSource.app = android_app;
    android_app->cmdPollSource.process = process_cmd;
    android_app->inputPollSource.id = LOOPER_ID_INPUT;
    android_app->inputPollSource.app = android_app;
    android_app->inputPollSource.process = process_input;

    ALooper* looper = ALooper_prepare(ALOOPER_PREPARE_ALLOW_NON_CALLBACKS);
    ALooper_addFd(looper, android_app->msgread, LOOPER_ID_MAIN, ALOOPER_EVENT_INPUT, NULL,
            &android_app->cmdPollSource);
    android_app->looper = looper;

    pthread_mutex_lock(&android_app->mutex);
    android_app->running = 1;
    pthread_cond_broadcast(&android_app->cond);
    pthread_mutex_unlock(&android_app->mutex);

    android_main(android_app);

    android_app_destroy(android_app);
    return NULL;
}

关键是后面android_main(android_app);这里就是调用了main.cpp中的void android_main(struct android_app* state)，到此正式进入了咱们的项目代码。

http://blog.csdn.net/lusing/article/details/59079114

进入main.cpp看看：

/**
 * This is the main entry point of a native application that is using
 * android_native_app_glue.  It runs in its own thread, with its own
 * event loop for receiving input events and doing other things.
 */
void android_main(struct android_app* state) {
    struct engine engine;

    memset(&engine, 0, sizeof(engine));
    state->userData = &engine;
    state->onAppCmd = engine_handle_cmd; //描述的是整个activity的生命周期
    state->onInputEvent = engine_handle_input;
    engine.app = state;

    // Prepare to monitor accelerometer
    engine.sensorManager = AcquireASensorManagerInstance(state);
    engine.accelerometerSensor = ASensorManager_getDefaultSensor(
                                        engine.sensorManager,
                                        ASENSOR_TYPE_ACCELEROMETER);
    engine.sensorEventQueue = ASensorManager_createEventQueue(
                                    engine.sensorManager,
                                    state->looper, LOOPER_ID_USER,
                                    NULL, NULL);

    if (state->savedState != NULL) {
        // We are starting with a previous saved state; restore from it.
        engine.state = *(struct saved_state*)state->savedState;
    }

    // loop waiting for stuff to do.
    
    while (1) {
        // Read all pending events.
        int ident;
        int events;
        struct android_poll_source* source;

        // If not animating, we will block forever waiting for events.
        // If animating, we loop until all events are read, then continue
        // to draw the next frame of animation.
        while ((ident=ALooper_pollAll(engine.animating ? 0 : -1, NULL, &events,
                                      (void**)&source)) >= 0) {

            // Process this event.
            if (source != NULL) {
                source->process(state, source);
            }

            // 输出加速度传感器的数据
            if (ident == LOOPER_ID_USER) {
                if (engine.accelerometerSensor != NULL) {
                    ASensorEvent event;
                    while (ASensorEventQueue_getEvents(engine.sensorEventQueue,
                                                       &event, 1) > 0) {
                        LOGI("accelerometer: x=%f y=%f z=%f",
                             event.acceleration.x, event.acceleration.y,
                             event.acceleration.z);
                    }
                }
            }

            // Check if we are exiting.
            if (state->destroyRequested != 0) {
                engine_term_display(&engine);
                return;
            }
        }
        //颜色渐变，输出到屏幕
        if (engine.animating) {
            // Done with events; draw next animation frame.
            engine.state.angle += .01f;
            if (engine.state.angle > 1) {
                engine.state.angle = 0;
            }

            // Drawing is throttled to the screen update rate, so there
            // is no need to do timing here.
            engine_draw_frame(&engine);
        }
    }
}

代码自带的注释说明了，这里是程序的进入点，运行在自己的线程，有自己队列来处理输入事件。

首先是指定了处理生命周期的方法engine_handle_cmd，处理输入的方法engine_handle_input。然后输准备监视加速度传感器。代码如下，逐行加了注释。

#include <dlfcn.h>
ASensorManager* AcquireASensorManagerInstance(android_app* app) {

  if(!app)
    return nullptr;

  //定义了一个函数指针类型，（*PF_GETINSTANCEFORPACKAGE）是类型，(const char *name)是参数，ASensorManager是返回值
  typedef ASensorManager *(*PF_GETINSTANCEFORPACKAGE)(const char *name);
  void* androidHandle = dlopen("libandroid.so", RTLD_NOW); //加载动态链接库，返回一个句柄
  PF_GETINSTANCEFORPACKAGE getInstanceForPackageFunc = (PF_GETINSTANCEFORPACKAGE)
      dlsym(androidHandle, "ASensorManager_getInstanceForPackage");//用上面的句柄，获取函数
  if (getInstanceForPackageFunc) {
    JNIEnv* env = nullptr;
    app->activity->vm->AttachCurrentThread(&env, NULL);//初始化env

    jclass android_content_Context = env->GetObjectClass(app->activity->clazz);
    jmethodID midGetPackageName = env->GetMethodID(android_content_Context,
                                                   "getPackageName",
                                                   "()Ljava/lang/String;");
    jstring packageName= (jstring)env->CallObjectMethod(app->activity->clazz,
                                                        midGetPackageName);//取到包名

    const char *nativePackageName = env->GetStringUTFChars(packageName, 0);
    ASensorManager* mgr = getInstanceForPackageFunc(nativePackageName);//拿到了ASensorManager
    env->ReleaseStringUTFChars(packageName, nativePackageName);//回收资源
    app->activity->vm->DetachCurrentThread();//解绑线程
    if (mgr) {//如果都没问题了
      dlclose(androidHandle);//关闭动态库连接
      return mgr;
    }
  }

  typedef ASensorManager *(*PF_GETINSTANCE)();
  PF_GETINSTANCE getInstanceFunc = (PF_GETINSTANCE)
      dlsym(androidHandle, "ASensorManager_getInstance");
  // by all means at this point, ASensorManager_getInstance should be available
  assert(getInstanceFunc);
  dlclose(androidHandle);

  return getInstanceFunc();
}

接着，就是把加速度传感器的数据输出到日志。

然后是实现屏幕的的颜色变化效果：

/**
 * Just the current frame in the display.
 */
static void engine_draw_frame(struct engine* engine) {
    if (engine->display == NULL) {
        // No display.
        return;
    }

    // Just fill the screen with a color.
    glClearColor(((float)engine->state.x)/engine->width, engine->state.angle,
                 ((float)engine->state.y)/engine->height, 1);
    glClear(GL_COLOR_BUFFER_BIT);

    eglSwapBuffers(engine->display, engine->surface);
}

glClearColor的参数是RGB和透明度，G是state.angle，而state.angle是在android_main方法中循环变换的，R和B与state.x和state.y有关，而state.x和state.y是在屏幕触摸处理方法中（engine_handle_input）被赋值的。

static int32_t engine_handle_input(struct android_app* app, AInputEvent* event) {
    struct engine* engine = (struct engine*)app->userData;
    if (AInputEvent_getType(event) == AINPUT_EVENT_TYPE_MOTION) {
        engine->animating = 1;
        engine->state.x = AMotionEvent_getX(event, 0);
        engine->state.y = AMotionEvent_getY(event, 0);
        return 1;
    }
    return 0;
}

到此就实现的全部功能。