如果你對本系列感興趣的話，可以看看上一篇，主要說的是如何在NDK使用多執行緒，還有就是基礎的java與c++的相互呼叫。
最近看了第二個官方示例，主要是說的不使用java程式碼，用c++寫一個activity。
整個專案down下來，結構如下。
<?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) -->
複製程式碼我們看到，這裡註冊了一個activity，程式碼在android.app.NativeActivity，它還有一個引數<meta-data android:name="android.app.lib_name"android:value="native-activity" />，根據這裡的註釋，我們的Activity是內建在NativeActivity中的，執行我們的NDK程式碼。所以android:value="native-activity"這個的值就是NDK程式碼的庫檔案的名字。我們去NativeActivity看看。
在NativeActivity.java的onCreate方法中我注意到以下程式碼：
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());
複製程式碼這個的程式碼並沒有使用META_DATA_FUNC_NAME = "android.app.func_name"引數，只使用了String META_DATA_LIB_NAME = "android.app.lib_name"引數。所以classLoader.findLibrary(libname);中的libname就是native-activity，而funcname的值並沒有改變仍是 "ANativeActivity_onCreate"。
我在CMakeLists.txt中發現了add_library(native-activity SHARED main.cpp)，這剛好符合上面的邏輯。同時CMakeLists.txt中還有兩行程式碼值得注意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)說明我們還用到了android_native_app_glue.c，但是暫時還不知道哪裡用到了，我們繼續往下看。
最後呼叫的loadNativeCode方法，把之前取到的引數傳了進去。我們看看loadNativeCode方法的實現，這個是個NDK方法，程式碼在系統原始碼中\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;
}
複製程式碼首先，程式碼中根據傳入的引數path，載入了lib，然後在lib中查詢方法，方法名自然就是我們傳入的ANativeActivity_onCreate。這個方法就在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);
}
複製程式碼在這個方法中繫結一些回撥，然後呼叫了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;
}
複製程式碼這裡起了一個執行緒，線上程上執行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);
複製程式碼看看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;
}
複製程式碼關鍵是後面android_main(android_app);這裡就是呼叫了main.cpp中的void android_main(struct android_app* state)，到此正式進入了我們們的專案程式碼。
/**
* 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; //描述的是整個activity的生命週期
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);
}
// 輸出加速度感測器的資料
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;
}
}
//顏色漸變，輸出到螢幕
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);
}
}
}
複製程式碼程式碼自帶的註釋說明了，這裡是程式的進入點，執行在自己的執行緒，有自己佇列來處理輸入事件。
首先是指定了處理生命週期的方法engine_handle_cmd，處理輸入的方法engine_handle_input。然後輸準備監視加速度感測器。程式碼如下，逐行加了註釋。
#include <dlfcn.h>
ASensorManager* AcquireASensorManagerInstance(android_app* app) {
if(!app)
return nullptr;
//定義了一個函式指標型別，（*PF_GETINSTANCEFORPACKAGE）是型別，(const char *name)是引數，ASensorManager是返回值
typedef ASensorManager *(*PF_GETINSTANCEFORPACKAGE)(const char *name);
void* androidHandle = dlopen("libandroid.so", RTLD_NOW); //載入動態連結庫，返回一個控制程式碼
PF_GETINSTANCEFORPACKAGE getInstanceForPackageFunc = (PF_GETINSTANCEFORPACKAGE)
dlsym(androidHandle, "ASensorManager_getInstanceForPackage");//用上面的控制程式碼，獲取函式
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);//回收資源
app->activity->vm->DetachCurrentThread();//解綁執行緒
if (mgr) {//如果都沒問題了
dlclose(androidHandle);//關閉動態庫連線
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();
}
複製程式碼接著，就是把加速度感測器的資料輸出到日誌。
然後是實現螢幕的的顏色變化效果：
/**
* 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);
}
複製程式碼glClearColor的引數是RGB和透明度，G是state.angle，而state.angle是在android_main方法中迴圈變換的，R和B與state.x和state.y有關，而state.x和state.y是在螢幕觸控處理方法中（engine_handle_input）被賦值的。
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;
}
複製程式碼到此就實現的全部功能。