如果你看遍了網上那些只是在C++裡面輸出一個 ‘ helloWorld ’ 的NDK教程的話，可以看看本系列的文章，本系列是通過NDK的運用的例子來學習NDK。
如果對這方面感興趣，可以看看前三篇。
Android鬼點子-通過Google官方示例學NDK（1）——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。
Android鬼點子-通過Google官方示例學NDK（2）——主要是說的不使用java程式碼，用c++寫一個activity。
Android鬼點子-通過Google官方示例學NDK（3）——這是一個opengl的例子。
第四個例子是展示的視訊解碼相關的內容，外加opengl部分內容。
程式碼在這裡。
主要的功能是播放在assets資料夾中的一段視訊。視訊可以暫停，繼續播放。
這個例子有個很有價值的點就是有一個c++實現的訊息佇列，也就是Looper。我讀過之後，收穫很多。
專案的結構如下：
先看Activity,NativeCodec.java中:
public void onCreate(Bundle icicle) {
super.onCreate(icicle);
setContentView(R.layout.main);
mGLView1 = (MyGLSurfaceView) findViewById(R.id.glsurfaceview1);
// set up the Surface 1 video sink
mSurfaceView1 = (SurfaceView) findViewById(R.id.surfaceview1);
mSurfaceHolder1 = mSurfaceView1.getHolder();
mSurfaceHolder1.addCallback(new SurfaceHolder.Callback() {
@Override
public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {
Log.v(TAG, "surfaceChanged format=" + format + ", width=" + width + ", height="
+ height);
}
@Override
public void surfaceCreated(SurfaceHolder holder) {
Log.v(TAG, "surfaceCreated");
if (mRadio1.isChecked()) {
setSurface(holder.getSurface());
}
}
@Override
public void surfaceDestroyed(SurfaceHolder holder) {
Log.v(TAG, "surfaceDestroyed");
}
});
// initialize content source spinner
Spinner sourceSpinner = (Spinner) findViewById(R.id.source_spinner);
ArrayAdapter<CharSequence> sourceAdapter = ArrayAdapter.createFromResource(
this, R.array.source_array, android.R.layout.simple_spinner_item);
sourceAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
sourceSpinner.setAdapter(sourceAdapter);
sourceSpinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
@Override
public void onItemSelected(AdapterView<?> parent, View view, int pos, long id) {
mSourceString = parent.getItemAtPosition(pos).toString();
Log.v(TAG, "onItemSelected " + mSourceString);
}
@Override
public void onNothingSelected(AdapterView parent) {
Log.v(TAG, "onNothingSelected");
mSourceString = null;
}
});
mRadio1 = (RadioButton) findViewById(R.id.radio1);
mRadio2 = (RadioButton) findViewById(R.id.radio2);
OnCheckedChangeListener checklistener = new CompoundButton.OnCheckedChangeListener() {
@Override
public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
···
if (isChecked) {
if (mRadio1.isChecked()) {
if (mSurfaceHolder1VideoSink == null) {
mSurfaceHolder1VideoSink = new SurfaceHolderVideoSink(mSurfaceHolder1);
}
mSelectedVideoSink = mSurfaceHolder1VideoSink;
mGLView1.onPause();
Log.i("@@@@", "glview pause");
} else {
mGLView1.onResume();
if (mGLView1VideoSink == null) {
mGLView1VideoSink = new GLViewVideoSink(mGLView1);
}
mSelectedVideoSink = mGLView1VideoSink;
}
switchSurface();
}
}
};
···
// native MediaPlayer start/pause
((Button) findViewById(R.id.start_native)).setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
if (!mCreated) {
if (mNativeCodecPlayerVideoSink == null) {
if (mSelectedVideoSink == null) {
return;
}
mSelectedVideoSink.useAsSinkForNative();
mNativeCodecPlayerVideoSink = mSelectedVideoSink;
}
if (mSourceString != null) {
mCreated = createStreamingMediaPlayer(getResources().getAssets(),
mSourceString);
}
}
if (mCreated) {
mIsPlaying = !mIsPlaying;
setPlayingStreamingMediaPlayer(mIsPlaying);
}
}
});
// native MediaPlayer rewind
((Button) findViewById(R.id.rewind_native)).setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
if (mNativeCodecPlayerVideoSink != null) {
rewindStreamingMediaPlayer();
}
}
});
}
複製程式碼這裡主要是一些控制元件的時間處理。比如暫停播放，讀取檔案，切換播放的SurfaceView，從頭播放等。
暫停（繼續）播放：setPlayingStreamingMediaPlayer(mIsPlaying);
讀取檔案：mCreated = createStreamingMediaPlayer(getResources().getAssets(),mSourceString);
切換播放的SurfaceView:當前播放的SurfaceView儲存在mNativeCodecPlayerVideoSink中，通過useAsSinkForNative()中呼叫setSurface(s)方法傳入到c++程式碼中。
從頭播放：rewindStreamingMediaPlayer()
退出：shutdown()
所以一共有下面5個jni方法：
public static native boolean createStreamingMediaPlayer(AssetManager assetMgr, String filename);
public static native void setPlayingStreamingMediaPlayer(boolean isPlaying);
public static native void shutdown();
public static native void setSurface(Surface surface);
public static native void rewindStreamingMediaPlayer();
複製程式碼上面的5個方法的實現都在native-codec-jni.cpp中，先看看讀取檔案的方法：
typedef struct {
int fd;
ANativeWindow* window;
AMediaExtractor* ex;
AMediaCodec *codec;
int64_t renderstart;
bool sawInputEOS;
bool sawOutputEOS;
bool isPlaying;
bool renderonce;
} workerdata;
workerdata data = {-1, NULL, NULL, NULL, 0, false, false, false, false};
jboolean Java_com_example_nativecodec_NativeCodec_createStreamingMediaPlayer(JNIEnv* env,
jclass clazz, jobject assetMgr, jstring filename)
{
LOGV("@@@ create");
// convert Java string to UTF-8
const char *utf8 = env->GetStringUTFChars(filename, NULL);
LOGV("opening %s", utf8);
off_t outStart, outLen;
int fd = AAsset_openFileDescriptor(AAssetManager_open(AAssetManager_fromJava(env, assetMgr), utf8, 0),
&outStart, &outLen);//開啟檔案
env->ReleaseStringUTFChars(filename, utf8);
if (fd < 0) {
LOGE("failed to open file: %s %d (%s)", utf8, fd, strerror(errno));
return JNI_FALSE;
}
data.fd = fd;
workerdata *d = &data;
AMediaExtractor *ex = AMediaExtractor_new();//負責將指定型別的媒體檔案從檔案中找到軌道，並填充到MediaCodec的緩衝區中
media_status_t err = AMediaExtractor_setDataSourceFd(ex, d->fd,
static_cast<off64_t>(outStart),
static_cast<off64_t>(outLen));
close(d->fd);
if (err != AMEDIA_OK) {
LOGV("setDataSource error: %d", err);
return JNI_FALSE;
}
int numtracks = AMediaExtractor_getTrackCount(ex);//獲取軌道數
AMediaCodec *codec = NULL;//負責媒體檔案的編碼和解碼工作
//log:input has 2 tracks
LOGV("input has %d tracks", numtracks);
for (int i = 0; i < numtracks; i++) {
AMediaFormat *format = AMediaExtractor_getTrackFormat(ex, i);
const char *s = AMediaFormat_toString(format);
//track 0 format: mime: string(video/avc), durationUs: int64(10000000), width: int32(480), height: int32(360), max-input-size: int32(55147), csd-0: data, csd-1: data}
//track 1 format: mime: string(audio/mp4a-latm), durationUs: int64(9914920), channel-count: int32(2), sample-rate: int32(44100), aac-profile: int32(2), bit-width: int32(16), pcm-type: int32(1), max-input-size: int32(694), csd-0: data}
LOGV("track %d format: %s", i, s);
const char *mime;
if (!AMediaFormat_getString(format, AMEDIAFORMAT_KEY_MIME, &mime)) {//在format中取出mime欄位
LOGV("no mime type");
return JNI_FALSE;
} else if (!strncmp(mime, "video/", 6)) {//獲取視訊所在軌道 注：strncmp相同返回0，比較6位 if:非0為真
// Omitting most error handling for clarity.
// Production code should check for errors.
AMediaExtractor_selectTrack(ex, i);//選中軌道
codec = AMediaCodec_createDecoderByType(mime);//通過mime建立解碼器
//配置解碼器
AMediaCodec_configure(codec, format, d->window, NULL, 0);
d->ex = ex;
d->codec = codec;
d->renderstart = -1;
d->sawInputEOS = false;
d->sawOutputEOS = false;
d->isPlaying = false;
d->renderonce = true;
//開始解碼
AMediaCodec_start(codec);
}
AMediaFormat_delete(format);
}
mlooper = new mylooper();
mlooper->post(kMsgCodecBuffer, d);
return JNI_TRUE;
}
複製程式碼在這個方法中，先找到了視訊的軌道，然後配置瞭解碼器，並把d->window與解碼器繫結。d->window是在setSurface方法中傳入的。
// set the surface
void Java_com_example_nativecodec_NativeCodec_setSurface(JNIEnv *env, jclass clazz, jobject surface)
{
// obtain a native window from a Java surface
if (data.window) {
ANativeWindow_release(data.window);
data.window = NULL;
}
data.window = ANativeWindow_fromSurface(env, surface);
LOGV("@@@ setsurface %p", data.window);
}
複製程式碼workerdata *d = &data中儲存了當前播放用到的一些標誌位。d->ex = ex視訊軌道，d->codec = codec解碼器，d->renderstart = -1渲染開始時間。
方法的最後mlooper = new mylooper(); mlooper->post(kMsgCodecBuffer, d);new了一個looper然後傳送了一個事件。例子中無論是暫停播放，開始播放，退出播放，還是每一幀的播放，都是通過mlooper->post(msg)的方法進行的，那麼看看這個looper是如何實現。
程式碼looper.cpp，關鍵部分加了註釋:
struct loopermessage;
typedef struct loopermessage loopermessage;
struct loopermessage {
int what;
void *obj;
loopermessage *next;
bool quit;
};
void* looper::trampoline(void* p) {
((looper*)p)->loop();
return NULL;
}
looper::looper() {
sem_init(&headdataavailable, 0, 0);//為0時此訊號量在程式間共享,訊號量的初始值
sem_init(&headwriteprotect, 0, 1);
pthread_attr_t attr;
pthread_attr_init(&attr);//執行緒預設配置
//起一個執行緒，執行緒上執行trampoline，引數是this，其實就是呼叫了loop()
pthread_create(&worker, &attr, trampoline, this);
running = true;
}
looper::~looper() {
if (running) {
LOGV("Looper deleted while still running. Some messages will not be processed");
quit();
}
}
//進入佇列
void looper::post(int what, void *data, bool flush) {
//組裝訊息
loopermessage *msg = new loopermessage();
msg->what = what;
msg->obj = data;
msg->next = NULL;
msg->quit = false;
addmsg(msg, flush);
}
void looper::addmsg(loopermessage *msg, bool flush) {
sem_wait(&headwriteprotect);//等待寫入訊息
loopermessage *h = head;
if (flush) {
//如果flush的話，先清空佇列
while(h) {
loopermessage *next = h->next;
delete h;
h = next;
}
h = NULL;
}
if (h) {
//如果佇列裡面有訊息
//指標移動到隊尾
while (h->next) {
h = h->next;
}
//在隊尾插入訊息
h->next = msg;
} else {
//如果佇列裡面沒有訊息，直接入隊
head = msg;
}
LOGV("post msg %d", msg->what);
sem_post(&headwriteprotect);
sem_post(&headdataavailable);
}
void looper::loop() {
while(true) {
// wait for available message
//阻塞當前執行緒直到訊號量headdataavailable的值大於0，解除阻塞後將headdataavailable的值-1
sem_wait(&headdataavailable);
// get next available message
sem_wait(&headwriteprotect);
loopermessage *msg = head;//有訊息了，取出第一個訊息
if (msg == NULL) {
LOGV("no msg");
sem_post(&headwriteprotect);//+1 可以寫入訊息了
continue;
}
head = msg->next;//把頭部指標指到下一條訊息
sem_post(&headwriteprotect);//+1 可以寫入訊息了
if (msg->quit) {//如果是退出的訊息
LOGV("quitting");
delete msg;
return;
}
LOGV("processing msg %d", msg->what);
handle(msg->what, msg->obj);//處理訊息
delete msg;
}
}
void looper::quit() {
LOGV("quit");
loopermessage *msg = new loopermessage();
msg->what = 0;
msg->obj = NULL;
msg->next = NULL;
msg->quit = true;//傳送一個退出的訊息
addmsg(msg, false);
void *retval;
pthread_join(worker, &retval);
sem_destroy(&headdataavailable); //釋放訊號量
sem_destroy(&headwriteprotect);
running = false;
}
void looper::handle(int what, void* obj) {
LOGV("dropping msg %d %p", what, obj);
}
複製程式碼大致的流程是起了一個單獨執行緒，然後在上面監視訊息佇列，有訊息入列void looper::addmsg(loopermessage *msg, bool flush)和讀取looper::loop()的操作。loop()中有一個while,一直監聽著佇列。取到了訊息，就呼叫looper::handle(int what, void* obj)進行處理。程式退出的時候呼叫looper::quit()結束讀取操作。
回到native-codec-jni.cpp中，這裡重寫了訊息的處理方法：
class mylooper: public looper {
virtual void handle(int what, void* obj);
};
static mylooper *mlooper = NULL;
void mylooper::handle(int what, void* obj) {
switch (what) {
case kMsgCodecBuffer:
doCodecWork((workerdata*)obj);
break;
case kMsgDecodeDone:
{
workerdata *d = (workerdata*)obj;
AMediaCodec_stop(d->codec);
AMediaCodec_delete(d->codec);
AMediaExtractor_delete(d->ex);
d->sawInputEOS = true;
d->sawOutputEOS = true;
}
break;
case kMsgSeek:
{
workerdata *d = (workerdata*)obj;
AMediaExtractor_seekTo(d->ex, 0, AMEDIAEXTRACTOR_SEEK_NEXT_SYNC);
AMediaCodec_flush(d->codec);
d->renderstart = -1;
d->sawInputEOS = false;
d->sawOutputEOS = false;
if (!d->isPlaying) {
d->renderonce = true;
post(kMsgCodecBuffer, d);
}
LOGV("seeked");
}
break;
case kMsgPause:
{
workerdata *d = (workerdata*)obj;
if (d->isPlaying) {
// flush all outstanding codecbuffer messages with a no-op message
d->isPlaying = false;
post(kMsgPauseAck, NULL, true);//清空佇列
}
}
break;
case kMsgResume:
{
workerdata *d = (workerdata*)obj;
if (!d->isPlaying) {
d->renderstart = -1;
d->isPlaying = true;
post(kMsgCodecBuffer, d);
}
}
break;
}
}
複製程式碼讀取了檔案之後，傳送了kMsgCodecBuffer訊息，在這個訊息的處理中呼叫了doCodecWork((workerdata*)obj);看看doCodecWork方法：
//https://www.cnblogs.com/jiy-for-you/p/7282033.html
//https://www.cnblogs.com/Xiegg/p/3428529.html
void doCodecWork(workerdata *d) {
ssize_t bufidx = -1;
if (!d->sawInputEOS) {
//獲取緩衝區，設定超時為2000毫秒
bufidx = AMediaCodec_dequeueInputBuffer(d->codec, 2000);
LOGV("input buffer %zd", bufidx);
if (bufidx >= 0) {
size_t bufsize;
//取到緩衝區輸入流
auto buf = AMediaCodec_getInputBuffer(d->codec, bufidx, &bufsize);
//開始讀取樣本
auto sampleSize = AMediaExtractor_readSampleData(d->ex, buf, bufsize);//d->ex已經設定了選中的軌道
if (sampleSize < 0) {
//如果讀到了尾部
sampleSize = 0;
d->sawInputEOS = true;
LOGV("EOS");
}
//以微秒為單位返回當前樣本的呈現時間。
auto presentationTimeUs = AMediaExtractor_getSampleTime(d->ex);//d->ex已經設定了選中的軌道
//將緩衝區傳遞至解碼器
AMediaCodec_queueInputBuffer(d->codec, bufidx, 0, sampleSize, presentationTimeUs,
d->sawInputEOS ? AMEDIACODEC_BUFFER_FLAG_END_OF_STREAM : 0);
//前進到下一個樣本
AMediaExtractor_advance(d->ex);
}
}
if (!d->sawOutputEOS) {
AMediaCodecBufferInfo info;
//緩衝區 第一步
auto status = AMediaCodec_dequeueOutputBuffer(d->codec, &info, 0);
if (status >= 0) {
if (info.flags & AMEDIACODEC_BUFFER_FLAG_END_OF_STREAM) {
LOGV("output EOS");
d->sawOutputEOS = true;
}
int64_t presentationNano = info.presentationTimeUs * 1000;
if (d->renderstart < 0) {
d->renderstart = systemnanotime() - presentationNano;
}
int64_t delay = (d->renderstart + presentationNano) - systemnanotime();
if (delay > 0) {
//延時操作
//如果緩衝區裡的可展示時間>當前視訊播放的進度，就休眠一下
usleep(delay / 1000);
}
//渲染 ,如果 info.size != 0 等於 true ，就會渲染到surface上 第二步
AMediaCodec_releaseOutputBuffer(d->codec, status, info.size != 0);
if (d->renderonce) {
d->renderonce = false;
return;
}
} else if (status == AMEDIACODEC_INFO_OUTPUT_BUFFERS_CHANGED) {
LOGV("output buffers changed");
} else if (status == AMEDIACODEC_INFO_OUTPUT_FORMAT_CHANGED) {
auto format = AMediaCodec_getOutputFormat(d->codec);
LOGV("format changed to: %s", AMediaFormat_toString(format));
AMediaFormat_delete(format);
} else if (status == AMEDIACODEC_INFO_TRY_AGAIN_LATER) {
//解碼當前幀超時
LOGV("no output buffer right now");
} else {
LOGV("unexpected info code: %zd", status);
}
}
if (!d->sawInputEOS || !d->sawOutputEOS) {
mlooper->post(kMsgCodecBuffer, d);//如果輸入或輸出沒有結束，就回撥自己
}
}
複製程式碼這裡主要是針對解碼器的輸入和輸出操作，註釋如有紕漏請指明。
如果沒有播放完，就會繼續呼叫mlooper->post(kMsgCodecBuffer, d);回撥自己。
如果使用者點選了暫停鍵，傳送kMsgPause訊息並清空訊息佇列，這裡就沒有再繼續回撥doCodecWork((workerdata*)obj)，所以播放就暫停了，但是播放的進度會記錄在workerdata *d = &data中(d->ex和d->codec)。當繼續播放時，會繼續傳送post(kMsgCodecBuffer, d)訊息。
如果點選重播按鈕，傳送mlooper->post(kMsgSeek, &data)訊息：
case kMsgSeek:
{
workerdata *d = (workerdata*)obj;
AMediaExtractor_seekTo(d->ex, 0, AMEDIAEXTRACTOR_SEEK_NEXT_SYNC);
AMediaCodec_flush(d->codec);
d->renderstart = -1;
d->sawInputEOS = false;
d->sawOutputEOS = false;
if (!d->isPlaying) {
d->renderonce = true;
post(kMsgCodecBuffer, d);
}
LOGV("seeked");
}
複製程式碼清除了進度AMediaExtractor_seekTo(d->ex, 0, AMEDIAEXTRACTOR_SEEK_NEXT_SYNC); AMediaCodec_flush(d->codec);，然後傳送post(kMsgCodecBuffer, d)訊息開始播放。
視訊的暫停和退出都是見到傳送訊息:
// set the playing state for the streaming media player
void Java_com_example_nativecodec_NativeCodec_setPlayingStreamingMediaPlayer(JNIEnv* env,
jclass clazz, jboolean isPlaying)
{
LOGV("@@@ playpause: %d", isPlaying);
if (mlooper) {
if (isPlaying) {
mlooper->post(kMsgResume, &data);
} else {
mlooper->post(kMsgPause, &data);
}
}
}
// shut down the native media system
void Java_com_example_nativecodec_NativeCodec_shutdown(JNIEnv* env, jclass clazz)
{
LOGV("@@@ shutdown");
if (mlooper) {
mlooper->post(kMsgDecodeDone, &data, true /* flush */);
mlooper->quit();
delete mlooper;
mlooper = NULL;
}
if (data.window) {
ANativeWindow_release(data.window);
data.window = NULL;
}
}
複製程式碼具體的操作都是在mylooper::handle(int what, void* obj)中處理的。
到此，視訊播放的部分大致結束。